Ž . Spectrochimica Acta Part B 55 2000 473]490 Permanent modification in electrothermal atomic absorption spectrometry } advances, anticipations and reality q Dimiter L. Tsalev a, U , Vera I. Slaveykova a , Leonardo Lampugnani b , Alessandro D’Ulivo b , Rositsa Georgieva c a Faculty of Chemistry, Uni ¤ ersity of Sofia, 1 James Bourchier Boule ¤ ard, Sofia 1126, Bulgaria b CNR, Istituto di Chimica Analitica Strumentale, Area della Ricerca di Pisa, Via Alfieri 1, Loc. San Cataldo, 56010 Ghezzano, Pisa, Italy c National Centre of Hygiene, Medical Ecology and Nutrition, Sofia 1431, Bulgaria Received 19 November 1999; accepted 2 March 2000 Abstract Permanent modification is an important recent development in chemical modification techniques which is promising in view of increasing sample throughput with ‘fast’ programs, reducing reagent blanks, preliminary elimination of unwanted modifier components, compatibility with on-line and in situ enrichment, etc. An overview of this approach based on the authors’ recent research and scarce literature data is given, revealing both success and Ž failure in studies with permanently modified surfaces carbides, non-volatile noble metals, noble metals on carbide . Ž . coatings, etc. , as demonstrated in examples of direct electrothermal atomic absorption spectrometric ETAAS Ž . applications to biological and environmental matrices and vapor generation VG ] ETAAS coupling with in-atomizer trapping of hydrides and other analyte vapors. Permanent modifiers exhibit certain drawbacks and limitations such as: poorly reproducible treatment technologies } eventually resulting in poor tube-to-tube repeatability and double or multiple peaks; impaired efficiency compared with modifier addition to each sample aliquot; relatively short lifetimes; limitations imposed on temperature programs, the pyrolysis, atomization and cleaning temperatures being set somewhat lower to avoid excessive loss of modifier; applicability to relatively simple sample solutions rather than to high-salt matrices and acidic digests; side effects of overstabilization, etc. The most important niches of application appear to be the utilization of permanently modified surfaces in coupled VG] ETAAS techniques, analysis of organic q This paper was presented at the CSI XXXI Pre-Symposium Electrothermal Atomization and Vaporization Techniques in AAS, OES and ICP-MS, held in Nevs ¸ ehir, Turkey, September 1999, and is published in the Special Issue dedicated to this symposium. U Corresponding author. Fax: q359-2-96-25-438. Ž . E-mail address: [email protected]fia.bg D.L. Tsalev 0584-8547r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 5 8 4 - 8 5 4 7 00 00194-4
Transcript
Ž .Spectrochimica Acta Part B 55 2000 473]490
Permanent modification in electrothermal atomicabsorption spectrometry } advances, anticipations
and reality q
Dimiter L. Tsalev a,U, Vera I. Slaveykovaa, Leonardo Lampugnanib,Alessandro D’Ulivob, Rositsa Georgievac
aFaculty of Chemistry, Uni ersity of Sofia, 1 James Bourchier Boule ard, Sofia 1126, BulgariabCNR, Istituto di Chimica Analitica Strumentale, Area della Ricerca di Pisa, Via Alfieri 1, Loc. San Cataldo,
56010 Ghezzano, Pisa, ItalycNational Centre of Hygiene, Medical Ecology and Nutrition, Sofia 1431, Bulgaria
Received 19 November 1999; accepted 2 March 2000
Abstract
Permanent modification is an important recent development in chemical modification techniques which ispromising in view of increasing sample throughput with ‘fast’ programs, reducing reagent blanks, preliminaryelimination of unwanted modifier components, compatibility with on-line and in situ enrichment, etc. An overview ofthis approach based on the authors’ recent research and scarce literature data is given, revealing both success and
Žfailure in studies with permanently modified surfaces carbides, non-volatile noble metals, noble metals on carbide. Ž .coatings, etc. , as demonstrated in examples of direct electrothermal atomic absorption spectrometric ETAAS
Ž .applications to biological and environmental matrices and vapor generation VG ]ETAAS coupling with in-atomizertrapping of hydrides and other analyte vapors. Permanent modifiers exhibit certain drawbacks and limitations suchas: poorly reproducible treatment technologies } eventually resulting in poor tube-to-tube repeatability and doubleor multiple peaks; impaired efficiency compared with modifier addition to each sample aliquot; relatively shortlifetimes; limitations imposed on temperature programs, the pyrolysis, atomization and cleaning temperatures beingset somewhat lower to avoid excessive loss of modifier; applicability to relatively simple sample solutions rather thanto high-salt matrices and acidic digests; side effects of overstabilization, etc. The most important niches of applicationappear to be the utilization of permanently modified surfaces in coupled VG]ETAAS techniques, analysis of organic
q This paper was presented at the CSI XXXI Pre-Symposium Electrothermal Atomization and Vaporization Techniques in AAS,OES and ICP-MS, held in Nevsehir, Turkey, September 1999, and is published in the Special Issue dedicated to this symposium.
