202A Volume 56, Number 8, 2002 focal point BY RALPH E. STURGEON* AND ZOLTA ´ N MESTER INSTITUTE FOR NATIONAL MEASUREMENT STANDARDS NATIONAL RESEARCH COUNCIL CANADA OTTAWA,ONTARIO, K1A 0R6, CANADA Analytical Applications of Volatile Metal Derivatives INTRODUCTION V apor generation is a wide- spread method used for the determination of metals, metalloids, and organometallic com- pounds. Contrary to common belief, a signi cant number of metal(oid)s form volatile species, many of which occur in nature. Vapor generation is a process wherein nonvolatile (usu- ally ionic-metallic or organometal- lic) compounds form volatile or se- mivolatile species through chemical, physical, or biological processes that may result in their transfer from a condensed phase to the gas phase. It is now well known that numerous volatile and semivolatile metal com- pounds are present in our environ- ment as a consequence of both an- thropogenic and natural processes. For example, metal hydrides can be formed in many environmental set- tings where reducing conditions pre- vail. Donard 1 noted release of tin hy- dride from a model seawater-sedi- ment system more than ten years ago. Several reports have also high- lighted detection of metal hydrides * Author to whom correspondence should be sent. arising from land ll and waste water treatment sites. Formation of metal hydrides in nature is primarily con- sidered to be the result of bacterial activity. 2 Other major sources for the volatilization of trace elements in- clude biotic and abiotic alkylation processes (usually methylation). Abiotic (chemical) methylation pro- cesses involve natural methyl-donor compounds, such as iodomethane, methylcobalamine, or humic sub- stances. Chemical methylation of tin in the marine environment has been reported by Weber et al. 3 Bio-meth- ylation of tin, 4 antimony, 5 selenium, 6 arsenic, 7 bismuth, 8 thallium, 9 and mercury 10 have additionally been documented. The possible formation of volatile metal chloride species in the natural environment is a largely neglected eld, but dispersion of or- ganometal halide species in nature may, in fact, be quite signi cant. 11 Feldmann also reported the presence of volatile molybdenum and tung- sten carbonyl species in municipal waste deposit sites. 12 A less known organometallic pollutant, the gaso- line antiknock additive methylcyclo- pentadienylmanganese tricarbonyl, has also been detected in various en- vironmental settings, including food products. 13–15 Finally, the formation and release of elemental vapors also occurs in our natural environment; ionic mercury can be reduced to el- emental mercury and widely dis- persed throughout the atmo- sphere. 16,17 Vapor generation, for the purposes of analytical application, encompas- es many of the same principal routes for syntheses as those found in na- ture, i.e., hydride generation, alkyl- ation, halide generation, generation of metal-carbonyls, and, in the case of mercury and cadmium, generation of elemental vapors. The interesting aspect of this parallel is that analyt- ical vapor generation methods were well-known long before volatile metal species had been detected and identi ed in the environment. The identi cation of volatile metals in nature has largely been accom- plished in the last decade due, in large part, to the increased selectivity and sensitivity of atomic spectro- scopic detectors used in this eld. Another major factor that has con- tributed to this is the ‘‘school of trace element speciation’’, which has pioneered molecular speci c detec- tors and various separation schemes enabling elucidation of the structures of such metal species.
Transcript
202A Volume 56 Number 8 2002
focal pointBY RALPH E STURGEON AND ZOLTAN MESTER
INSTITUTE FOR NATIONAL MEASUREMENT STANDARDS
NATIONAL RESEARCH COUNCIL CANADA
OTTAWA ONTARIO K1A 0R6 CANADA
Analytical Applicationsof Volatile Metal
DerivativesINTRODUCTION
V apor generation is a wide-spread method used for thedetermination of metals
metalloids and organometallic com-pounds Contrary to common beliefa signi cant number of metal(oid)sform volatile species many of whichoccur in nature Vapor generation isa process wherein nonvolatile (usu-ally ionic-metallic or organometal-lic) compounds form volatile or se-mivolatile species through chemicalphysical or biological processes thatmay result in their transfer from acondensed phase to the gas phase Itis now well known that numerousvolatile and semivolatile metal com-pounds are present in our environ-ment as a consequence of both an-thropogenic and natural processesFor example metal hydrides can beformed in many environmental set-tings where reducing conditions pre-vail Donard 1 noted release of tin hy-dride from a model seawater-sedi-ment system more than ten yearsago Several reports have also high-lighted detection of metal hydrides
Author to whom correspondence should besent
arising from land ll and waste watertreatment sites Formation of metalhydrides in nature is primarily con-sidered to be the result of bacterialactivity2 Other major sources for thevolatilization of trace elements in-clude biotic and abiotic alkylationprocesses (usually methylation)Abiotic (chemical) methylation pro-cesses involve natural methyl-donorcompounds such as iodomethanemethylcobalamine or humic sub-stances Chemical methylation of tinin the marine environment has beenreported by Weber et al3 Bio-meth-ylation of tin4 antimony5 selenium6
arsenic7 bismuth8 thallium9 andmercury10 have additionally beendocumented The possible formationof volatile metal chloride species inthe natural environment is a largelyneglected eld but dispersion of or-ganometal halide species in naturemay in fact be quite signi cant11
Feldmann also reported the presenceof volatile molybdenum and tung-sten carbonyl species in municipalwaste deposit sites12 A less knownorganometallic pollutant the gaso-line antiknock additive methylcyclo-pentadienylmanganese tricarbonylhas also been detected in various en-vironmental settings including food
products13ndash15 Finally the formationand release of elemental vapors alsooccurs in our natural environmentionic mercury can be reduced to el-emental mercury and widely dis-persed throughout the atmo-sphere1617
Vapor generation for the purposesof analytical application encompas-es many of the same principal routesfor syntheses as those found in na-ture ie hydride generation alkyl-ation halide generation generationof metal-carbonyls and in the caseof mercury and cadmium generationof elemental vapors The interestingaspect of this parallel is that analyt-ical vapor generation methods werewell-known long befo re volatilemetal species had been detected andidenti ed in the environment Theidenti cation of volatile metals innature has largely been accom-plished in the last decade due inlarge part to the increased selectivityand sensitivity of atomic spectro-scopic detectors used in this eldAnother major factor that has con-tributed to this is the lsquolsquoschool oftrace element speciationrsquorsquo which haspioneered molecular speci c detec-tors and various separation schemesenabling elucidation of the structuresof such metal species
APPLIED SPECTROSCOPY 203A
There is a fundamental drive toundertake vapor generation as it con-fers several signi cant advantagesfor analyses including (1) ef cientmatrix separation which often leadsto a reduction of interferences andbetter detection lim its (2 ) h ightransport ef ciency of analyte intothe atomic spectroscopic detectors(3) in some cases high selectivity topermit differentiation of chemicalspecies of a particular element and(4) enabling use of gas phase sepa-ration methods (ie gas chromatog-raphy) for speciation of some ele-ments In this article a short over-view of the most commonly used va-por generation methods is presentedNo attempt has been made to com-prehensively cite the literature onthis subject and while it is readilyrecognized that the majority of thepublications in this area relate to hy-dride generation a number of theless popular techniques are deliber-ately highlighted It is hoped that thisreview will serve to stimulate read-ers to contribute to the resurgence ofresearch currently giving rise to anexpansion of this technique to an in-creasing number analytes as well asexploration of alternative methodol-ogies for their sampling
HYDRIDE GENERATION
Hydride generation (HG) is theoldest method used for vaporizationof trace metals In the 1830s JamesMarsh developed a method (the socalled lsquoMarsh testrsquo) based on thechemical generation of arsenic hy-drides from various samples (mainlyforensic) The resultant arsine wasthermally decomposed and the me-tallic arsenic deposited onto a glasssurface forming a silvery mirrorSince the early 1970s hydride gen-eration rapidly gained acceptance inthe analytical community and hasbecome one of the more frequentlyused means of processing aqueoussamples for trace element analysis18
Separation of volatile insoluble hy-drides offers a route to the analysisof several important elements suffer-ing problems when determined byconventional methods The hydridesof Groups 14ndash16 are of particular
importance to analy tical atomicspectrometry for the determinationof As Bi Ge Pb Sb Se Sn Teand to some extent In and Tl forwhich limits of detection are readilyimproved by several orders of mag-nitude
As with other vapor-based tech-niques HG comprises several dis-tinct stages notably generation ofthe volatile analyte its collection(optional) and transfer to the atom-izer and lastly decomposition to thegaseous metal atoms (for excitationor ionization) Variations of each ofthese stages have been examined andit is evident that in contrast to manyother lsquolsquoestablishedrsquorsquo vapor generationtechniques hydride generation re-mains a dynamic eld of study withsigni cant research effort directedtowards improving the means ofgeneration the scope of elementsand species covered and the controlof interferences Current generationand collectiontransfer stages arethus considered below emphasis isplaced on novel approaches to gen-eration and the expansion of thesuite of elements amenable to vaporgeneration It is not the purpose ofthis article to discuss interferences oratomization reactionsdetection cellsexcept where these are considered todirectly impact the design of thegenerator
HYDRIDE GENERATIONTECHNIQUES
MetalndashAcid Reduction SystemsThis early technique is based on theuse of a metalndashacid reduction sys-tem such as the ZnndashHCl reaction asshown below
where E is the analyte element andm may or may not equal n This re-duction system appears capable ofgenerating AsH3 SbH3 and H2Sebut As (V) Sb (V) and Se (IV) needto be pre-reduced to their lower va-lence states by the addition of KI orSnCl2 before the hydrides areevolved Zinc metal is then added
and the hydrides along with excessH2 are generated The reaction isslow dif cult to automate suffersfrom large blanks due to impuritiesin the zinc and is inef cient as a re-sult of incomplete reaction andorentrapment of the hydride in the pre-cipitated zinc sludge Other metalndashacid reactions have been investigat-ed including Mg and TiCl3 or a slur-ry of Al reacted with HCl andH2SO4 A further major drawback ofthe metalndashacid reactions is that theyare limited to use with these analytes(occasionally with Bi and Te) Thesefactors along with the availability ofa more effective reducing agenthave served to all but eliminate useof this approach
Sodium Tetrahydroborate Acid-Reduction System This system isnow almost universally used for theproduction of hydrides18 and is basedon the (assumed) generation of na-scent hydrogen in the reaction ofNaBH4 with acid (this mechanismhas never been conclusively proven)
NaBH 1 3H O 1 HCl4 2
xreg H BO 1 NaCl 1 8H3 3m1Efrac34reg EH 1 H (excess) (2)n 2
Arsenic Bi Ge Pb Sb Se Sn TeIn and Tl can all be reduced to theirvolatile hydrides with NaBH4 Apartfrom this lsquolsquoclassicrsquorsquo suite of ele-ments it has recently been discov-ered that considerably more elementsmay be amenable to this vapor gen-eration reaction including Cd19 aswell as a number of transition andplatinum group metals which willbe addressed later
