[Comprehensive Analytical Chemistry] Sample Preparation for Trace Element Analysis Volume 41 ||...

Chapter 5

Detection methods for the quantitationof trace elements

Les Ebdon, Andrew S. Fisher, Maria Betti and Maurice Leroy

5.1 INTRODUCTION

This chapter serves as a general introduction to the methods of trace elementdetermination discussed throughout this book. Brief overviews of theseinstrumental techniques will be given, along with discussions of theiranalytical capabilities, requirements of the sample, sample throughput, figuresof merit and descriptions of the numerous methods of sample introduction. Inaddition, a brief overview of some of the sample preparation methods andsample manipulation procedures will also be given. Included in the chapter areselected examples of applications, although many of these will be treated inmore detail throughout later chapters in this volume.

It is worth noting that many of the sample introduction methods for themore frequently used atomic spectrometric techniques are common to all.Therefore, the description of the theory behind them will only be given onceand, thereafter, applications of each will be given for the other methods ofdetection.

5.2 CLASSICAL METHODS

Classical methods of analysis will be dealt with only very briefly here since it islargely outside the scope of this chapter. It should be noted, however, thattitrations are still an important part of an analyst’s armory because an“EDTA” titration readily provides traceability to a primary standard. Titrationscan be relatively time consuming and do not usually offer very great sensitivity,so are of limited use for many sample types. For those applications wherethe analyte is present at an appreciable concentration (contaminants at the0.01–5% m/m range in metallic samples, or even the major constituent), atitration can offer very accurate and precise results. As an example, inspectionof the certificate for the reference material BCS 177/2 lead-base white metal(available from the Bureau of Analysed Samples, Middlesbrough, UK)

Comprehensive Analytical Chemistry XLIMester and Sturgeon (Eds.)q 2003 Elsevier B.V. All rights reserved 117

indicates that lead (present at an average of 84.5%), antimony (10.1%) and tin(5.07%) were all determined by different titrimetric methods. In addition, threeanalysts determined arsenic (0.05%) by titration. A sound knowledge of thechemistry of both the sample and of the analytical method is required toprevent interferences.

A useful textbook covering many of the classical “wet-chemical” methods is“Vogel’s Textbook of Quantitative Chemical Analysis” [1] which contains anassortment of titrimetric, gravimetric, potentiometric, electrogravimetric,spectrophotometric and amperometric methods. As well as giving the basictheory behind each of the techniques, it also gives experimental details for someselected applications.

5.3 FLAME SPECTROMETRY

5.3.1 Introduction

Flame spectrometry, either atomic absorption spectrometry (AAS) or atomicemission spectrometry (AES), are amongst the most simple and inexpensive ofthe instrumental methods of trace element analysis. The cost of a basic AASinstrument can be less than US $10,000, although for the more powerful com-puter controlled instruments containing autosamplers, the cost can easily bedouble this. A flame photometer (a very basic AES instrument) that can be usedto determine analytes such as lithium, sodium and potassium costs even less.

5.3.2 Theory

A detailed description of the basic theory of AAS and AES is not required here;a detailed and theoretical description of the processes within the flame (orplasma for emission) may be found elsewhere [2].

The relaxation of electrons in an analyte atom from different excited energylevels populated by (thermal) flame processes back to the ground state willyield photons of light of different energy, i.e., the wavelength of the lightemitted will be different for each transition. A characteristic spectrum for eachelement will therefore arise.

The transition probability governs the sensitivity of a wavelength (ananalytical line). If the probability of a transition is low, then the number ofanalyte atoms (or ions) in which the electrons are excited to that energy levelwill also be low. This means that the number of atoms/ions emitting light at thecorresponding wavelength will be relatively few and hence the overallsensitivity will also be low. Therefore, to obtain a detectable signal, theconcentration of that analyte in the sample will have to be higher. Each line ofan analyte, therefore, has a different sensitivity. This can be useful analytically,because each line will have its own linear range. If the expected range of

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concentration of an analyte in a sample is known, then the most appropriateanalytical line may be chosen, thereby negating the need for large sampledilutions and hence keeping sample manipulation to a minimum anddecreasing the likelihood of dilution errors or contamination.