0584-8547r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 5 8 4 - 8 5 4 7 0 0 0 0 1 9 4 - 4
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solvents and extracts, concentrates and fractions obtained after enrichment andror speciation separations and directETAAS determinations of highly volatile analytes in relatively simple sample matrices. Q 2000 Elsevier Science B.V.All rights reserved.
w xChemical modification 1 , originally introducedw xwith the term ‘matrix modification’ 2 , has evolved
nowadays into a common, very popular methodo-logical approach in electrothermal atomic absorp-
Ž . w xtion spectrometry ETAAS 3]11 . According torecommendations of the International Union for
Ž . w xPure and Applied Chemistry IUPAC 1 : ‘Inorder to influence processes taking place in theatomizer in the desired way, reagents, calledchemical modifiers, may be added. These canhelp to retain the analyte to higher temperaturesduring pyrolysis, to remove unwanted concomi-tants or improve atomization in other way’. As
w xpointed out in some recent publications 7,11 , thescope of chemical modification tends to broadenwith gaining maturity; it is realized that chemicalmodifiers exhibit complex, multi-sided effects onall components of the electrothermal atomization
Žsystem the analyte, the matrix, the atomization. w xsurface and the gaseous phase 7 , hence, more
powerful, versatile in situ chemistry could be per-Žformed see Table 1 for a summary of the useful
effects, drawbacks and limitations of chemical. w xmodifiers 4,7 . The literature on this topic has
already amounted to over 2000 relevant publica-tions, and various aspects of chemical modifica-tion have been treated in much detail in several
w xrecent reviews articles and book chapters 3]11 .Among the topics discussed in these reviews arethe useful effects, drawbacks and limitations of
w xchemical modifiers 4,7,9 ; thermal stabilizationw xand ‘isoformation’ effects 7,10 ; the rational inte-
gration of chemical modification in analyticalw xprocedures 4,7,10,12 ; classification of modifiers
and analytes based on theoretical and empiricalw x w xapproaches 4,6,7 ; mechanisms 7]9,11,13 ;
guidelines for selection and optimization ofw xchemical modifier composition 4,6,7 ; and the
w x w xprogress made on mixed 4,7 , composite 4,7 andpermanent modifiers, in particular graphiteatomizers modified with high-melting carbidesw x9,10 and applications of modifiers to real sample
w xmatrices 3]5,7,10 .
2. Permanent modification — expectations
Although graphite atomizers modified withhigh-melting carbides have been known in ETAAS
w xfor almost three decades 9,10 , the term ‘perma-nent modification’ was introduced only recently
w xby Shuttler et al. 14 ; they applied a single, man-ual injection of 50 mg Pdq50 mg Ir on theintegrated L’vov platform of the transversely-
Ž .heated graphite atomizer THGA tube, makingpossible up to 300 complete cycles of hydridetrapping and atomization in hydride generationŽ .HG ]ETAAS determination of As, Bi and Sew x14 .
The idea of permanent modification appears tobe very attractive, considering some negative sideeffects and limitations of the classical chemicalmodification approach, viz. applying dissolved
Žmodifier additions to each sample aliquot Table.1 . Reagent blanks, originating from modifiers,
are pronounced for analytes such as Ag, Au, Cd,Cu, Mn, Pb and Zn, thus reducing the usefulworking range, impairing limits of detectionŽ .LOD and calling for additional purification ofreagents, particularly with large sample aliquotsand multiple injections. As an example, the prob-
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Table 1aSummary of the most important aimed effects and drawbacks of chemical modification
Aimed effectsŽ .Thermal stabilization of volatile analytes to higher pyrolysis temperatures Tpyr
e.g. such as As, Cd, Hg, Pb, Se, Tl, etc.Facilitating low-temperature atomization of certain analytes andror assisting the cleaning stage
e.g. increasing volatility of Cd, Pb or Zn in seawater matrix‘Isoformation’ of different analyte species
Ž . Ž . Ž .e.g. such as inorganic i and organo org arsenic i-As, org-As , org-Se, org-Sn; various oxidationŽ . Ž . Ž .states such as Se -II , Se IV and Se VI , etc.
Increasing volatility of unwanted concomitantse.g. removing chloride by NH NO , dilute HNO , H ]Ar, organic acids, etc.4 3 3 2
Thermal stabilization of some interferentse.g. P and PO with Ni or noble metals2
Ž .Facilitating in situ ashing ‘ashing aids’e.g. O or air alternate gas during ashing2
Insuring better contact between the sample and atomization surfacee.g. wetting agents, organic reagents, etc.
Ž .Increasing sensitivity in cases of inefficient atomization of certain analytes or analyte speciesŽ .Providing better more uniform conditions for different analytes in simultaneous multielement analysis
Improving resistance of atomization surface to attackrpermeatione.g. TaC, WC, NbC for acidic digests; organic extractsrsolvents, etc.
In situ speciation analysisŽ . Ž . Ž . Ž .e.g. by fractional vaporization of certain species, e.g. with systems Cr III rCr VI , Se IV rSe VI , etc.
Employing modified surfaces for in situ trapping generated volatile hydrides or other vapors, etc.
Limitations, drawbacks and side effectsReagent blanks
Ž .Longer temperature programs t , t typically requireddry pyrHydrolysis and compatibility problems
e.g. Ir, Nb, Pt, Ta, Ti, Zr need higher concentrations of acid; modifiers such as Si, W, Mo or V arestabilized in alkalinerammonia medium
Contaminationrmemory effectse.g. modifiers such as Cr, Cu, Mg, Mo, Ni, P, Pd, V, etc. cannot be determined as analytes at a laterstage
Impaired effectiveness of modifiers with real matrices: lower T ; larger modifier mass requiredpyrCorrosive action on graphite
e.g. modifiers such as NH NO , La, HNO , O , air, etc.4 3 3 2Overstabilization effects caused by copious amounts of modifiers or gradual accumulation of non-volatilemodifiers
Ž .e.g. Mn in Ir-coated graphite tube GT ; Mo in W-treated GT, etc.Background absorption problems
Ž .e.g. from some modifiers such as phosphate, Mg NO , etc.3 2Toxicity
e.g. noxious effects may prevent applications of certain potential modifiers: Cd, Hg, CS , thiourea,2Ž .Cr VI , etc.
Ample empirical information and large discrepancies in literature data
a w xFor details and references, see Tsalev and Slaveykova 7 .
lem with Cd blank from several common modi-fiers is illustrated in Fig. 1: most blank contribu-tions are higher than the characteristic mass, m ,o
Ž .in spite of high-purity i.e. expensive reagentsŽemployed. In situ purification of modifiers e.g.