Although initially added to acidi- ed samples in the form of solid pel-lets the reagent is now almost exclu-sively dosed into acidi ed sample so-lutions as a 005ndash10 (mv) solutionlending itself to ease of automationAlthough aqueous systems are themost frequently encountered the hy-drides of some elements (Sb Pb andSn) have also been generated directlyfrom non-aqueous media (chloro-form methyl isobutyl ketone and N-N 9-dimethylformamide DMF) by theaddition of NaBH4 DMF solution asthe reductant
As an alternative to use of solu-tions the reagent has also been usedin a heterogeneous fashion via im-mobilization onto a polymer (stronganion exchanger in the BH4
2 form)for packing into columns throughwhich acidi ed sample is pumped20
use in a packed membrane cell21 oron mobile paper strips onto whichacidi ed sample solutions aredosed22
Sodium tetrahydroborate cannotbe used with impunity the reagent iscapable of introducing contamina-tion and is relatively expensive Itsaqueous solution is unstable andshould be prepared prior to use al-though some stability is conferredthrough addition of NaOH (01ndash2)membrane ltration or refrigerationIn addition the derivatization reac-tion is subject to interferences fromconcomitants in the sample solutionand excessive amounts of H2 can beevolved which may alter the perfor-mance characteristics of some (plas-ma) detection systems
Thermochem ical GenerationThermochemical generation is basedon the injection of a thermosprayaerosol into a methanoloxygen ame where pyrolysis of the eluateoccurs with subsequent thermochem-ical derivatization of the analytesand their transfer to a cool H2 O2 dif-fusion ame for atomization (andAAS detection) The system has todate only been used for the specia-tion of arsenic in HPLC ef uent23
and is noted here for completenessonly as the methodology has notbeen popularized
Electrochemical GenerationElectrochemical hydride generationfor analytical purposes is a substan-tially novel laboratory techniquewhich is not yet fully established andis supported by relatively few pub-lications24 The hydride is generatedin the cathodic space of an electro-lytic cell with concurrent oxidationof water in the anodic compartmentAt least three sequential steps are in-volved in the process including re-duction and deposition of the analyteonto the surface of the cathode (Me)stepwise reaction of the depositedmetal with hydrogen co-generated
on the electrode and desorption ofthe analyte hydride These stepwisereactions can be summarized by thefollowing equation
Me-E 1 me2 1 mH3O1 reg EHm
1 mH2O 1 Me (3)
The design of the electrolytic gen-erator is paramount to success withthis approachmdasha high mass transferrate of the analyte to the cathode sur-face is required for optimum ef -ciency Batch sampling continuous ow and ow injection solution de-livery coupled with batch reactorsthin-layer membrane separator de-signs and tubular cells have beenexamined Several examples of sam-ple treatment manifolds are present-ed in Fig 1
Elimination of the need for a te-trahydroborate reagent and thelsquolsquocleanrsquorsquo reduction of the analytewith electrons is attractive in reduc-
ing the expense and analytical blankassociated with wet chemical hy-dride generation offering the allureof ever lower detection limits Ad-ditionally electrochemical genera-tion is reported to be less subject toanalyte oxidation state in uence ex-hibit greater freedom from interfer-ences arising from concomitant ele-ments and liberate less excess hy-drogen On the other hand produc-tion of a reproducible solid cathodesurface is dif cult and the perfor-mance of the device is highly depen-dent on the nature of the electrodematerial A number of materials havebeen examined including pyrolyticand vitreous graphite lead platinumsilver and amalgamated silver mo-lybdenum and cadmium Platinumis generally preferred as the anodeTo date arsenic gemanium seleni-um antimony and tin hydrides haveall been generated electrochemically
APPLIED SPECTROSCOPY 205A
Photo-induced Generation Pho-tocatalysis by titanium dioxide hasbeen widely studied in natural andsynthetic systems and numeroussacri cial agents have been added assensitizers to enhance quantumyields of reactions of interest Re-cently Kikuchi and Sakamoto 25 re-ported on the generation of H2Se fol-lowing UV-irradiation of an aqueousslurry of SeO4
22 in the presence ofTiO2 and formic acid Selenate is rst reduced to amorphous seleniumby photogenerated electrons andsubsequent oxidation of formic acidserves to raise the energy of the TiO2
conduction band such that reductionto selenide occurs
22 1 2SeO 1 8H 1 6e reg Se 1 4H O4 2
(4)1 2Se 1 2H 1 2e reg H Se2 (5)
Recent studies in our laboratoryindicate that mediation of the TiO2
catalyst is unnecessary in the vola-tilization of selenium from solutionlsquolsquodirectrsquorsquo generation can be achievedthrough UV-irradiation of a diluteformic acid solution spiked withSeO4
22 At this time it is not clearwhat the product of the reaction isphotolysis of formic acid is knownto yield CO CO2 H2 and H2O andit is possible that a stable Se(CO)2 isliberated The ef ciency of the pro-cess (estimated to be 70 at 10 ppbSeO4
22) suggests analytical utility26
By contrast studies by Amouroux etal27 indicated that formation of vol-atile selenium species via abioticmethylation in seawater subjected tosimulated sunlight conditions wasineffective for inorganic selenium
Vapor Generators Each of thegeneration systems noted above maybe operated in a batch continuousor ow injection format for sampleprocessing Examples of such mani-folds which essentially comprise thegenerator and gas-liquid phase sep-arator in a single unit are presentedin Fig 1 Although no in-depth anal-ysis of the relative bene ts of thesemodes of sample processing will beattempted it should be noted that ow injection systems are generallypreferable from the view point of au-
tomation throughput and minimi-zation of sample and reagent con-sumption as well as waste genera-tion precision control of interfer-ences and compatib ility withvarious detection systems Theevolved hydrides may be directlyswept to a suitable atomization cellfor detection or sequestered to effecta preconcentration prior to detectionthereby avoiding both dilution of theanalyte with evolved hydrogen andany transport gases as well as ulti-mately providing for a more lsquolsquosourcefriendlyrsquorsquo transport gas compositionwhen plasma cells are used for de-tection Practical and popular ap-proaches to enhancing sensitivityand detection power in this mannerare essentially limited to cryogenictrapping28 in liquid nitrogen or a dryiceacetone slush and in situ trappingin a heated graphite furnace29 Theseapproaches will be subsequently ad-dressed
Figure 2 summarizes the varietyof current hydride generator designsthat have been used for applicationsand research activities From a re-search perspective the most interest-ing hydride generation systems arethose that achieve rapid interactionof sample and reductant while pro-viding for instantaneous gas-liquidseparation as characterized by thenebulizer-based systems30 31 (Figs2B and 2C) and the moving reduc-tion bed technique 22 (Fig 2A) Thesedesigns offer the potential for reduc-tion or elimination of interferencesfrom concomitant species in solu-tion3031 and more signi cantly pro-vide an avenue for expansion of thescope of elemental coverage3 2ndash37
Successful generation of a number oflsquolsquounconventionalrsquorsquo volatile speciesby reaction of acidi ed sample so-lutions with sodium tetrahydroboratehas now been reported to include (inaddition to Hg and Cd and the clas-sical hydride forming elements) CuAg Au Zn Ni Pd Rh Pt Ti IrMn Co Fe and Cr Little is knownof these species except that they arerelatively unstable requiring rapidgasndashliquid or gasndashsolid separationtechniques and they appear to bemolecular in nature They are tenta-
tively suspected to exist as hydridesThe full scope of elements amenableto such reactions is currently un-known
Reported generation ef cienciesfor these elements have ranged fromgreater than 50 3334 to less than137 As research in this eld is onlyjust emerging the discovery of newspecies is commanding higher pri-ority than their in-depth characteriza-tion and as a consequence onlyminimal analytical use of this infor-mation has been reported to date34 Itis assumed that as these systems be-come better characterized and the ef- ciency of vapor generation is im-proved the methodology will bedriven from one of curiosity to ana-lytical application
ALKYLATION
Alkylation as a method for gen-eration of volatile forms of tracemetals became widespread when in-terest in the speciation of trace met-als emerged It is primarily used toconvert ionic nonvolatile organo-metallic compounds to volatile sat-urated species suitable for gas chro-matographic analysis The techniquewas inspired by synthetic organicchemists and as a consequence wasinitially con ned to use in non-aque-ous media based on the Grignard re-action Availability of sodium tetrae-thylborate in the late 1980s permit-ted ef cient alkylation reactions tobe conducted in aqueous systemsand placed the technique on a rmfoundation for growth
Grignard Alkylation Reactionof an alkyl halide with magnesiummetal in diethyl ether results in theformation of an alkylmagnesium ha-lide the so-called Grignard reagent(named after Victor Grignard whowon the Nobel Prize for its discov-ery in 1912) The transformation of(organo)metallic compounds intosaturated volatile species can beachieved by reaction with a Grignardreagent (R-MgX) in a suitable sol-vent as outlined in Eq 6
R-MgXR -M frac34frac34frac34frac34frac34reg R -M-R (6)x n1 x ndiethylethe r or THF
The R alkyl group may be a methyl-ethyl- propyl- butyl- pentyl- or
206A Volume 56 Number 8 2002
focal point
reg
FIG 2 Hydride generation systems (A) Mov-ing bed generator (reproduced with permis-sion from Ref 22 (B) modied meinhard con-centric nebulizer (C) cross-ow nebulizer (D)tubular electrochemical hydride generator (re-produced with permission from Ref 106) and(E) membrane
hexyl-moiety X is usually chlorideiodide or bromide ion Alkylationvia the Grignard reaction has beenwidely used to effect derivatizationof organotin38ndash40 alkyllead41 anti-mony42 and germanium43 prior totheir determination The Grignard re-action is generally performed in con-junction with an organic solvent ex-traction of the analyte because theGrignard reagent is rapidly hydro-lyzed in water Grignard derivatiza-tion permits the determination of anumber of organometallic species invarious environmental matrices (wa-ter sediment biota) with high deriv-atization yields and excellent repro-ducibility However the procedure isquite lengthy requires several sam-ple manipulation steps which in-crease the risk of contamination de-composition and losses and signif-icantly increases the cost of analysis
Aqueous Phase Alkylation Asnoted earlier sodium tetraethylbo-rate-based ethylation (NaBEt4) is themost commonly used alkylation re-action in the inorganic analyticalspeciation eld Its reaction withmetal or organometallic compoundscan be described using the followingsimpli ed reaction scheme
n1R -M 1 nNaB(C H )x 2 5 4
reg R -M-(C H )x 2 5 n
11 nB(C H ) 1 nNa (7)2 5 3
The principal advantage of NaBEt4
based derivatization is that it can beachieved in aqueous media the nat-ural medium for most environmentaland biological samples and there isthus no need to change phases aswith the use of Grignard reagentsThe reagent has been used for thederivatization of alkyllead4445 or-ganotin46ndash49 and selenium 5051 spe-cies Fernandez et al52 recently re-viewed the various derivatization ap-
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
1 O F X Donard and J H Weber Nature(London) 323 339 (1988)
2 K Michalke E B Wickenheiser MMehring A V Hirner and R HenselAppl Environ Microbiol 66 2791(2000)
3 J H Weber Mar Chem 65 67 (1999)4 J S White J M Tobin and J J Coo-
ney Can J Microbiol 45 541 (1999)5 P Andrewes W R Cullen and E Pol-
ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
Chem 58 173 (1997)7 W R Cullen H Li S A Pergantis G
K Eigendorf and A A Mosi Appl Or-ganomet Chem 9 507 (1995)
8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
11 Z Mester and R E Sturgeon EnvironSci Technol 36 1198 (2002)
12 J Feldmann J Environ Monit 1 33(1999)
13 F Yang and Y K Chau Analyst (Cam-bridge UK) 124 71 (1999)
14 M S Fragueiro F Alava Moreno ILavilla and C Bendicho SpectrochimActa Part B 56 215 (2001)
15 D S Forsyth and L Dusseault FoodAdd Contam 14 301 (1997)
16 M Leermakers W Baeyens R Ebin-ghaus J Kuballa and H H Kock Wa-ter Air Soil Pollut 97 257 (1997)
17 C Brosset and E Lord Water Air SoilPollut 91 187 (1996)
18 J Dedina and D L Tsalev HydrideGeneration Atomic Absorption Spec-trometry (John Wiley and Sons Chich-ester UK 1995)
19 H G Infante M L F Sanchez and ASanz Medel J Anal At Spectrom 11571 (1996)
20 S Tesfalidet and K Irgum Anal Chem61 2079 (1989)
21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
51 M Guidotti J AOAC Int 83 1082(2000)
52 R G Fernandez M M Bayon J I GAlonso and A Sanz Medel J MassSpectrom 35 639 (2000)
53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
APPLIED SPECTROSCOPY 203A
There is a fundamental drive toundertake vapor generation as it con-fers several signi cant advantagesfor analyses including (1) ef cientmatrix separation which often leadsto a reduction of interferences andbetter detection lim its (2 ) h ightransport ef ciency of analyte intothe atomic spectroscopic detectors(3) in some cases high selectivity topermit differentiation of chemicalspecies of a particular element and(4) enabling use of gas phase sepa-ration methods (ie gas chromatog-raphy) for speciation of some ele-ments In this article a short over-view of the most commonly used va-por generation methods is presentedNo attempt has been made to com-prehensively cite the literature onthis subject and while it is readilyrecognized that the majority of thepublications in this area relate to hy-dride generation a number of theless popular techniques are deliber-ately highlighted It is hoped that thisreview will serve to stimulate read-ers to contribute to the resurgence ofresearch currently giving rise to anexpansion of this technique to an in-creasing number analytes as well asexploration of alternative methodol-ogies for their sampling
HYDRIDE GENERATION
Hydride generation (HG) is theoldest method used for vaporizationof trace metals In the 1830s JamesMarsh developed a method (the socalled lsquoMarsh testrsquo) based on thechemical generation of arsenic hy-drides from various samples (mainlyforensic) The resultant arsine wasthermally decomposed and the me-tallic arsenic deposited onto a glasssurface forming a silvery mirrorSince the early 1970s hydride gen-eration rapidly gained acceptance inthe analytical community and hasbecome one of the more frequentlyused means of processing aqueoussamples for trace element analysis18
Separation of volatile insoluble hy-drides offers a route to the analysisof several important elements suffer-ing problems when determined byconventional methods The hydridesof Groups 14ndash16 are of particular
importance to analy tical atomicspectrometry for the determinationof As Bi Ge Pb Sb Se Sn Teand to some extent In and Tl forwhich limits of detection are readilyimproved by several orders of mag-nitude
As with other vapor-based tech-niques HG comprises several dis-tinct stages notably generation ofthe volatile analyte its collection(optional) and transfer to the atom-izer and lastly decomposition to thegaseous metal atoms (for excitationor ionization) Variations of each ofthese stages have been examined andit is evident that in contrast to manyother lsquolsquoestablishedrsquorsquo vapor generationtechniques hydride generation re-mains a dynamic eld of study withsigni cant research effort directedtowards improving the means ofgeneration the scope of elementsand species covered and the controlof interferences Current generationand collectiontransfer stages arethus considered below emphasis isplaced on novel approaches to gen-eration and the expansion of thesuite of elements amenable to vaporgeneration It is not the purpose ofthis article to discuss interferences oratomization reactionsdetection cellsexcept where these are considered todirectly impact the design of thegenerator
HYDRIDE GENERATIONTECHNIQUES
MetalndashAcid Reduction SystemsThis early technique is based on theuse of a metalndashacid reduction sys-tem such as the ZnndashHCl reaction asshown below
where E is the analyte element andm may or may not equal n This re-duction system appears capable ofgenerating AsH3 SbH3 and H2Sebut As (V) Sb (V) and Se (IV) needto be pre-reduced to their lower va-lence states by the addition of KI orSnCl2 before the hydrides areevolved Zinc metal is then added
and the hydrides along with excessH2 are generated The reaction isslow dif cult to automate suffersfrom large blanks due to impuritiesin the zinc and is inef cient as a re-sult of incomplete reaction andorentrapment of the hydride in the pre-cipitated zinc sludge Other metalndashacid reactions have been investigat-ed including Mg and TiCl3 or a slur-ry of Al reacted with HCl andH2SO4 A further major drawback ofthe metalndashacid reactions is that theyare limited to use with these analytes(occasionally with Bi and Te) Thesefactors along with the availability ofa more effective reducing agenthave served to all but eliminate useof this approach
Sodium Tetrahydroborate Acid-Reduction System This system isnow almost universally used for theproduction of hydrides18 and is basedon the (assumed) generation of na-scent hydrogen in the reaction ofNaBH4 with acid (this mechanismhas never been conclusively proven)
NaBH 1 3H O 1 HCl4 2
xreg H BO 1 NaCl 1 8H3 3m1Efrac34reg EH 1 H (excess) (2)n 2
Arsenic Bi Ge Pb Sb Se Sn TeIn and Tl can all be reduced to theirvolatile hydrides with NaBH4 Apartfrom this lsquolsquoclassicrsquorsquo suite of ele-ments it has recently been discov-ered that considerably more elementsmay be amenable to this vapor gen-eration reaction including Cd19 aswell as a number of transition andplatinum group metals which willbe addressed later
Although initially added to acidi- ed samples in the form of solid pel-lets the reagent is now almost exclu-sively dosed into acidi ed sample so-lutions as a 005ndash10 (mv) solutionlending itself to ease of automationAlthough aqueous systems are themost frequently encountered the hy-drides of some elements (Sb Pb andSn) have also been generated directlyfrom non-aqueous media (chloro-form methyl isobutyl ketone and N-N 9-dimethylformamide DMF) by theaddition of NaBH4 DMF solution asthe reductant
As an alternative to use of solu-tions the reagent has also been usedin a heterogeneous fashion via im-mobilization onto a polymer (stronganion exchanger in the BH4
2 form)for packing into columns throughwhich acidi ed sample is pumped20
use in a packed membrane cell21 oron mobile paper strips onto whichacidi ed sample solutions aredosed22
Sodium tetrahydroborate cannotbe used with impunity the reagent iscapable of introducing contamina-tion and is relatively expensive Itsaqueous solution is unstable andshould be prepared prior to use al-though some stability is conferredthrough addition of NaOH (01ndash2)membrane ltration or refrigerationIn addition the derivatization reac-tion is subject to interferences fromconcomitants in the sample solutionand excessive amounts of H2 can beevolved which may alter the perfor-mance characteristics of some (plas-ma) detection systems
Thermochem ical GenerationThermochemical generation is basedon the injection of a thermosprayaerosol into a methanoloxygen ame where pyrolysis of the eluateoccurs with subsequent thermochem-ical derivatization of the analytesand their transfer to a cool H2 O2 dif-fusion ame for atomization (andAAS detection) The system has todate only been used for the specia-tion of arsenic in HPLC ef uent23
and is noted here for completenessonly as the methodology has notbeen popularized
Electrochemical GenerationElectrochemical hydride generationfor analytical purposes is a substan-tially novel laboratory techniquewhich is not yet fully established andis supported by relatively few pub-lications24 The hydride is generatedin the cathodic space of an electro-lytic cell with concurrent oxidationof water in the anodic compartmentAt least three sequential steps are in-volved in the process including re-duction and deposition of the analyteonto the surface of the cathode (Me)stepwise reaction of the depositedmetal with hydrogen co-generated
on the electrode and desorption ofthe analyte hydride These stepwisereactions can be summarized by thefollowing equation
Me-E 1 me2 1 mH3O1 reg EHm
1 mH2O 1 Me (3)
The design of the electrolytic gen-erator is paramount to success withthis approachmdasha high mass transferrate of the analyte to the cathode sur-face is required for optimum ef -ciency Batch sampling continuous ow and ow injection solution de-livery coupled with batch reactorsthin-layer membrane separator de-signs and tubular cells have beenexamined Several examples of sam-ple treatment manifolds are present-ed in Fig 1
Elimination of the need for a te-trahydroborate reagent and thelsquolsquocleanrsquorsquo reduction of the analytewith electrons is attractive in reduc-
ing the expense and analytical blankassociated with wet chemical hy-dride generation offering the allureof ever lower detection limits Ad-ditionally electrochemical genera-tion is reported to be less subject toanalyte oxidation state in uence ex-hibit greater freedom from interfer-ences arising from concomitant ele-ments and liberate less excess hy-drogen On the other hand produc-tion of a reproducible solid cathodesurface is dif cult and the perfor-mance of the device is highly depen-dent on the nature of the electrodematerial A number of materials havebeen examined including pyrolyticand vitreous graphite lead platinumsilver and amalgamated silver mo-lybdenum and cadmium Platinumis generally preferred as the anodeTo date arsenic gemanium seleni-um antimony and tin hydrides haveall been generated electrochemically
APPLIED SPECTROSCOPY 205A
Photo-induced Generation Pho-tocatalysis by titanium dioxide hasbeen widely studied in natural andsynthetic systems and numeroussacri cial agents have been added assensitizers to enhance quantumyields of reactions of interest Re-cently Kikuchi and Sakamoto 25 re-ported on the generation of H2Se fol-lowing UV-irradiation of an aqueousslurry of SeO4
22 in the presence ofTiO2 and formic acid Selenate is rst reduced to amorphous seleniumby photogenerated electrons andsubsequent oxidation of formic acidserves to raise the energy of the TiO2
conduction band such that reductionto selenide occurs
22 1 2SeO 1 8H 1 6e reg Se 1 4H O4 2
(4)1 2Se 1 2H 1 2e reg H Se2 (5)
Recent studies in our laboratoryindicate that mediation of the TiO2
catalyst is unnecessary in the vola-tilization of selenium from solutionlsquolsquodirectrsquorsquo generation can be achievedthrough UV-irradiation of a diluteformic acid solution spiked withSeO4
22 At this time it is not clearwhat the product of the reaction isphotolysis of formic acid is knownto yield CO CO2 H2 and H2O andit is possible that a stable Se(CO)2 isliberated The ef ciency of the pro-cess (estimated to be 70 at 10 ppbSeO4
22) suggests analytical utility26
By contrast studies by Amouroux etal27 indicated that formation of vol-atile selenium species via abioticmethylation in seawater subjected tosimulated sunlight conditions wasineffective for inorganic selenium
Vapor Generators Each of thegeneration systems noted above maybe operated in a batch continuousor ow injection format for sampleprocessing Examples of such mani-folds which essentially comprise thegenerator and gas-liquid phase sep-arator in a single unit are presentedin Fig 1 Although no in-depth anal-ysis of the relative bene ts of thesemodes of sample processing will beattempted it should be noted that ow injection systems are generallypreferable from the view point of au-