In atomic absorption, the amount of light absorbed from an incident beam oflight is proportional to the number of analyte atoms in the optical path, andhence to its concentration in the sample. As for all absorption-basedtechniques, the path length also has an effect on the sensitivity. Typically, a10 cm path length is used for an air–acetylene flame, but there is no reasonwhy a smaller path length (5 cm, used for nitrous oxide–acetylene), could notbe used, which would lead to half the absorbance of that using the largerburner. This also has the effect of extending the linear range by a factor of two.As in atomic emission, numerous wavelengths are available for each analyteand these will each confer a different sensitivity. A comprehensive theorybehind atomic absorption may be found elsewhere [3]. It is worth noting thattrue spectral interferences for atomic absorption are very rare. This is becauseof the “lock and key” effect of the incident radiation and the analyte atoms.Theoretically, there are no other atoms present in the atom cell that shouldabsorb the radiation and hence false high signals should not be obtained.Unfortunately, the presence of particulate matter or of some molecules maylead to absorption of the light. Under circumstances such as these, erroneouslyhigh signals may then be obtained.

5.3.3 Instrumentation

As discussed above, a light source is required to excite the analyte atoms whenusing the AAS technique. There are two common types of light source, of whichthe line source hollow cathode lamp (HCL) is the more frequently used. It has acup-shaped cathode coated internally with (or fabricated from) the analyte ofinterest. Often, only one element is used per lamp, but multi-element lamps arealso commercially available that may contain two, three or perhaps even five orsix elements. Although multi-element lamps are more expensive than singleelement ones, they have the advantage of being less expensive than investingin five or six individual lamps. Their disadvantage is that often compromiseoperating conditions must be used, which may have an unfavorable effect onthe signal-to-noise ratio and linear range for some of the analytes. The otherlight source commonly used is the electrodeless discharge lamp (EDL). Theseare more expensive to purchase but offer an increased light intensity and forsome analytes, e.g., arsenic, and provide for higher sensitivity and enhanceddetection limit.

Sample is usually, but not always (see Section 5.3.6), introduced to theinstrument as a liquid via a nebulizer/spray chamber assembly. As the gas usedfor combustion passes the end of a capillary, a pressure drop is obtained. If theother end of the capillary is immersed in a liquid sample, it will be drawn

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through the capillary by the Venturi effect. As it enters the gas stream rushingacross the end of the capillary, the stream of liquid is shattered into a nebular(an aerosol or mist of droplets). This nebular then enters a spray chamberwhere the larger droplets are separated from the smaller ones by a combinationof an impact bead (that helps shatter the droplets into smaller droplets) and aseries of baffles. The smallest droplets are then carried in the gas flow towardsthe atom cell whilst the larger ones pass to waste under the influence of gravity.For flame spectrometry, typically 10–15% of the sample reaches the atom cellwhilst 85–90% is wasted.

Once the sample aerosol enters the atom cell, the flame desolvates theaerosol and then dissociates the salts present into their constituent atoms. Theatoms of the analyte will then absorb the light emitted from the light sourceand the amount of light absorbed can be related to the concentration of theanalyte in the sample. There are several flame types that can be used, the mostcommon being a mixture of air and acetylene. These can be mixed in severaldifferent proportions, including fuel rich (here a yellow flame that has reducingproperties is produced), fuel lean (a blue flame that is chemically oxidizing) orstoichiometric (an intermediate flame that is blue but also has yellow “feathers”at its base). The type of flame chemistry used will depend on the analyte, andshould be optimized for every element determined. The temperatures of theseflames range between 1700 and 2200 K, which is sufficient to dissociate themajority of compounds. A hotter flame, e.g., nitrous oxide–acetylene (2500–2700 K) may be necessary for the more refractory compounds. Again, differentproportions of nitrous oxide and acetylene may be required for optimaldetermination of different analytes. Other flames used, albeit less frequently,include a hydrogen diffusion flame and a methane flame. The former has theadvantage of being very optically clean at lower wavelengths, which willimprove the signal-to-noise characteristics for wavelengths such as 193.7 nm(As), 196 nm (Se), 213.9 nm (Zn) and 217 nm (Pb). The methane flame is usefulwhen the sample matrix may contain a very high concentration of a componentthat forms an explosive acetylide compound (e.g., Ag or Cu).