.for volatile analytes such as Cd, Hg or Zn bytheir thermal pre-treatment entails prolongedtemperature programs and inadequate samplethroughput rates. Permanent modification offersbetter possibilities to control or obviate some
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Fig. 1. Reagent blank levels for Cd in direct ETAAS with ‘end-capped’ THGA, originating from some typical chemical modifiers, asŽ .compared with the characteristic mass, m in pg .o
other limitations of dissolved modifier applica-tions: prolonged temperature programs, back-ground problems, compatibility on mixing withsample solutions, carryover, large diversity ofmodifier compositions, etc. Some interfering mod-ifier components such as Cly ions in the determi-nation of Hg, Tl and In are thus eliminated priorto sample injection.
The number of potential permanent modifiersis confined to approximately 15]16 elements: the
Ž .high-boiling noble metals Ir, Pd, Pt, Rh, Ru andŽthe elements forming refractory, ‘metal-like’ Hf,
.Mo, Nb, Re, Ta, Ti, V, W, Zr or ‘covalent’Ž .carbides B, Si . Noteworthy, the ‘salt-like’ car-
bides of Ce, La, Sc, Y and the lanthanoids areimpractical because of their easy hydrolysis bywater or dilute acids upon subsequent injectionsof sample solutions, resulting in corrosion ofgraphite surfaces, defoliation of pyrolytic graphitecoatings, sensitivity drift, impaired precision and
w xother adverse effects 4,7,9,10 . Some noble met-Ž .als of moderate volatility Ag, Au, Pd have found
occasional application upon blending withŽ w xanother, less volatile metal Pd]Ir 14 , Pd]Rh
w x w x.25 , Au]Rh 26 or otherwise upon their thermalŽstabilization on carbide-coated surfaces Pd]Zr,
. w xPd]W 15,27 ; in both cases, however, they are
gradually vaporized and re-distributed within theatomizer, thus entailing sensitivity drift, limitedlifetime of coating and memory effects.
Our experience with permanent modification isw xmainly based on studies of the Zr]Ir 15]20 ,
w x w x w xW]Ir 15]17,20 , Zr]Pd 15 and W]Pd 15-treated platforms, as well as on comparativestudies of some series of modifiers such as
w x w xPd]Rh]Ru 28 and V]Zr]Mo]W 4,6 . Detailsof coating procedures by multiple successive in-jections of Zr or W salt solutions, followed bythermal treatments and application of iridium are
w xgiven elsewhere 15 . Reproducibility of thesetreatments is illustrated in Table 2 as m andoRSD figures for ETAAS measurements withcadmium. It is noteworthy that the impairment ofm after G1300 firings is rather due to aging ofothe graphite tube, viz. enlargement of the injec-tion hole, increasing porosity of tube walls andcorrosion of end caps, than to changes in thepermanently modified platform. Some directETAAS applications of the W]Ir permanent
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490 477
Table 2Ž .Reproducibility of characteristic mass m in pg in peak area measurements with differently treated THGA tubes with integratedo
a Forty picograms of Cd in 0.2% vrv HNO ; T 5008C, except for 3508C with untreated pyrocoated platforms. Treatment with3 pyrw x110 mg Zr and 8 mg Ir or 240 mg W and 8 mg Ir as detailed in Tsalev et al. 15 . RSD in parentheses for three to five subsequent
injections.
modifier to the determination of several volatileŽ .elements in certified reference materials CRM
are shown in Table 3, exhibiting good agreementwith certified contents.
Examples of recent analytical applications ofpermanent modification are compiled in Tables 4and 5. Literature data are briefly outlined, com-mented and divided into two groups of applica-tions, depending on the complexity of the samplematrix. In Table 4 direct ETAAS analyses of
w x w xintact samples 22 , digests 16,26,42 , slurriesw x w x21,23,24 , organic solvents 37,39,40 and extractsw x31,36,37,39,40 are given, including the trouble-
w xsome chlorinated solvents 36,40 and other con-Žcentrates, resulting, e.g. from ion exchange eluate
w x.fractions 35 or coprecipitation separationsw x29,32 . Table 5 is devoted to vapor generationŽ .VG ]ETAAS with in-atomizer trapping of hy-
drides and other analyte vapors on permanentlyŽ .modified surfaces graphite tubes or platforms
w x12,76,77 .It is worth mentioning that most previous
Žobservations on carbide-treated atomizers over.300 publications have been considered during
preparation of this overview but they are notnecessarily listed in Table 4, due to the availabil-ity of an exhaustive recent review by Volynskyw x9,10 . Moreover, many early applications of car-bide-treated graphite tubes, e.g. those claiming‘drastic sensitivity improvement’ or ‘better inter-ference control’ compared with uncoated graphitetubes, are obsolete nowadays with modern, moreefficient electrothermal atomization methodologybased on the stabilized temperature platform fur-
Ž . w xnace STPF concept 78 with graphite tubes andplatforms coated with dense, high-quality pyro-
Table 3Examples of direct ETAAS analysis of CRMs with permanent modification employing Ir]W-treated integrated platforms of THGA
a NRCC, National Research Council of Canada, Ottawa, Canada; BCR, Institute for Reference Materials and Measurements,Geel, Belgium; NIST, National Institute of Standards and Technology, Gaithersburg, MD, USA.
b Details of sample decomposition procedures and treatment of platforms with 240 mg W and 2 mg Ir as detailed in Tsalev et al.w x16 .
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aŽ .Examples of permanent modification in ETAAS direct injection and procedures involving enrichment other than VG with in-atomizer trapping
Analyte Matrix Modifier Comment Ref.