tomation throughput and minimi-zation of sample and reagent con-sumption as well as waste genera-tion precision control of interfer-ences and compatib ility withvarious detection systems Theevolved hydrides may be directlyswept to a suitable atomization cellfor detection or sequestered to effecta preconcentration prior to detectionthereby avoiding both dilution of theanalyte with evolved hydrogen andany transport gases as well as ulti-mately providing for a more lsquolsquosourcefriendlyrsquorsquo transport gas compositionwhen plasma cells are used for de-tection Practical and popular ap-proaches to enhancing sensitivityand detection power in this mannerare essentially limited to cryogenictrapping28 in liquid nitrogen or a dryiceacetone slush and in situ trappingin a heated graphite furnace29 Theseapproaches will be subsequently ad-dressed
Figure 2 summarizes the varietyof current hydride generator designsthat have been used for applicationsand research activities From a re-search perspective the most interest-ing hydride generation systems arethose that achieve rapid interactionof sample and reductant while pro-viding for instantaneous gas-liquidseparation as characterized by thenebulizer-based systems30 31 (Figs2B and 2C) and the moving reduc-tion bed technique 22 (Fig 2A) Thesedesigns offer the potential for reduc-tion or elimination of interferencesfrom concomitant species in solu-tion3031 and more signi cantly pro-vide an avenue for expansion of thescope of elemental coverage3 2ndash37
Successful generation of a number oflsquolsquounconventionalrsquorsquo volatile speciesby reaction of acidi ed sample so-lutions with sodium tetrahydroboratehas now been reported to include (inaddition to Hg and Cd and the clas-sical hydride forming elements) CuAg Au Zn Ni Pd Rh Pt Ti IrMn Co Fe and Cr Little is knownof these species except that they arerelatively unstable requiring rapidgasndashliquid or gasndashsolid separationtechniques and they appear to bemolecular in nature They are tenta-
tively suspected to exist as hydridesThe full scope of elements amenableto such reactions is currently un-known
Reported generation ef cienciesfor these elements have ranged fromgreater than 50 3334 to less than137 As research in this eld is onlyjust emerging the discovery of newspecies is commanding higher pri-ority than their in-depth characteriza-tion and as a consequence onlyminimal analytical use of this infor-mation has been reported to date34 Itis assumed that as these systems be-come better characterized and the ef- ciency of vapor generation is im-proved the methodology will bedriven from one of curiosity to ana-lytical application
ALKYLATION
Alkylation as a method for gen-eration of volatile forms of tracemetals became widespread when in-terest in the speciation of trace met-als emerged It is primarily used toconvert ionic nonvolatile organo-metallic compounds to volatile sat-urated species suitable for gas chro-matographic analysis The techniquewas inspired by synthetic organicchemists and as a consequence wasinitially con ned to use in non-aque-ous media based on the Grignard re-action Availability of sodium tetrae-thylborate in the late 1980s permit-ted ef cient alkylation reactions tobe conducted in aqueous systemsand placed the technique on a rmfoundation for growth
Grignard Alkylation Reactionof an alkyl halide with magnesiummetal in diethyl ether results in theformation of an alkylmagnesium ha-lide the so-called Grignard reagent(named after Victor Grignard whowon the Nobel Prize for its discov-ery in 1912) The transformation of(organo)metallic compounds intosaturated volatile species can beachieved by reaction with a Grignardreagent (R-MgX) in a suitable sol-vent as outlined in Eq 6
R-MgXR -M frac34frac34frac34frac34frac34reg R -M-R (6)x n1 x ndiethylethe r or THF
The R alkyl group may be a methyl-ethyl- propyl- butyl- pentyl- or
206A Volume 56 Number 8 2002
focal point
reg
FIG 2 Hydride generation systems (A) Mov-ing bed generator (reproduced with permis-sion from Ref 22 (B) modied meinhard con-centric nebulizer (C) cross-ow nebulizer (D)tubular electrochemical hydride generator (re-produced with permission from Ref 106) and(E) membrane
hexyl-moiety X is usually chlorideiodide or bromide ion Alkylationvia the Grignard reaction has beenwidely used to effect derivatizationof organotin38ndash40 alkyllead41 anti-mony42 and germanium43 prior totheir determination The Grignard re-action is generally performed in con-junction with an organic solvent ex-traction of the analyte because theGrignard reagent is rapidly hydro-lyzed in water Grignard derivatiza-tion permits the determination of anumber of organometallic species invarious environmental matrices (wa-ter sediment biota) with high deriv-atization yields and excellent repro-ducibility However the procedure isquite lengthy requires several sam-ple manipulation steps which in-crease the risk of contamination de-composition and losses and signif-icantly increases the cost of analysis
Aqueous Phase Alkylation Asnoted earlier sodium tetraethylbo-rate-based ethylation (NaBEt4) is themost commonly used alkylation re-action in the inorganic analyticalspeciation eld Its reaction withmetal or organometallic compoundscan be described using the followingsimpli ed reaction scheme
n1R -M 1 nNaB(C H )x 2 5 4
reg R -M-(C H )x 2 5 n
11 nB(C H ) 1 nNa (7)2 5 3
The principal advantage of NaBEt4
based derivatization is that it can beachieved in aqueous media the nat-ural medium for most environmentaland biological samples and there isthus no need to change phases aswith the use of Grignard reagentsThe reagent has been used for thederivatization of alkyllead4445 or-ganotin46ndash49 and selenium 5051 spe-cies Fernandez et al52 recently re-viewed the various derivatization ap-
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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2 K Michalke E B Wickenheiser MMehring A V Hirner and R HenselAppl Environ Microbiol 66 2791(2000)
3 J H Weber Mar Chem 65 67 (1999)4 J S White J M Tobin and J J Coo-
ney Can J Microbiol 45 541 (1999)5 P Andrewes W R Cullen and E Pol-
ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
Chem 58 173 (1997)7 W R Cullen H Li S A Pergantis G
K Eigendorf and A A Mosi Appl Or-ganomet Chem 9 507 (1995)
8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
11 Z Mester and R E Sturgeon EnvironSci Technol 36 1198 (2002)
12 J Feldmann J Environ Monit 1 33(1999)
13 F Yang and Y K Chau Analyst (Cam-bridge UK) 124 71 (1999)
14 M S Fragueiro F Alava Moreno ILavilla and C Bendicho SpectrochimActa Part B 56 215 (2001)
15 D S Forsyth and L Dusseault FoodAdd Contam 14 301 (1997)
16 M Leermakers W Baeyens R Ebin-ghaus J Kuballa and H H Kock Wa-ter Air Soil Pollut 97 257 (1997)
17 C Brosset and E Lord Water Air SoilPollut 91 187 (1996)
18 J Dedina and D L Tsalev HydrideGeneration Atomic Absorption Spec-trometry (John Wiley and Sons Chich-ester UK 1995)
19 H G Infante M L F Sanchez and ASanz Medel J Anal At Spectrom 11571 (1996)
20 S Tesfalidet and K Irgum Anal Chem61 2079 (1989)
21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
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53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
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58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
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65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
As an alternative to use of solu-tions the reagent has also been usedin a heterogeneous fashion via im-mobilization onto a polymer (stronganion exchanger in the BH4
2 form)for packing into columns throughwhich acidi ed sample is pumped20
use in a packed membrane cell21 oron mobile paper strips onto whichacidi ed sample solutions aredosed22
Sodium tetrahydroborate cannotbe used with impunity the reagent iscapable of introducing contamina-tion and is relatively expensive Itsaqueous solution is unstable andshould be prepared prior to use al-though some stability is conferredthrough addition of NaOH (01ndash2)membrane ltration or refrigerationIn addition the derivatization reac-tion is subject to interferences fromconcomitants in the sample solutionand excessive amounts of H2 can beevolved which may alter the perfor-mance characteristics of some (plas-ma) detection systems
Thermochem ical GenerationThermochemical generation is basedon the injection of a thermosprayaerosol into a methanoloxygen ame where pyrolysis of the eluateoccurs with subsequent thermochem-ical derivatization of the analytesand their transfer to a cool H2 O2 dif-fusion ame for atomization (andAAS detection) The system has todate only been used for the specia-tion of arsenic in HPLC ef uent23
and is noted here for completenessonly as the methodology has notbeen popularized
Electrochemical GenerationElectrochemical hydride generationfor analytical purposes is a substan-tially novel laboratory techniquewhich is not yet fully established andis supported by relatively few pub-lications24 The hydride is generatedin the cathodic space of an electro-lytic cell with concurrent oxidationof water in the anodic compartmentAt least three sequential steps are in-volved in the process including re-duction and deposition of the analyteonto the surface of the cathode (Me)stepwise reaction of the depositedmetal with hydrogen co-generated
on the electrode and desorption ofthe analyte hydride These stepwisereactions can be summarized by thefollowing equation
Me-E 1 me2 1 mH3O1 reg EHm
1 mH2O 1 Me (3)
The design of the electrolytic gen-erator is paramount to success withthis approachmdasha high mass transferrate of the analyte to the cathode sur-face is required for optimum ef -ciency Batch sampling continuous ow and ow injection solution de-livery coupled with batch reactorsthin-layer membrane separator de-signs and tubular cells have beenexamined Several examples of sam-ple treatment manifolds are present-ed in Fig 1
Elimination of the need for a te-trahydroborate reagent and thelsquolsquocleanrsquorsquo reduction of the analytewith electrons is attractive in reduc-
ing the expense and analytical blankassociated with wet chemical hy-dride generation offering the allureof ever lower detection limits Ad-ditionally electrochemical genera-tion is reported to be less subject toanalyte oxidation state in uence ex-hibit greater freedom from interfer-ences arising from concomitant ele-ments and liberate less excess hy-drogen On the other hand produc-tion of a reproducible solid cathodesurface is dif cult and the perfor-mance of the device is highly depen-dent on the nature of the electrodematerial A number of materials havebeen examined including pyrolyticand vitreous graphite lead platinumsilver and amalgamated silver mo-lybdenum and cadmium Platinumis generally preferred as the anodeTo date arsenic gemanium seleni-um antimony and tin hydrides haveall been generated electrochemically
APPLIED SPECTROSCOPY 205A
Photo-induced Generation Pho-tocatalysis by titanium dioxide hasbeen widely studied in natural andsynthetic systems and numeroussacri cial agents have been added assensitizers to enhance quantumyields of reactions of interest Re-cently Kikuchi and Sakamoto 25 re-ported on the generation of H2Se fol-lowing UV-irradiation of an aqueousslurry of SeO4
22 in the presence ofTiO2 and formic acid Selenate is rst reduced to amorphous seleniumby photogenerated electrons andsubsequent oxidation of formic acidserves to raise the energy of the TiO2
conduction band such that reductionto selenide occurs
22 1 2SeO 1 8H 1 6e reg Se 1 4H O4 2
(4)1 2Se 1 2H 1 2e reg H Se2 (5)
Recent studies in our laboratoryindicate that mediation of the TiO2
catalyst is unnecessary in the vola-tilization of selenium from solutionlsquolsquodirectrsquorsquo generation can be achievedthrough UV-irradiation of a diluteformic acid solution spiked withSeO4
22 At this time it is not clearwhat the product of the reaction isphotolysis of formic acid is knownto yield CO CO2 H2 and H2O andit is possible that a stable Se(CO)2 isliberated The ef ciency of the pro-cess (estimated to be 70 at 10 ppbSeO4
22) suggests analytical utility26