The analyte wavelength used for the measurement process is usuallyisolated with the use of a low-resolution monochromator, since in AAS theresolution of the instrument is essentially derived from the narrow wavelengthoutput of the line source.

Once the wavelength of light of interest has been isolated, it may bedetected using a photomultiplier tube or solid-state electronic device (such as acharge coupled or charge injection device or diode array). These convertphotons to an electrical signal, the magnitude of which may be related to theconcentration of the analyte within the sample. Since atomic absorption is aratio technique, i.e., a comparison is made of the initial light intensity with theintensity after absorption by the analyte has occurred, there are no units.Tuned electronic circuits are used to ensure that light produced by emission

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processes arising from the analyte within the flame do not interfere with themeasurement of atomic absorption.

Atomic emission spectrometry utilizes all of the above procedures with theexception of the HCL light source. A monochromator is usually used to isolatethe wavelengths of interest but very low-resolution optical filters may be usedin the less expensive flame photometers. Analyte atoms thermally excited bythe flame emit multiple wavelengths of light, one of which is isolated anddetected. Since the advent of inductively coupled plasma instrumentation foratomic emission, the flame emission technique has been in decline and is nowrarely used.

A more detailed description of the instrumentation used and of the pro-cesses occurring within it may be obtained in several other publications [2,3].

5.3.4 Interferences and background correction techniques

The majority of interferences that are encountered are either physical orchemical in nature, although a few spectral interferences arising frommolecular species also exist. Transport efficiency of the sample through thesample introduction system can lead to interferences if standards are notclosely matrix matched with the samples. For example, differences in viscositybetween samples and standards result in different nebulization efficiencies. Ifless of the sample reached the flame, an underestimate of the trueconcentration of the analytes would be made. If the concentration of theanalyte is sufficiently high, it may be possible to dilute the sample such that thedissolved solid content becomes negligible. If the analyte cannot be diluted,then it may be necessary to perform a standard additions analysis. Manyanalysts are not overly keen to use the standard additions technique, because itmeans that the same sample must be analyzed up to four times with differentadded concentrations, thereby lengthening the analytical process fourfold. Inaddition, the volume of sample used will be increased fourfold, which may beproblematic if only a limited supply is available.

There are several types of interference that may occur in flamespectroscopy. Chemical interferences may cause either depressions or enhance-ments in the signal, depending on the particular interferent. If a species ispresent in the sample that will combine with the analyte to form a less volatilecompound that is difficult to dissociate in the flame, then a depression in signalmay occur, an example being the presence of phosphate during thedetermination of calcium. There are various methods for overcoming this,including adjusting the nebulizer so that smaller droplets are produced;making observations higher in the flame so that the less volatile compound hasa longer time to become dissociated; using a releasing agent (e.g., lanthanum),that preferentially combines with the phosphate, and using a chelating agent(e.g., EDTA) to complex with the analyte so that it cannot combine with thephosphate. The addition of other chemicals to the samples may lead to

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contamination and error, so possibly the most simple and reliable method ofovercoming this type of interference is the use of a hotter flame, i.e., nitrousoxide–acetylene. Signal depressions may also occur if the analyte becomesoccluded in a refractory compound, such as is the case of the oxides of rare earthelements, uranium or zirconium. Again, the use of a hotter flame usuallyovercomes this problem. Signal enhancements are much rarer, but an exampleis the formation of an EDTA complex for calcium. The complex is more volatilethan many other calcium compounds. Matrix matching may overcome this, butthe use of a hotter flame will ensure that all of the analyte in both standardsand samples is atomized. Similarly, if an analyte is occluded into a volatilecompound (e.g., ammonium chloride), the atomization of the analyte may beenhanced. Again, matrix matching usually provides a solution to the problem.