Ž . Ž . Ž . w xAs Environmental Zr; Zr-coated GT; HCl Both As III and As V coprecipitated with hydrated Zr IV oxide; AsCl 293waters fractionally volatilized at 1100]14008C upon conc. HCl addition; Tvap
Ž .14008C for As Vw xCd Aqueous model Ir; Pd; Pd]Rh; Rh T 800, 900, 1000 and 11008C with Ir, Pd, Pd]Rh, Rh, respectively; 25vap
y1Ž Ž .solutions 30 mg l electroplated good long-term performance up to 520 firingsNaCl in 0.1%
.vrv HClŽ . Ž . w xCd Fish slurry 250 mg W]200 mg Rh vs. Pd] Hot injection 1008C ; T 4508C; T 12508C; coating lifetime approximately 23pyr at
Ž .Mg NO and phosphate- 250]300 firings, with reconditioning platform treatment thereafter; LOD3 2y1containing modifiers 1.4 ng g ; Zeeman STPF
Ž w xGa Al-base alloy, river W-treated platform T 5008C; T 23008C; addition of Ni modifier recommended m 8 and 30pyr at o.water 3 pg in the absence and presence of Ni, respectively ; LOD 6 pg
Ž . w xGe Plant Ginseng, etc. Aqueous NH in Mo-treated GT After extraction with phenylfluorone]isopropylacetone] 313N,N-dimethylformamide; T 9008Cpyr
w xHg Soil Au; Ir; Pd; Rh; Au]Rh; Pd or Au]Rh can serve as permanent modifiers; m 80]220 pg; 26oelectroplated Pd or Rh Zeeman STPF
Ž . w xMn Zn Y and Y-treated GT Coprecipitation with Y OH at pH 9.0]11.3 323Ž w xPb Water rain, river, W-, Nb-, Hf- and Zr-treated GTs W-treated tube recommended, with 5 mg Pd additions to each sample 33
. Ž .snow, tap qPd modifier additions aliquot; T 14008C; lifetime approximately 250 firings; m 12 pg; LODpyr oy1Ž . Ž . Ž . w xSe Aq. solutions BN-coated GT vs. Pd]Mg NO 0.02 mg l Se IV , Se VI and selenomethionine studied; in-situ pre- 343 2
Ž . Ž .NaCl added modifier addition atomization separation of Se IV as volatile piazselenol possible on BN-coated tubes
Ž . Ž . w xPb Sediment slurry 250 mg W]200 mg Rh vs. Hot injection 1008C ; T 9008C; T 17508C; coating lifetime approximately 24pyr aty1Ž .Pd]Mg NO 250 firings; LOD 61 ng g ; Zeeman STPF3 2
w xSi Serum W-treated pyrocoated GT Anion exchange HPLC]ETAAS for speciation; 600 ml fractions collected; 3510 ml injections T 12008C; T 26008Cpyr at
w xSn Fruits, vegetables Mg; Pd; NH H PO ; Ti- or Speciation of tricyclohexyltin hydroxide after CHCl extraction; 364 2 4 3Zr-treated GT Zeeman STPF or Ti-treated tubes
Ž . w xAs, Pb Sediment As, Pb ; 2 mg Ir on Zr-treated integrated Bomb-decomposed samples in dilute HNO ; T 800 and 14008C; T 1600 163 pyr aty1Ž .plant Pb platform and 21008C for Pb and As, respectively; Zeeman THGA; 10]30 h
w xAs, Bi, Cd, Aq. solutions 2 mg Ir on Zr- or W-treated Double peaks for Bi and Te with Ir]W treated platforms; Zeeman STPF 15Ž .Pb, Sb, platform permanent modifier
Se, Te,Tl
Ž . Ž . w xAs, Bi, Cd, Organic solvents and PdCl MTOA or PtCl MTOA APDC]MIBK extraction; better thermal stabilization on W-treated GT 374 2 6 2Pb, Sb, extracts on W-treated GTsSe, Sn,Te
Ž . w xAs, Se, Si Aq. solutions Pd; Rh as permanent modifiers Electroplating of modifiers detailed; T 1300, 1200 and 12008C on Pd and 38vap1450, 1400 and 16008C on Rh for As, Se and Si, resp.
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Ž .Table 4 Continued
Analyte Matrix Modifier Comment Ref.
Ž . w xCd, Co, Cu, Organic solvents W-treated GT; Pd for Cd and Pb Smaller amount of Pd needed for MIBK vs. aqueous solutions: 39Fe, Ni, Pb and extracts 0.2 vs. 10]15 mg, resp.
Ž . w xCd, Co, Pb Chlorinated PdCl MTOA ]MIBK extracts; T 800 and 10008C for Cd and Pb with 20 ng Pd modifier; T 500, 1500 402 2 vap pyrŽ .solventsrextracts: W-treated GT and 8008C with W-treated GTs recommended
CCl , CHCl , C H Cl4 3 2 2 2Ž w xCd, Mn, Pb, Aq. solutions Ir deposited by cathodic T 800, 1200 and 14008C for Cd, Pb and Se 41vap
.Se, V sputteringw xCd, Pb, Se Blood Ir-sputtered GTs Blood digests in dilute HNO ; T impaired to 500, 750 and 4008C in 423 vap
digests vs. 800, 1200 and 14008C in dilute HNO for Cd, Pb and Se, resp.3Ž . w xCd, Pb, Se Biol. CRMs slurry 200 mg W]210 mg Ir; 300 mg W] T 12008C for Pb and Se on W]Ir; graphite tube lifetime increased by 21vap
Ž .400 mg Ru; vs. Pd]Mg NO 50]100% vs. pyrocoated platforms; THGA3 2w xCd, Pb, Se Water 250 mg W]200 mg Rh vs. Pd] T 500, 800 and 12008C for Cd, Pb and Se, resp.; coating lifetime 300]350 22pyr
Ž . Ž .Mg NO and Mg NO ] firings; tube lifetime increased by 50]100%; m 1.1, 30 and 42 pg, resp.;3 2 3 2 oNH H PO Zeeman STPF4 2 4
Table 5aExamples of VG]ETAAS with in-atomizer trapping procedures, involving permanent modification
Analyte Matrix Modifier Comment Ref.