By contrast studies by Amouroux etal27 indicated that formation of vol-atile selenium species via abioticmethylation in seawater subjected tosimulated sunlight conditions wasineffective for inorganic selenium
Vapor Generators Each of thegeneration systems noted above maybe operated in a batch continuousor ow injection format for sampleprocessing Examples of such mani-folds which essentially comprise thegenerator and gas-liquid phase sep-arator in a single unit are presentedin Fig 1 Although no in-depth anal-ysis of the relative bene ts of thesemodes of sample processing will beattempted it should be noted that ow injection systems are generallypreferable from the view point of au-
tomation throughput and minimi-zation of sample and reagent con-sumption as well as waste genera-tion precision control of interfer-ences and compatib ility withvarious detection systems Theevolved hydrides may be directlyswept to a suitable atomization cellfor detection or sequestered to effecta preconcentration prior to detectionthereby avoiding both dilution of theanalyte with evolved hydrogen andany transport gases as well as ulti-mately providing for a more lsquolsquosourcefriendlyrsquorsquo transport gas compositionwhen plasma cells are used for de-tection Practical and popular ap-proaches to enhancing sensitivityand detection power in this mannerare essentially limited to cryogenictrapping28 in liquid nitrogen or a dryiceacetone slush and in situ trappingin a heated graphite furnace29 Theseapproaches will be subsequently ad-dressed
Figure 2 summarizes the varietyof current hydride generator designsthat have been used for applicationsand research activities From a re-search perspective the most interest-ing hydride generation systems arethose that achieve rapid interactionof sample and reductant while pro-viding for instantaneous gas-liquidseparation as characterized by thenebulizer-based systems30 31 (Figs2B and 2C) and the moving reduc-tion bed technique 22 (Fig 2A) Thesedesigns offer the potential for reduc-tion or elimination of interferencesfrom concomitant species in solu-tion3031 and more signi cantly pro-vide an avenue for expansion of thescope of elemental coverage3 2ndash37
Successful generation of a number oflsquolsquounconventionalrsquorsquo volatile speciesby reaction of acidi ed sample so-lutions with sodium tetrahydroboratehas now been reported to include (inaddition to Hg and Cd and the clas-sical hydride forming elements) CuAg Au Zn Ni Pd Rh Pt Ti IrMn Co Fe and Cr Little is knownof these species except that they arerelatively unstable requiring rapidgasndashliquid or gasndashsolid separationtechniques and they appear to bemolecular in nature They are tenta-
tively suspected to exist as hydridesThe full scope of elements amenableto such reactions is currently un-known
Reported generation ef cienciesfor these elements have ranged fromgreater than 50 3334 to less than137 As research in this eld is onlyjust emerging the discovery of newspecies is commanding higher pri-ority than their in-depth characteriza-tion and as a consequence onlyminimal analytical use of this infor-mation has been reported to date34 Itis assumed that as these systems be-come better characterized and the ef- ciency of vapor generation is im-proved the methodology will bedriven from one of curiosity to ana-lytical application
ALKYLATION
Alkylation as a method for gen-eration of volatile forms of tracemetals became widespread when in-terest in the speciation of trace met-als emerged It is primarily used toconvert ionic nonvolatile organo-metallic compounds to volatile sat-urated species suitable for gas chro-matographic analysis The techniquewas inspired by synthetic organicchemists and as a consequence wasinitially con ned to use in non-aque-ous media based on the Grignard re-action Availability of sodium tetrae-thylborate in the late 1980s permit-ted ef cient alkylation reactions tobe conducted in aqueous systemsand placed the technique on a rmfoundation for growth
Grignard Alkylation Reactionof an alkyl halide with magnesiummetal in diethyl ether results in theformation of an alkylmagnesium ha-lide the so-called Grignard reagent(named after Victor Grignard whowon the Nobel Prize for its discov-ery in 1912) The transformation of(organo)metallic compounds intosaturated volatile species can beachieved by reaction with a Grignardreagent (R-MgX) in a suitable sol-vent as outlined in Eq 6
R-MgXR -M frac34frac34frac34frac34frac34reg R -M-R (6)x n1 x ndiethylethe r or THF
The R alkyl group may be a methyl-ethyl- propyl- butyl- pentyl- or
206A Volume 56 Number 8 2002
focal point
reg
FIG 2 Hydride generation systems (A) Mov-ing bed generator (reproduced with permis-sion from Ref 22 (B) modied meinhard con-centric nebulizer (C) cross-ow nebulizer (D)tubular electrochemical hydride generator (re-produced with permission from Ref 106) and(E) membrane
hexyl-moiety X is usually chlorideiodide or bromide ion Alkylationvia the Grignard reaction has beenwidely used to effect derivatizationof organotin38ndash40 alkyllead41 anti-mony42 and germanium43 prior totheir determination The Grignard re-action is generally performed in con-junction with an organic solvent ex-traction of the analyte because theGrignard reagent is rapidly hydro-lyzed in water Grignard derivatiza-tion permits the determination of anumber of organometallic species invarious environmental matrices (wa-ter sediment biota) with high deriv-atization yields and excellent repro-ducibility However the procedure isquite lengthy requires several sam-ple manipulation steps which in-crease the risk of contamination de-composition and losses and signif-icantly increases the cost of analysis
Aqueous Phase Alkylation Asnoted earlier sodium tetraethylbo-rate-based ethylation (NaBEt4) is themost commonly used alkylation re-action in the inorganic analyticalspeciation eld Its reaction withmetal or organometallic compoundscan be described using the followingsimpli ed reaction scheme
n1R -M 1 nNaB(C H )x 2 5 4
reg R -M-(C H )x 2 5 n
11 nB(C H ) 1 nNa (7)2 5 3
The principal advantage of NaBEt4
based derivatization is that it can beachieved in aqueous media the nat-ural medium for most environmentaland biological samples and there isthus no need to change phases aswith the use of Grignard reagentsThe reagent has been used for thederivatization of alkyllead4445 or-ganotin46ndash49 and selenium 5051 spe-cies Fernandez et al52 recently re-viewed the various derivatization ap-
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
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59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
APPLIED SPECTROSCOPY 205A
Photo-induced Generation Pho-tocatalysis by titanium dioxide hasbeen widely studied in natural andsynthetic systems and numeroussacri cial agents have been added assensitizers to enhance quantumyields of reactions of interest Re-cently Kikuchi and Sakamoto 25 re-ported on the generation of H2Se fol-lowing UV-irradiation of an aqueousslurry of SeO4
22 in the presence ofTiO2 and formic acid Selenate is rst reduced to amorphous seleniumby photogenerated electrons andsubsequent oxidation of formic acidserves to raise the energy of the TiO2
conduction band such that reductionto selenide occurs
22 1 2SeO 1 8H 1 6e reg Se 1 4H O4 2
(4)1 2Se 1 2H 1 2e reg H Se2 (5)
Recent studies in our laboratoryindicate that mediation of the TiO2
catalyst is unnecessary in the vola-tilization of selenium from solutionlsquolsquodirectrsquorsquo generation can be achievedthrough UV-irradiation of a diluteformic acid solution spiked withSeO4
22 At this time it is not clearwhat the product of the reaction isphotolysis of formic acid is knownto yield CO CO2 H2 and H2O andit is possible that a stable Se(CO)2 isliberated The ef ciency of the pro-cess (estimated to be 70 at 10 ppbSeO4
22) suggests analytical utility26
By contrast studies by Amouroux etal27 indicated that formation of vol-atile selenium species via abioticmethylation in seawater subjected tosimulated sunlight conditions wasineffective for inorganic selenium
Vapor Generators Each of thegeneration systems noted above maybe operated in a batch continuousor ow injection format for sampleprocessing Examples of such mani-folds which essentially comprise thegenerator and gas-liquid phase sep-arator in a single unit are presentedin Fig 1 Although no in-depth anal-ysis of the relative bene ts of thesemodes of sample processing will beattempted it should be noted that ow injection systems are generallypreferable from the view point of au-
tomation throughput and minimi-zation of sample and reagent con-sumption as well as waste genera-tion precision control of interfer-ences and compatib ility withvarious detection systems Theevolved hydrides may be directlyswept to a suitable atomization cellfor detection or sequestered to effecta preconcentration prior to detectionthereby avoiding both dilution of theanalyte with evolved hydrogen andany transport gases as well as ulti-mately providing for a more lsquolsquosourcefriendlyrsquorsquo transport gas compositionwhen plasma cells are used for de-tection Practical and popular ap-proaches to enhancing sensitivityand detection power in this mannerare essentially limited to cryogenictrapping28 in liquid nitrogen or a dryiceacetone slush and in situ trappingin a heated graphite furnace29 Theseapproaches will be subsequently ad-dressed
Figure 2 summarizes the varietyof current hydride generator designsthat have been used for applicationsand research activities From a re-search perspective the most interest-ing hydride generation systems arethose that achieve rapid interactionof sample and reductant while pro-viding for instantaneous gas-liquidseparation as characterized by thenebulizer-based systems30 31 (Figs2B and 2C) and the moving reduc-tion bed technique 22 (Fig 2A) Thesedesigns offer the potential for reduc-tion or elimination of interferencesfrom concomitant species in solu-tion3031 and more signi cantly pro-vide an avenue for expansion of thescope of elemental coverage3 2ndash37
Successful generation of a number oflsquolsquounconventionalrsquorsquo volatile speciesby reaction of acidi ed sample so-lutions with sodium tetrahydroboratehas now been reported to include (inaddition to Hg and Cd and the clas-sical hydride forming elements) CuAg Au Zn Ni Pd Rh Pt Ti IrMn Co Fe and Cr Little is knownof these species except that they arerelatively unstable requiring rapidgasndashliquid or gasndashsolid separationtechniques and they appear to bemolecular in nature They are tenta-
tively suspected to exist as hydridesThe full scope of elements amenableto such reactions is currently un-known
Reported generation ef cienciesfor these elements have ranged fromgreater than 50 3334 to less than137 As research in this eld is onlyjust emerging the discovery of newspecies is commanding higher pri-ority than their in-depth characteriza-tion and as a consequence onlyminimal analytical use of this infor-mation has been reported to date34 Itis assumed that as these systems be-come better characterized and the ef- ciency of vapor generation is im-proved the methodology will bedriven from one of curiosity to ana-lytical application
ALKYLATION
Alkylation as a method for gen-eration of volatile forms of tracemetals became widespread when in-terest in the speciation of trace met-als emerged It is primarily used toconvert ionic nonvolatile organo-metallic compounds to volatile sat-urated species suitable for gas chro-matographic analysis The techniquewas inspired by synthetic organicchemists and as a consequence wasinitially con ned to use in non-aque-ous media based on the Grignard re-action Availability of sodium tetrae-thylborate in the late 1980s permit-ted ef cient alkylation reactions tobe conducted in aqueous systemsand placed the technique on a rmfoundation for growth
Grignard Alkylation Reactionof an alkyl halide with magnesiummetal in diethyl ether results in theformation of an alkylmagnesium ha-lide the so-called Grignard reagent(named after Victor Grignard whowon the Nobel Prize for its discov-ery in 1912) The transformation of(organo)metallic compounds intosaturated volatile species can beachieved by reaction with a Grignardreagent (R-MgX) in a suitable sol-vent as outlined in Eq 6