Ionization interferences occur mainly for the alkali metals that have anexceptionally low first ionization potential (IP). Since flame spectroscopyusually determines atoms (either by atomic absorption or by atomic emission),the formation of ions may lead to problems because these will not absorb or emitat the same wavelength. If, for example, sodium is to be determined by eitheratomic absorption or emission, it is usually necessary to add a highconcentration of another easily ionized element such as potassium or cesium(assuming that these are not amongst the analytes), to the samples, standardsand blanks. These will become ionized in the flame, producing a large excess ofelectrons that then force the ionization equilibrium to favor the neutral analytespecies. Samples are more likely to contain other easily ionizable elements thanpure aqueous standards, and therefore the extent of ionization will be less thanfor the standards. Unless a large excess of ionization buffer is added to allsamples, standards and blanks, an overestimate of the analyte concentrationcould result.

Spectral interference, caused by direct line overlap, is negligible for flamespectroscopy. However, since molecules exhibit a much wider wavelength bandof absorption/emission, these can occasionally prove to be problematic.Examples include phosphate and sulfate interferences on the arsenic andselenium lines at 193.7 and 196 nm, respectively. Similarly, small particulatematter within the flame may attenuate the light beam, leading to anerroneously high signal unless it is corrected for by a method of backgroundcorrection.

There are several types of background correction systems used for AAS.These include the deuterium lamp (continuum source), Zeeman effect andSmith-Hieftje systems, and each has been available in commercial instrumen-tation. Each background correction system has its own relative advantages anddisadvantages. A detailed description of their operation is not necessary here,but may be found in the literature [3]. However, a brief description of theadvantages and disadvantages of each is appropriate. The deuterium lamp hasrelatively ineffective output above 350 nm and so the most sensitive resonanceline for chromium (at 357.9 nm) may occasionally be problematic if particulate

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material is present in the flame. The deuterium lamp is, however, fairlyinexpensive to purchase and to operate and provides an adequate correction forthose analytes that have a primary wavelength in the UV region. Since thelarge majority of background absorption phenomena occur below 350 nm, it isadequate for most applications. Another disadvantage of this continuum sourcesystem is that the beam from this light source must follow an identical opticalpath to the beam from the HCL. The Smith-Hieftje correction system operateson the principle of self-reversal, i.e., if the source HCL is pulsed to much higheroperating current, the narrow emission line profiles are broadened and suffersome self-reversal. The analyte atoms absorb only a fraction of this broadenedline radiation whereas the background absorption is unaffected. By operatingthe HCL at normal current (2–25 mA) and at much higher current (.100 mA)in a rapidly oscillating manner, estimates of the total absorbance (atomic andbackground) and the background absorbance may be made by subtracting thetwo absorbance signals. The technique works fairly well for many types ofinterference, but has several drawbacks. These include the shortened lifetimeof the HCL, the “assumption” that the atomic absorption during the highcurrent pulse is negligible, which leads to reduced analytical sensitivity, andincreased curvature of the calibration curve. The Zeeman backgroundcorrection system is used almost exclusively for electrothermal AAS(ET-AAS), but will be included here amongst the other background correctiontechniques for completeness. It is a technique that uses a powerful magnet(approximately 1 T) to separate the normal single atomic line profile intoseveral different components, as described in more detail elsewhere [3,4]. Thesignificant advantage of this type of correction system is that it is capable ofcorrecting much larger background signals than any of the other methods.Unfortunately, it also suffers from decreased sensitivity and increasedcurvature (and ultimately complete roll-over) of the calibration function.

5.3.5 Conventional nebulization

The process by which conventional nebulization occurs and some of thepotential problems that may arise (i.e., different viscosity of samples andstandards leading to different nebulization efficiency and hence, differentsensitivity) have been described previously and several other factors need to bediscussed. The sample uptake rate for conventional nebulization in a typicalflame AAS/AES instrument is between 4 and 8 ml min21. This may usually bechanged by careful adjustment of the nebulizer. Fortunately, the time requiredfor the analyte to pass through the nebulizer/spray chamber system, into theburner head and then into the flame, is only about a second. Therefore,measurements may be made only 2–5 s (depending on the integration timeused) after sample introduction commences. The amount of sample consumedwill depend on the number of replicate readings taken, but usually, a volume of3–4 ml is adequate to determine an analyte. However, for most instruments,