Ž . w xAs Aquatic plant, Ir-treated GT 100 mg Ir On-line UV photooxidation or MWD with FI-HG]ETAAS for ‘first-order’ 43biological tissues, speciation; LOD 0.14 ngurine, water
w xAs Aq. solutions Ir-treated integrated platform FI-HG-ETAAS; effect of sample volume on LOD evaluated; T 4008C; 44collŽ .T 21008C; larger volumes )1 ml do not improve LODsat
Ž . w xAs Water Ir-, W- and Zr-treated platforms FI-HG]ETAAS; selective medium reactions for total As, As III , 45Ž . Ž . Ž .hot-spring, sea i-As III qV and As III qDMA elaborated; Zr-treated platform
y1recommended; T 10008C; T 20008C; m 31 pg and LOD 20 ng lcoll at oŽ .for total As ; Zeeman THGA
w xCd Seawater Ir-, W- and W-treated GTs FI-VG]trapping]ETAAS; T 208C; Ir coating recommended; 46colly1m 3 pg; LOD 4 ng lo
w xGe Aq. solutions Pd-, Zr- and Pd]Zr-coated GTs Sample injection vs. HG]trapping]ETAAS; T 8008C 27collŽ . Ž . w xGe Rock, sediment, Pd, Ir, Ir]Pd, Pd]Mg NO ; HG]trapping]ETAAS; T 400]5008C noble metal or 500]6008C 473 2 coll
Ž .steel coatings: Nb, Ta, W, Zr carbide ; m 10]54 pgpw xHg Water Ir permanent modifier or Au]Pt FI-CVT]ETAAS; T 1508C 48coll
gauzew xHg Water 120 mg Ir permanent modifier CVT]trapping]ETAAS; T 1508C, T 9008C; coating lifetime 500 49coll at
firings; LOD 70 pg; Zeeman STPFy1 w xHg Water GT with Au]Pt gauze insert Automated CF-VG]ETAAS; T 7508C; LOD 15 pg or 1.5 ng l in 10 ml 50at
y1Ž . w xHg Water fresh, sea Ir-, W- and Zr-treated GTs CVT]ETAAS; Ir preferred; m 240 pg; LOD 60 ng l 51ow xPb Aq. solutions Pd-treated GT; Zr-treated GT Direct ETAAS or HG]trapping]ETAAS; kinetics studied; weak Pb]Zr 52
y1Ž .interaction E 45]48 kcal molaw xPb Coal fly ash, mussel, Zr-treated GT HG]trapping]ETAAS; T 3008C; m 52.8 pg, LOD 242 pg 53coll o
tea, waterw xPb Seawater Ir-, W- and Zr-treated platforms FI-HG]ETAAS; T 20, 400 and 208C for Ir, W and Zr, resp.; m 70, 54coll o
y1100 and 90 pg, resp.; Ir recommended; T 18008C; LOD 60 ng l ;aty140 h ; Zeeman THGA
209 w xPb Sediment Ir; Pd]Ir; Ir]Mg; W- and HG]trapping]ETAAS; Pb tracer study; best sensitivity with Ir; m 55oZr-coatings 21 pg; LOD 0.25 ng
Ž . Ž . Ž . w xPb Water river, sea , 150 mg Ir conditioned at 11008C FI]ethylation with NaB C H ; tetraethyllead trapped at 3008C; LOD 562 5 4fish, lobster 12 pg; Zeeman STPF
w xSb Sediments 2 mg Ir on Zr-treated platform FI-HG]ETAAS after bomb decomposition 18w xSe Aq. solutions 50 mg Ir as permanent modifier FI-HG]trapping]ETAAS; STPF 57w xSe Aq. solutions Pd]Ir-treated platform HG-trapping]ETAAS; T 2508C; hydride atomization studied; STPF 58coll
Ž . w xSe Industrial sewage Ir-treated platform 150 mg FI ethylation with NaBEt ; trapping of Et Se; ETAAS; selective 594 2Ž .sludge determination of Se IV ; T 7508C; T 20008C; LOD 0.05]0.08 ngcoll at
Ž . w xSe Nutritional 50 mg Ir permanent modifier FI-HG]trapping]ETAAS; T 2508C 60collsupplement formula
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Ž .Table 5 Continued
Analyte Matrix Modifier Comment Ref.
Ž . Ž . w xSe Soil Pd-treated GT GC]trapping]ETAAS for speciation of CH Se, C H Se and 613 2 2 5 2Ž .CH Se with m 18, 12 and 30 pg, resp.; T 500, 700 and 7008C, resp.3 2 2 p coll
y y w xSe Urine 120 mg Ir FI-HG]ETAAS after off-line digestion with Br rBrO ; T 2508C; 623 colly1LOD 3 mg l
w xSe Vitaminic complex Zr-treated platform vs. Pd-treated Zr preferred; treatment by injection preferred to impregnation at reduced 63containing Na SeO or non-treated platform pressure for simplicity, speed and better reproducibility; lifetime2 3
approximately 300 firings; T 4008C; T 25008C; m 133 pg;coll at oy1LOD 0.23 mg l
Ž . w xSe Seawater Ir-treated platform Speciation protocol: Se IV determined without pre-treatment; total Se after 64y1pre-reduction in 5 mol l HCl at 1008C or UV photolysis at pH 10 for
y130 min; FI-HG]ETAAS; T 3008C; LOD 1.5 ng lcollw x]SH Proteins 8 mg Ir on W-treated integrated Thiol groups of several proteins, modified with p-hydroxymercuribenzoate 65
groups platform and separated by hydrophobic interaction chromatography, determined byFI-CV]ETAAS upon Br ]HBr decomposition of chromatographic2fractions; T 2508C; T 13008Ccoll at
w xSn Aq. solutions 100 mg Ir permanent modifier FI-HG]trapping]ETAAS for i-Sn and butyltins; T 5008C; complete 66,67collisoformation of species not possible; STPF
w xSn Low-alloy steel Nb-, Ta-, W- and Zr-treated GTs HG]ETAAS; T F 6008C; LOD 25 pg; m 17 and 20 pg for Zr- and W- 68coll oor platforms treated GT, resp., preferred for better sensitivity and long-term stability
Ž . w xSn Seawater Zr-treated GT HG]trapping]ETAAS; T 5008C; m 14 and 20 pg for i-Sn II and 69coll oq qBu Sn , resp.; Bu Sn speciation after extraction in CH Cl3 3 2 2
w xAs, Se Aq. solutions Zr-treated GT vs. Pd modifier HG]trapping]ETAAS; T 800]900 and 600]8008C, resp.; m 43 and 70coll paddition 77 pg, resp.