R-MgXR -M frac34frac34frac34frac34frac34reg R -M-R (6)x n1 x ndiethylethe r or THF
The R alkyl group may be a methyl-ethyl- propyl- butyl- pentyl- or
206A Volume 56 Number 8 2002
focal point
reg
FIG 2 Hydride generation systems (A) Mov-ing bed generator (reproduced with permis-sion from Ref 22 (B) modied meinhard con-centric nebulizer (C) cross-ow nebulizer (D)tubular electrochemical hydride generator (re-produced with permission from Ref 106) and(E) membrane
hexyl-moiety X is usually chlorideiodide or bromide ion Alkylationvia the Grignard reaction has beenwidely used to effect derivatizationof organotin38ndash40 alkyllead41 anti-mony42 and germanium43 prior totheir determination The Grignard re-action is generally performed in con-junction with an organic solvent ex-traction of the analyte because theGrignard reagent is rapidly hydro-lyzed in water Grignard derivatiza-tion permits the determination of anumber of organometallic species invarious environmental matrices (wa-ter sediment biota) with high deriv-atization yields and excellent repro-ducibility However the procedure isquite lengthy requires several sam-ple manipulation steps which in-crease the risk of contamination de-composition and losses and signif-icantly increases the cost of analysis
Aqueous Phase Alkylation Asnoted earlier sodium tetraethylbo-rate-based ethylation (NaBEt4) is themost commonly used alkylation re-action in the inorganic analyticalspeciation eld Its reaction withmetal or organometallic compoundscan be described using the followingsimpli ed reaction scheme
n1R -M 1 nNaB(C H )x 2 5 4
reg R -M-(C H )x 2 5 n
11 nB(C H ) 1 nNa (7)2 5 3
The principal advantage of NaBEt4
based derivatization is that it can beachieved in aqueous media the nat-ural medium for most environmentaland biological samples and there isthus no need to change phases aswith the use of Grignard reagentsThe reagent has been used for thederivatization of alkyllead4445 or-ganotin46ndash49 and selenium 5051 spe-cies Fernandez et al52 recently re-viewed the various derivatization ap-
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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2 K Michalke E B Wickenheiser MMehring A V Hirner and R HenselAppl Environ Microbiol 66 2791(2000)
3 J H Weber Mar Chem 65 67 (1999)4 J S White J M Tobin and J J Coo-
ney Can J Microbiol 45 541 (1999)5 P Andrewes W R Cullen and E Pol-
ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
Chem 58 173 (1997)7 W R Cullen H Li S A Pergantis G
K Eigendorf and A A Mosi Appl Or-ganomet Chem 9 507 (1995)
8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
11 Z Mester and R E Sturgeon EnvironSci Technol 36 1198 (2002)
12 J Feldmann J Environ Monit 1 33(1999)
13 F Yang and Y K Chau Analyst (Cam-bridge UK) 124 71 (1999)
14 M S Fragueiro F Alava Moreno ILavilla and C Bendicho SpectrochimActa Part B 56 215 (2001)
15 D S Forsyth and L Dusseault FoodAdd Contam 14 301 (1997)
16 M Leermakers W Baeyens R Ebin-ghaus J Kuballa and H H Kock Wa-ter Air Soil Pollut 97 257 (1997)
17 C Brosset and E Lord Water Air SoilPollut 91 187 (1996)
18 J Dedina and D L Tsalev HydrideGeneration Atomic Absorption Spec-trometry (John Wiley and Sons Chich-ester UK 1995)
19 H G Infante M L F Sanchez and ASanz Medel J Anal At Spectrom 11571 (1996)
20 S Tesfalidet and K Irgum Anal Chem61 2079 (1989)
21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
51 M Guidotti J AOAC Int 83 1082(2000)
52 R G Fernandez M M Bayon J I GAlonso and A Sanz Medel J MassSpectrom 35 639 (2000)
53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
206A Volume 56 Number 8 2002
focal point
reg
FIG 2 Hydride generation systems (A) Mov-ing bed generator (reproduced with permis-sion from Ref 22 (B) modied meinhard con-centric nebulizer (C) cross-ow nebulizer (D)tubular electrochemical hydride generator (re-produced with permission from Ref 106) and(E) membrane
hexyl-moiety X is usually chlorideiodide or bromide ion Alkylationvia the Grignard reaction has beenwidely used to effect derivatizationof organotin38ndash40 alkyllead41 anti-mony42 and germanium43 prior totheir determination The Grignard re-action is generally performed in con-junction with an organic solvent ex-traction of the analyte because theGrignard reagent is rapidly hydro-lyzed in water Grignard derivatiza-tion permits the determination of anumber of organometallic species invarious environmental matrices (wa-ter sediment biota) with high deriv-atization yields and excellent repro-ducibility However the procedure isquite lengthy requires several sam-ple manipulation steps which in-crease the risk of contamination de-composition and losses and signif-icantly increases the cost of analysis
Aqueous Phase Alkylation Asnoted earlier sodium tetraethylbo-rate-based ethylation (NaBEt4) is themost commonly used alkylation re-action in the inorganic analyticalspeciation eld Its reaction withmetal or organometallic compoundscan be described using the followingsimpli ed reaction scheme
n1R -M 1 nNaB(C H )x 2 5 4
reg R -M-(C H )x 2 5 n
11 nB(C H ) 1 nNa (7)2 5 3
The principal advantage of NaBEt4
based derivatization is that it can beachieved in aqueous media the nat-ural medium for most environmentaland biological samples and there isthus no need to change phases aswith the use of Grignard reagentsThe reagent has been used for thederivatization of alkyllead4445 or-ganotin46ndash49 and selenium 5051 spe-cies Fernandez et al52 recently re-viewed the various derivatization ap-
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
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8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
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21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
51 M Guidotti J AOAC Int 83 1082(2000)
52 R G Fernandez M M Bayon J I GAlonso and A Sanz Medel J MassSpectrom 35 639 (2000)
53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
APPLIED SPECTROSCOPY 207A
FIG 2 Continued
proaches used for methylmercurydetermination Morabito et al53 com-pared various derivatization meth-ods including ethylation by sodiumtetraethylborate and Zu aurre etal54 compared different alkylationmethods for trimethyllead determi-nation One of the main limitationsof the ethylation technique is that it
cannot be applied to the speciationof ethyl ligand containing speciesFor example reaction of triethylleadand inorganic lead w ith NaBEt4
yields the same product ie tetra-ethyllead Yu and Pawliszyn45 over-came this limitation by utilizing deu-terated NaBEt4 for the derivatizationof organolead compounds The ap-
plication of the propyl-5556 and phen-ylborate5758 reagent also appears tobe a promising approach
HALIDE GENERATION
Halide generation aims to achievethe formation of volatile metal(oid)-and organometal-halide species 59
The chemistry involved is generallyquite straightforward the halideforming species are simply exposedto a signi cant excess of a halide inthe form of a salt or acid (such assodium chloride HCl) but in somecases high acidity is also requiredThe generated metal halide speciescan then be directly introduced intoa detector Alternatively pre-concen-tration on sorbent material can beundertaken Inorganic arsenicmono- and dimethylarsenic60 ger-manium61 tributyltin62 methylmer-cury63 silicon64ndash66 and gallium haveall been subject to halide generationfor analytical purposes The highboiling point and thermal instabilityof these species generally makesthem unsuitable for gas chromato-graphic analysis At the same timethe high boiling points facilitateroom temperature trapping as op-posed to cryotrapping necessary forhydrides This approach is also freefrom classical transition metal inter-ferences typical of hydride genera-tion methods
COLD VAPOR GENERATION
Cold vapor generation for deter-mination of mercury was rst de-scribed in the late 1960s after whichit became the leading method formercury determination Cold vaporgeneration is based on the reductionof mercury with a reducing agentsuch as sodium tetrahydroborate ortin chloride to generate the elemen-tal state Since the vapor pressure ofthe resultant elemental mercury isquite high it can be readily separat-ed (purged) from the aqueous matrixand introduced into a detector Be-cause the analyte is already in atomicfrom there is no need for an addi-tional atomization step permittingsimple and sensitive room tempera-ture sources such as atomic absorp-tion and uorescence to be used
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
208A Volume 56 Number 8 2002
focal point
In most cases tin chloride is fa-vored over sodium tetrahydroborateas reductant because of the relativesimplicity of its puri cation (if traceconcentrations are targeted) and be-cause no hydrogen production is in-volved that can in uence the stabil-ity of plasma-based detectors How-ever tin chloride reduction is slowerand requires additional purge gas toeffect the gasndashliquid separation pro-cess The tin chloride based reduc-tion system is by itself not capableof reducing organomercury speciesOrganomercurials can be reduced toelemental mercury with a high con-centration of sodium hydroxide andcupric sulfate in combination withtin chloride Sodium tetrahydrobor-ate is capable of reducing both in-organic and organomercury speciesHowever the resulting gaseous prod-ucts are different with inorganicmercury being reduced to elementalmercury but the organomercurialsbeing transformed to hydride spe-cies The different chemical natureof the products facilitates the furtherisolationspeciation of mercury usinggas chromatography For measure-ment of total mercury the differingchemical nature of the product maybe disadvantageous
Until recently it was widely ac-knowledged that the only metal ca-pable of being generated as a mon-atomic vapor at room temperaturefor analytical purposes was mercuryHowever in 1995 two independentresearch groups reported on the de-tection of cadmium cold vapor fol-lowing reaction of the aquo-ion inacidic medium with tetrahydrobo-rate6768 The reaction ef ciency wasenhanced in the presence of organicreagents such as thiourea68 or a ves-icle inducing reagent (organized me-dium) such as didodecyldimethylam-monium bromide69 to the extent thatoverall ef ciencies for generation ofCd 0 reached 75 It was postulated 69
that the reaction intermediate may beCdH2 transportable over signi cantdistances at room temperaturewhere under the in uence of the UVexcitation from a hollow cathodelamp it ultimately decomposes toyield the measurable Cd 0 It should
be noted that there are numerous ear-lier reports that volatile cadmiumspecies (presumably the hydride) canbe generated by reaction of the aquo-ion with tetrahydroborate Applica-tion of cold vapor generation foranalysis of Cd in several certi ed en-vironmental reference materials wassuccessfully undertaken by Guo andGuo68
OTHER DERIVATIZATIONMETHODS
In the late 1960s researchers wereexploring use of chelating agents tofoster formation of volatile metalcompounds Among the most popu-lar for th is purpose were b-di-ketones monothio b-diketones b-keto amines dithiocarbamates di-thiophosphates and crown ethersThe vapor pressures of these metalcomplexes are suf ciently high thattheir gas chromatographic separationand detection could be achievedWith the availability of electron cap-ture detectors (ECD) additionalcomplexing agents were designedwith halide containing ligands to en-hance the sensitivity of determina-tion of metals by ECD detecionCurrently the popularity of com-plexation based volatilization has de-creased likely due to the improvedperformance of atomic spectroscopicdetectors that can now address manyof the interferences intended to beavoided by these matrix separationapproaches Using chelation-basedgas chromatography determinationsof Be Al Cr Ni Fe Cu Co Lu ErMg Ca Ba Nd Gd Sc Se Mn ZrHg Zn Ga Rh Ru Ho Dy Tb VYb Tb Th U Pb Cd and Pt havebeen reported70 Chromatographicseparation of the species was origi-nally required because conventionalGC detectors cannot differentiate thevarious metal chelates However byusing advanced solid phase trappingsuch as solid phase microextraction(SPME) or solid phase extraction(SPE) thermal desorption and directin troduction in to atomic spectro -scopic detectors (ICPs AAS) che-lation-based metal volatilization maywell attract considerable attention inthe near future For example volatile