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flame AAS is a single element technique, i.e., the operating conditions will haveto be changed for another analyte, 3–4 ml of sample is required for everyanalyte. This may prove problematic if 10–20 analytes need to be determinedand only 25 ml of sample is available. It should be noted, however, that somemodern instruments have a rapidly scanning monochromator (2000 nm min21)and specialized valves that enable a very rapid change of the flame chemistry.These attributes, combined with a series of fast rotating mirrors to changebetween different HCLs, enable very rapid sequential determinations to bemade. Such an instrument offers significant improvements in analysis time,although sample consumption may be marginally higher.

Conventional nebulization into flame AAS is, as discussed above, veryrapid. After the measurement of one sample or standard, a washout period isnecessary in which water, dilute acid or a chemical matched to the matrix of thesamples is used to ensure that there is no signal carry over between samples.Depending on the matrix and the analyte, this requires anywhere from just afew seconds to in excess of a minute. The washout may have to be especiallylong if the sample contains a very high concentration of dissolved salts. Oncethese samples enter the burner head, they will desolvate and, unless a longwash period is used, there is a chance that the salts will start to block theburner head. This would result in several effects, including a reduction in thesensitivity (because if gaps start appearing in the flame, the path length iseffectively decreased) and excessive signal drift. Occasionally, the burner maybe cleaned by gentle scraping with a non-combustible item, e.g., a stainlesssteel spatula but often, flame extinction followed by dismantling of the burnerassembly is necessary to clean it. This would obviously lead to an increase inthe analysis time. A single element may be determined in only a few secondsper sample, and therefore a batch of 20 samples could be analyzed for oneanalyte in less than 10 min. Using the rapid sequential instruments, samplethroughput for several analytes can be improved significantly.

Many modern instruments come equipped with an autosampler, whichfacilitates the unattended operation of the instrument, thereby maximizingsample throughput with minimal human intervention. Many modern instru-ments have software that enables the complete analysis to be pre-programmed,incorporating assorted quality control measures, e.g., check standards,collection of data from multiple replicates and the calculation of mean values,standard deviations and precision, etc. Some instruments have a moveableturret in which three or four HCLs may be inserted. The software then controlsthe monochromator, changing to the wavelength necessary for each analyte.

The figures of merit of flame techniques tend to be the least impressive ofthe standard instrumental techniques, but are still adequate for manyapplications. The limit of detection will depend on several factors, includingthe analyte itself. Some analytes, e.g., magnesium or cadmium, are extremelysensitive, whereas others, e.g., lead, are not. Other analytes that are extremelyrefractory, such as tantalum and tungsten, offer relatively poor sensitivity.

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The limit of detection will also depend on which analytical line of the analyte isbeing measured. As described previously, numerous analytical lines may beused and each has a different sensitivity, LOD, calibration range, etc. The LODobtained using each of the lines will also depend on the instrumentalparameters used, as each element has an optimal flame chemistry, HCLcurrent, spectral bandpass, viewing height, etc., and unless the optimalconditions are used, the optimal figures of merit will not be obtained. A list oflimits of detection for numerous elements is given in Table 5.1, for which it hasbeen assumed that the most sensitive analytical line is used for each analyteunder optimal conditions. It should be noted that the figures given in Table 5.1relate to liquid samples. If a solid has been dissolved or digested, then a dilutionhas occurred and the LOD related to the solid would have to be re-calculated.

For atomic absorption measurements, the linear range usually spans 1.5–2orders of magnitude before there is a departure from linearity. Therefore,assuming that several standards are prepared that cover this range and thatsteps have been taken to overcome potential interferences, reliable data shouldbe obtained. Any sample that has an absorbance greater than the mostconcentrated standard should be diluted so that it comes into the workingrange. If many or all samples contain a concentration of an analyte that isabove the most concentrated standard, it would be less time consuming to usean alternative wavelength and prepare a more appropriate calibration rangethan to dilute perhaps 50 samples. An alternative method is to rotate theburner head slightly. This has the effect of decreasing the path length, i.e.,fewer atoms are in the light beam at any one instant, and therefore thelinearity may be extended (at the expense of sensitivity). In such a case, it willstill be necessary to prepare another standard that contains an analyteconcentration closer to that expected in the sample to ensure linearity.