w xAs, Se Aq. high-salt Ir-treated integrated platform FI-HG]ETAAS; effect of salting-out and gas-liquid separator studied; 71Ž . Ž . Ž . Ž .solutions T 4008C As or 2508C Se ; T 21008C As or 20008C Se ;coll at
Ž .NaCl; K SO Zeeman THGA2 4w xAs, Se Seawater Pd]Ir-treated GT Electrochemical HG]trapping]ETAAS 72w xAs, Bi, Sb Low-alloy steel Ir; Ir]Mg; Ir]Pd; carbide-treated FI-HG]ETAAS with in-situ trapping on permanently-modified GTs or tubes 73
Ž .surfaces Nb, Ta, W, Zr with integrated platforms; Zr preferred for lifetime over 400 cycles;m 16, 9 and 15 pg and LODs 15, 27 and 10 pg, resp.o
w xAs, Bi, Se Aq. solutions 50 mg Pd]50 mg Ir FI-HG]ETAAS with in-situ trapping; permanent modifier useful up to 14300 cycles; m 45, 76 and 61 pg, resp.o
Ž . w xBi, Se Aq. solutions Ir-treated platform 100 mg Simultaneous FI-HG]ETAAS; T 2508C; T 22008C: m 55 and 41 pg; 74coll at oLOD 0.13 and 0.19 mg ly1 , resp.
w xSe, Te Low-alloy steel Ir, Ir]Mg, Ir]Pd; carbide-treated FI-HG]ETAAS and CF-HG]ETAAS; T -6008C; m 11 and 12 pg; 75coll oŽ .surfaces Nb, Ta, W, Zr LOD 11 and 7 pg on Ir coating in FI and CF modes, resp.
w xAs, Bi, Sb, Aq. solutions 2 mg Ir on a Zr-treated platform HG]trapping]ETAAS with permanent modification; effect of L-cysteine on 17Sn inorganic and organoelement species studied
()
D.L
.Tsale¨
etal.rSpectrochim
icaA
ctaP
artB:A
tomic
Spectroscopy55
2000473
]490482
Ž .Table 5 Continued
Analyte Matrix Modifier Comment Ref.
Ž w xAs, Bi, Sb, Aq. solutions Bi, 2 mg Ir on Zr- or W-treated HG]trapping]ETAAS; trappingrstabilization of org.-As, org.-Sn and 16y1. ŽSe, Sn, Te Te , seawater As, platform org.-Se species also studied; 16]28 h
.Sb, Se ; sedimentŽ .As, Sb, Se, Sn
aAbbre¨iations: Aq., aqueous; CF, continuous flow; CV, cold vapor; CVT, cold vapor technique; E , activation energy; FI, flow injection; GC, gas chromatography;am , characteristic mass for peak height measurements; MWD, microwave decomposition; resp., respectively; org.-, organo-; T , atomization temperature; T ,p at coll
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490 483
lytic graphite layers, high heating rates, trans-versely heated atomizers, more efficient chemicalmodifiers, etc.
Inserts placed in the graphite atomizer, such asboats, platforms, or linings, made of refractoryŽ . Ž w x.Ta or noble metals e.g. Au]Pt gauze 48,50
Žhave found very limited application e.g. for trap-w x.ping Hg vapors 48,50 and are considered to be
outside the scope of this overview.Direct ETAAS applications of permanent mod-
Žification to real samples are rather scarce Table.4 . For best thermal stabilization of volatile ana-
lytes, preference is given to the least volatileŽ .noble metals Ir, Rh, Ru , either alone or on
graphite surfaces treated with carbide-formingŽ . w xmetals W, Zr , such as the pairs W]Ir 16,21 ,w x w x w xW]Ru 21 , W]Rh 22]24 and Zr]Ir 15]18 .
The analysis of organic extracts, chromatographiceffluents and other concentrates is convenientlyperformed in graphite atomizers treated with car-
w xbide-forming permanent modifiers: Mo 31 , Tiw x w x w x w x Ž w x.36 , Y 32 , W 35,39 , Zr 29 , etc. review 10 ,which provide better conditions for cleavingchemical bonds of organoelement species andcomplexed analyte elements.
Coprecipitation separations, e.g. withw xZrO .nH O 29 , utilizing a modifier component2 2
as a carrier, are combined in a rational mannerwith subsequent quantitation on a Zr-treated tube
Ž . Ž .or platform } more examples with Zr IV , La IIIŽ .and Y III hydroxides and other carriers, e.g. for
As, P, Sb and other analytes, may be found in ourw xprevious reviews 4,7 .Ž .Efficiency of noble metal modifiers is often
improved when applied on carbide-treatedatomizers, providing higher T , more dispersevapand uniform modifier distribution, and better
Žlong-term stability see Fig. 2 for Ir distributionon an untreated vs. a W]Ir-treated platform upon
.400 firings , synergistic and catalytic effects andw xother positive assets 7]10,13 } for example Ni,
ŽPd or Pt on W-treated tubes or platforms Ni]Ww x w x w x.30 , Pd]W 33,37,39,40 , Pt]W 37 .