and semivolatile thermally unstablemetallic compounds can now bemeasured by SPMEndashthermal desorp-tionndashICP-MS63 Combined withmain stream hydride and ethyl gen-eration methods the broad scope ofchelation-based techniques mayserve to substantially expand analyt-ical application of volatile metal de-rivatives
Osmium can be vaporized by ox-idation to its highest valence stateOs (VIII) Generation of volatile os-mium tetraoxide by using strong ox-idizing agents such as H2O2 is apopular means of Os determinationas it can be purged from solution andtransported to a detector Inductivelycoupled plasma multicollector high-resolution mass spectrometry for ex-ample has become a method ofchoice for the determination of theisotopic composition of osmium ingeological samples71
The feasibility of the generation ofmetal carbonyl species for analyticalpurposes has been demonstrated byseveral authors Nickel tetracarbonylcan be formed by reaction of ele-mental nickel with carbon monoxideat ambient temperature and pres-sure72 Rigin73 has also reported onthe similar generation of iron nickeland cobalt carbonyls under labora-tory conditions but no additionalstudies have corroborated these re-ports The generation of carbonylspecies has not become very popularprobably because it requires the han-dling of toxic CO gas RecentlyHuang et al35 demonstrated that avolatile nickel compound (possiblythe hydride72) can be readily gener-ated by a simple reaction of Ni21
with NaBH4
TRAPPINGPRECONCENTRATIONSAMPLE INTRODUCTION
Volatile metals collected from ourenvironment or synthesized in thelaboratory can be directly introducedinto a suitable detector and identi- edquanti ed or rst trapped andpreconcentrated in or on an appro-priate medium Depending on thedetection power needed as well as
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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212A Volume 56 Number 8 2002
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28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
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33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
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36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
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40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
51 M Guidotti J AOAC Int 83 1082(2000)
52 R G Fernandez M M Bayon J I GAlonso and A Sanz Medel J MassSpectrom 35 639 (2000)
53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
APPLIED SPECTROSCOPY 209A
the stability of the volatile speciesan informed choice can be made
Direct Sample Introduction Inmany cases direct introduction ofvolatile metals into a detector is thesimplest approach Following a con-densed phase generation process thevolatile compounds are simply lib-erated into the gas phase and astream of gas is used to introducethem into the detection system Di-rect gaseous sample introduction re-quires ful lment of several criteria(1) availability of a detector ( eldanalyses are generally not feasible)(2) suf ciently high concentrationsof analyte in the gas phase to permitdirect detection (not usually the casefor ultratrace analysis or for environ-mental air sampling) and (3) com-pounds co-generated or present withthe analyte (eg hydrogen boronhydride HCl and high sodium ionconcentration during hydride gener-ation resulting from an inef cientgasndashliquid separator or CO2 and N2
present in air samples) are compati-ble with the available detection sys-tem
In Situ Trapping The major im-pediments to enhancing performanceof conventional hydride and cold va-por generation detection techniquesare that the species are often dilutedby the co-evolved hydrogen as wellas any gas used to ensure phase sep-aration and transport thereby de-creasing the sensitivity of determi-nation Further many hydrides arenot ef ciently atomized in popularheated quartz tube sources (egGeH4)
In situ trapping techniques whichcouple the graphite furnace with hy-dride generation (and element alkyl-ation) permit signi cant enhance-ment in relative detection powerover conventional batch and contin-uous generation approaches for theultratrace determination of metallichydrides 29 The graphite furnace isused to decompose the volatile hy-dride and trap the analyte species onthe tube surface or on a surface pre-treated with palladium or a perma-nent modi er such as iridium Thiseffects a clean rapid separation fromthe matrix as well as convenient
concentration of the analyte In thismanner large sample volumes maybe addressed using either batch or ow injection processing This meth-odology offers substantial advantag-es over conventional furnace orquartz tube analytical techniques in-cluding simplicity of operation ex-ible use of sample volumes auto-mation a substantial increase in de-tection power and elimination of at-omization interferences In effectthe technique provides a useful si-multaneous separationndashconcentrationstep prior to electrothermal atomiza-tion or vaporization for tandemsource atomic spectrometry
Sorbent Trapping Sorbent trap-ping of gaseous metals involves theapplication of a solid or liquid ex-traction phase or an extraction phaseimmobilized onto a solid supportThe sequestered volatile species arethen desorbed (usually thermally)from the extraction phase and intro-duced into a detector
Column Preconcentration Theprimary set-up for sorbent trappingremains column preconcentra tionThe phases typically used includeactivated carbon Tenax and variousgas chromatographic stationaryphases Trapping can be accom-plished at cryogenic or room tem-perature depending on the volatilitystability of the analyte The mostwidespread use of this setup is forthe trapping of mercury on goldColumn trapping has also been usedin purge and trap GC systems forcollection of derivatized organome-tallic compounds The use of acti-vated carbon traps has also been de-scribed for the preconcentration ofmetal hydrides
Solid Phase M icroextractionThe principal advantage of solidphase microextraction accrues fromthe small volume of the extractionphase usually less than 1 mL whichcan be a high molecular weight poly-meric lsquoliquidrsquo or a porous solid sor-bent typically having a high surfacearea A small diameter fused silica ber coated with the extractionphase is mounted in a syringe-likedevice for protection and ease ofhandling The needle serves to con-
veniently pierce septa during sampleextraction and desorption operationsUsing the syringe-like mechanism ofthe holder unit the ber can be ex-truded from the needle to expose theextraction phase to the sample (head-space or liquid) After the samplingperiod the same mechanism can beused to retract the ber inside theneedle During the extraction and de-sorption periods the ber is thus ex-posed outside of the needle duringtransfer of the SPME unit to a de-sorp tion apparatus the polymericend of the ber is inside the protec-tive needle
Most elemental speciation appli-cations of SPME are based on directextraction of volatile metals andmetalloids from the headspace aboveaqueous samples or after derivatiza-tion Mester et al74 recently re-viewed the application of SPMEtechniques for determination of or-ganometallic species The typical de-rivatization method used for tinlead and mercury species is ethyla-tion using NaBEt4 reagent Usuallyvolatile metals collected on theSPME ber are thermally desorbedand introduced into a gas chromato-graph for separation
Several researchers have reportedresults for the headspace samplingdetermination of metal hydrides us-ing SPME techniques Jiang and co-workers7576 developed a samplingmethod for organomercury speciesbased on hydride generation usingKBH4 reagent Mester et al77 testedthe compatibility of two different -bers (and two different extractionphenomena) for sampling volatilemetal hydrides coupled with ICP-MS detection An adsorption-basedcarboxen coating provided bettersensitivity than an absorption-basedextraction with a liquid type polydi-methylsiloxane polymeric coating(PDMS) The success of absorptivesampling con rmed the relativelyhigh stability of the studied metalhydrides (As Se Sn and Sb) be-cause they survived diffusion intoand release from the polymeric liq-uid
Guidotti et al5178 described thedetermination of Se (IV) by head-
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
1 O F X Donard and J H Weber Nature(London) 323 339 (1988)
2 K Michalke E B Wickenheiser MMehring A V Hirner and R HenselAppl Environ Microbiol 66 2791(2000)
3 J H Weber Mar Chem 65 67 (1999)4 J S White J M Tobin and J J Coo-
ney Can J Microbiol 45 541 (1999)5 P Andrewes W R Cullen and E Pol-
ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
Chem 58 173 (1997)7 W R Cullen H Li S A Pergantis G
K Eigendorf and A A Mosi Appl Or-ganomet Chem 9 507 (1995)
8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
11 Z Mester and R E Sturgeon EnvironSci Technol 36 1198 (2002)
12 J Feldmann J Environ Monit 1 33(1999)
13 F Yang and Y K Chau Analyst (Cam-bridge UK) 124 71 (1999)
14 M S Fragueiro F Alava Moreno ILavilla and C Bendicho SpectrochimActa Part B 56 215 (2001)
15 D S Forsyth and L Dusseault FoodAdd Contam 14 301 (1997)
16 M Leermakers W Baeyens R Ebin-ghaus J Kuballa and H H Kock Wa-ter Air Soil Pollut 97 257 (1997)
17 C Brosset and E Lord Water Air SoilPollut 91 187 (1996)
18 J Dedina and D L Tsalev HydrideGeneration Atomic Absorption Spec-trometry (John Wiley and Sons Chich-ester UK 1995)
19 H G Infante M L F Sanchez and ASanz Medel J Anal At Spectrom 11571 (1996)
20 S Tesfalidet and K Irgum Anal Chem61 2079 (1989)
21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
49 D Amouroux E Tessier and O F XDonard Environ Sci Technol 34 988(2000)
50 R Allabashi J Rendl and M Grasser-bauer Freseniusrsquo J Anal Chem 360723 (1998)
51 M Guidotti J AOAC Int 83 1082(2000)
52 R G Fernandez M M Bayon J I GAlonso and A Sanz Medel J MassSpectrom 35 639 (2000)
53 R Morabito P Massanisso and P Que-vauviller Trends Anal Chem 19 113(2000)
54 R Zu aurre B Pons and C Nerin JChromatogr A 779 299 (1997)
55 K Bergmann and B Neidhart J Sepa-ration Sci 24 221 (2001)
56 M Heisterkamp and F C Adams JAnal At Spectrom 14 1307 (1999)
57 Y Cai S Monsalud and K G FurtonChromatographia 52 82 (2000)
58 Y Cai S Monsalud R Jaffe and R DJones J Chromatogr A 876 147(2000)
59 P Smichowski and S V Farias Micro-chem J 67 147 (2000)
60 Z Mester and R E Sturgeon J AnalAt Spectrom 16 470 (2001)
61 X W Guo and X M Guo Anal ChimActa 330 237 (1996)
62 Z Mester R E Sturgeon J W Lam PS Maxwell and L Peter J Anal AtSpectrom 16 1313 (2001)
63 Z Mester J Lam R Sturgeon and J
Pawliszyn J Anal At Spectrom 15837 (2000)
64 A L Molinero L Martinez A Villa-real and J R Castillo Talanta 45 1211(1998)
65 A L Molinero A Morales A Villa-real and J R Castillo Freseniusrsquo JAnal Chem 358 599 (1997)
66 G Pignalosa N Cabrera A Mollo IPortillo V Rouco and L VazquezSpectrochim Acta Part B 56 1995(2001)
69 A Sanz Medel M C Valdes-Hevia yTemprano N Bordel Garc otilde a and M RFernandez de la Campa Anal Chem67 2216 (1995)
70 G Schwedt Chromatographic Methodsin Inorganic Analysis Dr Alfred Hu-thig Ed (Verlag Heidelberg Germany1981)
71 D R Hassler B Peucker Ehrenbrinkand G E Ravizza Chem Geol 166 1(2000)
72 R E Sturgeon S N Willie and S SBerman J Anal At Spectrom 4 443(1989)
73 V I Rigin Anal Chim Acta 283 895(1993)
74 Z Mester R Sturgeon and J Pawli-szyn Spectrochim Acta Part B 56 233(2001)
75 B He G B Jiang and Z M Ni JAnal At Spectrom 13 1141 (1998)
76 B He and G B Jiang Freseniusrsquo JAnal Chem 365 615 (1999)
77 Z Mester R E Sturgeon and J WLam J Anal At Spectrom 15 1461(2000)