The possibilities for speciation analysis withETAAS at sub-nanogram levels are attractive, yet
w xrather limited 29,34]36 , e.g. after separations byw x w xion exchange 35 , extraction 35,36,69 or by per-
forming clever, in situ chemical reactions for frac-
Fig. 2. SEM images showing the iridium distribution on aŽ .pyrolytic-graphite coated platform top; 10 mg Ir vs. iridium
Žmapping on an aged W]Ir-treated pyrocoated platform bot-w x .tom; 240 mg W and 10 mg Ir 15 after 400 firings at 19008C .
Ž . w x Ž . w xtional volatilization of As III 29 or Se IV 34Ž .as AsCl or Se IV piazselenol, respectively. VG3
techniques offer better speciation capabilities, e.g.w x w x w xfor As 16,17,43,45 , Se 59,61,64 and Sn 16,17,69
} see Table 5.w xAs expected, ‘hot injections’ 23,24 and multi-
w xple injections 16 are more conveniently per-formed with permanently modified tubes or plat-forms.
Undoubtedly, permanent modifiers have foundan almost ideal, straightforward application in
Ž .VG]ETAAS Table 5 , with in-atomizer trappingw xof inorganic hydrides 12,76,77,79 , vapors of Hg
w x w x48]51,80 and Cd 46 , ethylated derivatives of Pb
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490484
w x w x56 and Se 59 , substituted methylated hydridesw xof As 16,17,45 , substituted alkyltin hydrides
w x w x16,17,66,67 , and alkyl selenides 61 . Analyticalperformance, advantages and limitations of thistechnique have been discussed in depth in recent
w xreviews 12,77 . VG]ETAAS is nowadays success-w xfully commercialized 79 by two instrument man-
Ž .ufacturers Perkin Elmer and Analytik Jena and,due to permanent modification with Ir, offers
Žautomation with high sampling frequencies 16]40y1 w x.h , depending on the sample volume 16,54 ,
competitive LODs down to a few nanograms perliter, speciation potentialities, long lifetime of
Žcoatings 300]800 firings, depending on modifier.and temperature program , and other extra assets
Admittedly, permanent modifiers do not showas straightforward performance as anticipated.They often exhibit certain drawbacks and limita-tions. Current technologies for producing ‘perma-nent’ modifier residues are far from perfection,reproducibility and reliable quality control. For
w xthe sake of simplicity, the ‘modifier’ method 10is commonly applied, i.e. deposition of a modify-
Žing solution by injection as a single aliquot or by.multiple successive injections . Accordingly,
‘treated’ rather than ‘coated’ graphite surfacesand ‘prolonged’ rather than ‘permanent’ mode of
w xaction are provided 9,10 . Comparative studies ofdifferent techniques for permanent modificationare documented: e.g. ‘modifier’ injection pre-ferred to impregnation at reduced pressure for
Žsimplicity, speed and better reproducibility Zr-w x.treated platform for Se determination 63 ; elec-
w x w x w xtroplating of Ir 80 , Pd 26,38 or Rh 25,38preferred to the ‘modifier’ method; and cathodicsputtering of Ir preferred to the ‘modifier’ ap-
w xproach 41,42 . Both technologies, cathodic sput-w x w xtering of Ir 41,42 and electroplating of Rh 25,38 ,
have demonstrated more uniform deposits, bettersurface coverage and competitive long-term
w xstability } e.g. up to 520 firings for Cd 25 andw x750 firings for Se 41 .
Ž .Scanning electron microscopy SEM pho-w xtographs 19,20,22,38,41 reveal imperfect, discon-
tinuous ‘coating’ which tends to replicate the sur-face morphology of the graphite substrate, e.g.with preferential coverage of and redistributionto the valleys of the cauliflower-like surface struc-
w xture of the pyrolytic graphite coating 41 . Themicrographs in Figs. 2]4 illustrate some typicaleffects: better dispersion and more uniform dis-tribution of Ir on a carbide-treated platform, pro-viding higher thermal stability of the resulting
Ž .‘mixed’ modifier lower volatility of Ir , betterefficiency and adequate lifetime of the W]Ir per-
Ž .manent modifier Fig. 2 ; different morphologyand incomplete, non-uniform surface coverage for
ŽW]Ir and Zr]Ir treated pyrolytic platforms Fig..3 ; and effects of aging, eventually resulting in
apparent spatial re-distribution of modifierresidues after 400 firings } ‘shrinking’ and per-
Ž .haps also in-depth penetration Fig. 4 .It is reasonable to expect that various surface
sites may contribute in a different manner to theprocesses of hydride sequestration or sample so-lution pre-atomization behavior and, further-more, on analyte vaporizationratomization fromdifferent sites. Consequently, an incidence of peakshape distortion and even appearance of double-peak atomization signals has been reported withmany types of permanent modifiers: carbide-
w x w xtreated 20,27,68 , Ir-sputtered 41 and Ir]W-w xtreated graphite atomizers 16,20 . For example,
persistent double peaks with relatively good re-peatability of peak times have been observed for
w xBi and Te with W]Ir-treated platforms 16,20Ž .see Fig. 5 and for Ag, Bi and Te on W-treated
w xplatforms 20 . These effects are less pronouncedin HG]ETAAS for Bi and Te on W]Ir-treated
w xplatforms 16 and are almost negligible for Cd inboth direct ETAAS and HG]ETAAS. They are
w xnot observed with Zr]Ir-treated platforms 15 ,Žalso not for other volatile analytes examined As,
. w xPb, Sb, Se, Sn 15,16 . The ‘low-temperature’Ž .species e.g. Bi-1 and Te-1 in Fig. 5 result from
vaporizationratomization of these analytes fromthe uncoated pyrolytic graphite surface, whereas
Ž .the ‘high-temperature’ pulses Bi-2 and Te-2 areassociated with thermally stabilized portions of
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490 485
Fig. 3. SEM photographs showing different surface mor-Ž .phology and incomplete surface coverage of W]Ir- top and
Ž .W]Zr-treated bottom pyrocoated platforms; 240 mg W; 110w xmg Zr; 10 mg Ir as detailed in Tsalev et al. 15 .
these analytes on W]Ir, as discussed in morew xdetail elsewhere 16,20 .
The efficiency of thermal stabilization is oftenimpaired with permanent modifiers vs. modifier
w xadditions to each sample aliquot 7 , and more-over in the presence of real matrices. Thus
w xRademeyer et al. 42 observed vaporization tem-Ž .peratures T of 500, 750 and 4008C for Cd, Pbvap
and Se in blood digests in dilute HNO vs. 8008C,312008C and 14008C, respectively, in dilute HNO3only, in spite of employing a high-quality coatingof sputtered iridium.
Promising results with slurry samples have been
Fig. 4. SEM photograph showing differences in surface cover-Ž .age with new top and aged W]Ir-treated pyrocoated plat-
Ž . w xforms bottom . 240 mg W and 10 mg Ir 15 , after 400 firings.at 19008C .
reported recently by two groups from Brazil: Cdw x w xin fish 23 , Pb in sediment 24 and Cd, Pb and Se
w xin biological CRMs 21 . However, analyticalŽ .lifetimes of the W]Rh deposit 250q200 mg
were approximately 250]300 firings, thus callingfor re-conditioning treatment with W and Rhw x23,24 . Under favorable conditions, this treat-
w xment could be repeated twice 23 or even fourw xtimes 22 , thus reducing analytical cost, as shown
in the economic evaluation made by Lima et al.w x Ž24 . Such considerations on cost of consumablesby users and manufacturers are not necessarily in
.harmony!
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490486
Fig. 5. Typical double peaks for two volatile analytes atomized from an W]Ir-treated integrated platform of the THGA. Dashedw xline, 4 ng Bi; solid line, 2 ng Te. Platform treated with 240 mg W and 10 mg Ir as detailed in Tsalev et al. 15 . T 8008C; Tpyr at
16008C.
In real analytical cases, lifetime of permanentmodifiers depends strongly not only on the type
Ž .and mass of matrix and acid s but also on thetemperature program: pyrolysis, atomization and
Žcleaning temperatures T , T and T , respec-pyr at cl.tively and corresponding times of the individual
steps as well as on stop-flow conditions. Owing toŽ .the simplicity or rather absence of matrix, the
VG]ETAAS technique with in-atomizer trappingw xprovides lifetimes of 600]700 thermal cycles 16
or even )1000 for cadmium, with T , T , Tcoll pyr atand T , as low as 3508C, 5008C, 13008C andcl
Fig. 6. Examples of ‘isoformation’ potentials of a permanent modifier, Zr]Ir on an integrated platform of THGA, for somew xorganoelement species of As, Se and Sn. 110 mg Zr and 2 mg Ir applied as in Tsalev et al. 15 .
( )D.L. Tsale¨ et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 2000 473]490 487
20008C, respectively. Noteworthy, the potentiallifetime of a permanent modifier such as Zr]Ir orW]Ir on an integrated platform, may well exceed
w xthat of the THGA tube itself in HG]ETAAS 16 .Ž .Compromize lower settings of T , T and Tpyr at cl
are recommended in order to avoid excessive lossand redistribution of modifier, e.g. F14008C,20508C and 21008C for Zr]Ir and F14008C,21008C and 21508C for W-Ir on the integrated
w xplatform of the THGA 15,16 .The ‘isoformation’ potential of permanent
Žmodifiers towards different analyte species see.Fig. 6 for As, Se and Sn on Zr]Ir are an impor-
w xtant current issue, e.g. for As 16,17,45 , Sew x w x16,17,61 , or Sn 16,17,66,67,69 , although theymay happen not to be as good as anticipatedw x16,17,34,66,67 , and may be more difficult to con-trol compared with the more versatile approachbased on properly blended dissolved modifier ad-
w xditions 7 .
5. Conclusion
Application scope, advantages and limitationsof permanent modification have been summa-rized and discussed. While being very attractiveand potentially promising, this methodology isstill far from straightforward performance androutine applicability, except in hyphenatedVG]ETAAS assays. The scope of permanentmodification is unlikely to become as broad asthat of classical chemical modification ap-proaches, as far as difficult matrices, numerousanalytes and diverse analytical tasks areconcerned. Nevertheless, some important, opti-mal application niches can be already defined:VG]ETAAS; analysis of organic extracts, eluatesand other concentrates in hyphenated ETAASsystems, in particular for on-line pre-concentra-tion and speciation in flow injection systems; insitu enrichment by multiple and hot injections;
Ž .utilization of noble metal modifiers on carbide-treated atomization surfaces, etc. The apparentlack of interest of instrument manufacturers andproducers of graphite parts to permanent modifi-cation cannot be overlooked } presumably be-
cause of technological and quality control dif-ficulties, limited demand and economic considera-tions. More research in this field would beexpected, in particular on improvement and uni-fication of coating technologies, performance ver-ification in real analytical conditions, and provid-ing more evidence on metrological, technical andeconomical merits of permanent modification.
Acknowledgements
Financial support to D.L.T. and V.I.S. by theConsiglio Nazionale delle Richerche, Istituto diChimica Analitica Strumentale, Pisa, Italy and toD.L.T. by the Organizing Committee of the XXXIColloquium Spectroscopicum Internationale,Ankara, Turkey, is gratefully acknowledged.
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