78 M Guidotti G Ravaioli and M VitaliJ High Resolut Chromatogr 22 414(1999)
79 T Gorecki and J Pawliszyn AnalChem 68 3008 (1996)
80 J P Snell W Frech and Y ThomassenAnalyst (Cambridge UK) 121 1055(1996)
81 B Kolb J Chromatogr A 842 163(1999)
82 C M Tseng A de Diego F M MartinD Amouroux and O F X Donard JAnal At Spectrom 12 743 (1997)
83 C M Tseng A de Diego F M Martinand O F X Donard J Anal At Spec-trom 12 629 (1997)
84 J Sanz M Perez M T Martinez andM Plaza Talanta 50 149 (1999)
85 J Sanz M Perez M T Martinez andM Plaza Talanta 51 849 (2000)
86 J Sanz Asensio M T Martinez SoriaM Plaza Medina and M Perez ClavijoAnal Chim Acta 409 171 (2000)
87 S Cabredo J Galban and J Sanz Tal-anta 46 631 (1998)
88 R Eiden H F Scholer and M GastnerJ Chromatogr A 809 151 (1998)
89 T Kaise M Ogura T Nozaki K Sai-toh T Sakurai C Matsubara C Watan-
APPLIED SPECTROSCOPY 213A
abe and K Hanaoka Appl OrganometChem 11 297 (1997)
90 A G Howard and C Salou AnalChim Acta 333 89 (1996)
91 J L Burguera M Burguera C Rivasand P Carrero Talanta 45 531 (1998)
92 A G Howard and C Salou J Anal AtSpectrom 13 683 (1998)
93 U Pyell A Dworschak E Nitschkeand B Neidhart Freseniusrsquo J AnalChem 363 495 (1999)
94 J Y Cabon and N Cabon Freseniusrsquo JAnal Chem 368 484 (2000)
95 I R Pereiro A Wasik and R LobinskiAnal Chem 70 4063 (1998)
96 C M Tseng A de Diego H Pinaly DAmouraoux and O F X Donard JAnal At Spectrom 13 755 (1998)
97 A de Diego C M Tseng T StoichevD Amouroux and O F X Donard JAnal At Spectrom 13 623 (1998)
98 K Haas and J Feldmann Anal Chem72 4205 (2000)
99 C Pecheyran C R Quetel F M MLecuyer and O F X Donard AnalChem 70 2639 (1998)
100 D Amouroux E Tessier C Pecheyranand O F X Donard Anal Chim Acta377 241 (1998)
101 C Pecheyran D Amouroux and O F
X Donard J Anal At Spectrom 13615 (1998)
102 C Pecheyran B Lalere and O F X Don-ard Environ Sci Technol 34 27 (2000)
103 J Feldmann I Koch and W R CullenAnalyst (Cambridge UK) 123 815(1998)
104 J Feldmann and W R Cullen EnvironSci Technol 31 2125 (1997)
105 X D Tian H Emteborg and F C Ad-ams J Anal At Spectrom 14 1807(1999)
106 E Bolea F Laborda M A Belarra andJ R Castillo Spectrochim Acta Part B56 2347 (2001)
210A Volume 56 Number 8 2002
focal point
FIG 3 Schematic of the SPME thermal desorption ICP-MS interface
space SPME-GCMS following de-rivatization based on either piaselen-ol formation or ethylation Themethod was applied to Se (IV) andSe (VI) speciation by determiningthe Se (IV) content rst followed bythe total selenium concentration afterconverting all selenium species tothe Se (IV) oxidation state
Organometallic species that arenormally saturated (non-ionic) andsuf ciently volatile can be sampledby SPME and determined by GCwithout derivatization Gorecki andPawliszyn79 described a simple sam-pling procedure for tetraethylleadSimilar studies were performed bySnell et al80 for determination of di-methylmercury in the headspace ofnatural gas condensates
Mester et al63 recently describedan SPME method for methylmercuryand butyltin62 determination basedon the relatively high vapor pressureof methylmercury chloride and tri-butyltin chloride species HeadspaceSPME sampling was performedabove a methylmercury or butyltinsolution that was previously saturat-ed with sodium chloride A slightlypolar solid coating (PDMSDVB)was used for sampling Sample in-troduction in to an ICP-MS wasachieved with a unique thermal de-sorption interface consisting of aheated glass-lined splitless type GCinjector placed directly at the base ofthe torch to minimize the length ofthe transfer-line This arrangementprovided for fast desorption and high
sample introduction ef ciency Aschematic of the thermal desorptionICP-MS interface is shown in Fig 3This unit provides a very versatileinterface which can be connected tovirtually any atomic spectroscopicsource
Cryogenic Trapping Cold trapsare used to accomplish two goalsanalyte enrichment and solute bandconcentration Most cold traps arehome-made and are operated manu-ally requiring skill and experienceof the operator Kolb81 recently re-viewed headspace sampling meth-ods including cryotrapping Amongmany others it is an excellent start-ing point for headspace samplingtechniques Donardrsquos research grouppioneered the adaptation of cryotrap-ping technology to speciation stud-ies8283 and it is now widely used inthe speciation eld especially forpreconcentration of volatile met-al(oid) species fo llowing hydridegenerationethylation derivatizationTypical trapping temperatures are inthe 2150 to 2196 8C range obtainedwith liquid nitrogen cooling Cry-otrapping can also be used as ameans of sampling volatile met-al(oid) species in the environmentafter which they can be introducedinto a separation system (typicallyGC) following fast heat-up Alter-natively the cryotrap can function asa crude lsquolsquoseparationrsquorsquo device wherewith ramped heating the analytesdesorb from the trap in accordancewith their boiling points
Cryogenic sample collection hasbeen used for the determination oforganotin species after hydride gen-eration84ndash87 ethylation with sodiumtetraethylborate88 and for the sam-pling of saturated alkyltin speciesfrom sea water and air49 It has alsobeen extensively applied in specia-tion studies of arsenic89ndash94 and mer-cury8295ndash97 Volatile metal(oid) spe-cies have been detected in air9899
water49100ndash102 and in land ll gas sam-ples8103 using cryogenic samplingMost of these studies employed mul-tielement detectors (mainly ICP-MS)for simultaneous monitoring of thevolatile species of these elementsVolatile metal carbonyl species
APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
1 O F X Donard and J H Weber Nature(London) 323 339 (1988)
2 K Michalke E B Wickenheiser MMehring A V Hirner and R HenselAppl Environ Microbiol 66 2791(2000)
3 J H Weber Mar Chem 65 67 (1999)4 J S White J M Tobin and J J Coo-
ney Can J Microbiol 45 541 (1999)5 P Andrewes W R Cullen and E Pol-
ishchuk Chemosphere 41 1717 (2000)6 D Amouroux and O F X Donard Mar
Chem 58 173 (1997)7 W R Cullen H Li S A Pergantis G
K Eigendorf and A A Mosi Appl Or-ganomet Chem 9 507 (1995)
8 J Feldmann E M Krupp D Glinde-mann A V Hirner and W R CullenAppl Organomet Chem 13 739(1999)
9 O F Schedlbauer and K G Heumann
212A Volume 56 Number 8 2002
focal point
Appl Organomet Chem 14 330(2000)
10 R E Farrell P M Huang and J J Ger-mida Appl Organomet Chem 12 613(1998)
11 Z Mester and R E Sturgeon EnvironSci Technol 36 1198 (2002)
12 J Feldmann J Environ Monit 1 33(1999)
13 F Yang and Y K Chau Analyst (Cam-bridge UK) 124 71 (1999)
14 M S Fragueiro F Alava Moreno ILavilla and C Bendicho SpectrochimActa Part B 56 215 (2001)
15 D S Forsyth and L Dusseault FoodAdd Contam 14 301 (1997)
16 M Leermakers W Baeyens R Ebin-ghaus J Kuballa and H H Kock Wa-ter Air Soil Pollut 97 257 (1997)
17 C Brosset and E Lord Water Air SoilPollut 91 187 (1996)
18 J Dedina and D L Tsalev HydrideGeneration Atomic Absorption Spec-trometry (John Wiley and Sons Chich-ester UK 1995)
19 H G Infante M L F Sanchez and ASanz Medel J Anal At Spectrom 11571 (1996)
20 S Tesfalidet and K Irgum Anal Chem61 2079 (1989)
21 S Tesfalidet and K Irgum Freseniusrsquo JAnal Chem 341 532 (1991)
22 X D Tian Z X Zhuang B Chen andX R Wang Analyst (Cambridge UK)123 627 (1998)
23 J S Blais G M Momplaisir and W DMarshall Anal Chem 62 1161 (1990)
24 E Denkhaus A Gulloch X M Guoand B Huang J Anal At Spectrom 16870 (2001)
25 E Kikuchi and H Sakamoto J Electro-chem Soc 147 4589 (2000)
26 X M Guo Z Mester and R E Stur-geon Anal Chem paper submitted(2002)
27 D Amouroux C Pecheyran and O FX Donard Appl Organomet Chem 14236 (2000)
28 C Dietz Y Madrid C Camara and PQuevauviller J Anal At Spectrom 141349 (1999)
29 H Matusiewicz and R E SturgeonSpectrochim Acta Part B 51 377(1996)
30 G H Tao and R E Sturgeon Spectro-chim Acta Part B 54 481 (1999)
31 W W Ding and R E Sturgeon AnalChem 69 527 (1997)
32 C Moor J W H Lam and R E Stur-geon J Anal At Spectrom 15 143(2000)
33 A S Luna R E Sturgeon and R Cde Campos Anal Chem 72 3523(2000)
34 R E Sturgeon J Liu V J Boyko andV T Luong Anal Chem 68 1883(1996) X Duan R L McLaughlin ID Brindle and A Conn J Anal AtSpectrom 17 227 (2002)
35 X M Guo B L Huang Z H Sun RQ Ke Q Q Wang and Z B Gong
Spectrochim Acta Part B 55 943(2000)
36 P Pohl and W Zyrnicki Anal ChimActa 429 135 (2001)
37 Y L Feng J W Lam and R E Stur-geon Analyst (Cambridge UK) 1261833 (2001)
38 S Diez L Ortiz and J M BayonaChromatographia 52 657 (2000)
39 G Binato G Biancotto R Piro and RAngeletti Freseniusrsquo J Anal Chem361 333 (1998)
40 I Fernandez-Escobar M Gibert AMesseguer and J M Bayona AnalChem 70 3703 (1998)
41 J R Baena S Cardenas M Gallegoand M Valcarcel Anal Chem 72 1510(2000)
42 M B dela Calle Guntinas and F C Ad-ams J Chromatogr A 764 169 (1997)
43 G B Jiang and F C Adams AnalChim Acta 337 83 (1997)
44 N Mikac M Branica Y Wang and RM Harrison Environ Sci Technol 30499 (1996)
45 X M Yu and J Pawliszyn Anal Chem72 1788 (2000)
46 P W Looser M Berg K Fent J Muhl-emann and R P Schwarzenbach AnalChem 72 5136 (2000)
47 J R Encinar J Alonso and A SanzMedel J Anal At Spectrom 15 1233(2000)
48 B S Tselentis and E S Tzannatos Fre-senius Environ Bull 9 499 (2000)
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APPLIED SPECTROSCOPY 213A
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X Donard J Anal At Spectrom 13615 (1998)
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APPLIED SPECTROSCOPY 211A
FIG 4 Periodic Table of the elements indicating elements that can be volatilized at atmospheric pressure and room temperature asgreen uoride red chelates yellow cold vapor light green carbonyl orange oxide and blue hyrides
[Ni(CO)4 Mo(CO)6 and W(CO)6]have also been detected in fermen-tation gases from municipal sewagetreatment plants using cryogenic gassamplingtrapping12104
One of the main advantages ofcryogenic trapping compared to oth-er preconcentration methods is thatunstab lereactive species do notcome into contact with any liquid orsolid sorbent material signi cantlyreducing the possibility of their al-terationdecomposition and also min-imizing the possibility of introducingcontaminants via the solvent usedwith other techniques By contrastcryotrapping technology despite itsclear advantages remains unpopularespecially for eld application be-cause of its bulkiness and the needfor handling liquid nitrogen
CONCLUSION
The eld of vapor generation forelemental analysis is broad and con-tinues to expand with the discoveryof new species and methods for theirgeneration and detection Several ofthe more classical techniques havebecome stagnant and are little usedfor analytical purposes whereas oth-
ers have experienced a resurgence ofinterest Developments in chemicalhydride generation halide genera-tion and alkylation reactions areserving to signi cantly expand ana-lytical capabilities in the elds oftrace element analysis and organo-metallic speciation As illustrated byFig 4 a signi cant fraction of themetals and semi-metals throughoutthe Periodic Table can now be ad-dressed by vapor generation tech-niques and the scope of elementscovered continues to grow It is pos-sible that a number of mature ap-proaches relating to chelation withsuch ligands as the b-ketonates andcarbamates may experience a resur-gence in utility when coupled withmodern more suitable samplingtechniques such as SPE and SPME
With the continued improvementin sampling and detection approach-es it is now feasible to detect andin some cases also to identify vola-tile metals in the environment a tasklargely impossible only a few yearsago The scope of volatile metalmeasurement in air will likely sig-ni cantly expand in the near futureproviding a better understanding of
the global bio-geochemical cycle oftrace elements for which the releaseof volatile metal(oid)s into our eco-systems is not fully appreciated Thewidespread use of organometalliccompounds ranging from volatilegasoline additives (Mn and Pb) toplasticizers (Sn) and drugs (Li PtAu V etc) may also pose conse-quences for our health in that alter-native uptake routes via the respira-tory system may have to be takeninto consideration
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