The precision expected from a flame instrument (as with any instrumentalmethod) will obviously depend on the concentration being measured. If theconcentration is close to the LOD, then precision will be poor when comparedwith a concentration further up the linear range. For the latter example, aprecision of 0.1–2% relative standard deviation (RSD) is typical.

As discussed previously, the majority of samples introduced via conven-tional nebulization must be liquid based and that the transport efficiency isusually between 10 and 15% for aqueous based samples. This figure willdepend, however, on the nature of the sample. The presence of appreciableamounts of dissolved solid is likely to decrease this value. If the sample ispresent in an organic solvent, then the nebulization characteristics will differmarkedly. Water has a fairly high surface tension and viscosity and a low vaporpressure. Organic solvents tend to have a lower surface tension and viscosityand a higher vapor pressure. This means that they more efficiently form anaerosol, resulting in an increased transport efficiency to the flame and enhancedsensitivity. It is therefore extremely important to prepare standards in the samesolvent as that used to dissolve the samples. The presence of organic solvents in

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the flame will radically change the flame chemistry and it is therefore importantto optimize the flame conditions using the solvent of interest if optimalsensitivity is to be obtained. Organometallic standards, which are often basedon the cyclohexylbutyrates, are available commercially and are soluble in manyorganic solvents. If the solvent is methanol, then many inorganic standardsthat are stabilized in nitric or hydrochloric acid are soluble.

TABLE 5.1

Limits of detection using flame AAS under optimum conditions with the most sensitiveline

Analyte LOD (mg l21) Analyte LOD (mg l21)

Ag 2 Mo 20

Al 30 Na 0.2

As 300 Nb 2000

Ba 20 Nd 1000

Be 1 Ni 10

Bi 50 P 40,000

Ca 1 Pb 10

Cd 2 Pd 10

Co 5 Pr 10,000

Cr 6 Pt 100

Cs 4 Rb 10

Cu 3 Ru 100

Dy 30 Sb 40

Er 50 Sc 50

Eu 1.5 Se 500

Fe 6 Si 300

Ga 100 Sn 100

Gd 2000 Sr 2

Ge 200 Ta 2000

Hf 2000 Tb 700

Hg 200 Te 30

Ho 40 Th

In 40 Ti 100

Ir 500 Tl 20

K 3 Tm 20

La 2000 U 40,000

Li 2 V 100

Lu 300 W 1000

Mg 0.3 Y 200

Mn 2 Zn 1

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Although the large majority of samples introduced into flame atomicspectrometric instrumentation is liquid based, the introduction of solidmaterials is possible. This is usually achieved through the preparation ofslurries. A slurry is a suspension of a very finely ground sample in a liquidmedium which is usually a dispersant to prevent the particles fromflocculating. The subject will be dealt with in far more detail in a later section(Section 5.3.6.6). Briefly, the sample is ground so that the particle size isequivalent to the droplet size in the aerosol formed by the nebulizer. Thetransport efficiency of the slurry particles should therefore be equivalent toaqueous standards and therefore, simple aqueous standards (or standardsmatrix matched with the dispersant used for the slurry) can be used forcalibration.

A plethora of other pre-concentration and matrix separation techniques,including solid phase extractions, liquid–liquid extraction, co-precipitation,flotation and evaporation are available and these will be discussed in moredetail in other chapters throughout the book.

5.3.6 Alternative methods of sample introduction

There are a number of alternative methods that may be used to introducesamples for flame spectrometry. Some of these offer increased sensitivity andothers help overcome potential interferences, thereby yielding more reliableresults or better long-term stability.

5.3.6.1 Chemical vapor generationChemical vapor generation as a method of sample introduction is discussed indetail by Cai in this volume. The topic has also been reviewed by Tsalev [5] andby Howard [7]. Although primarily applicable to elements such as arsenic,selenium, antimony, tellurium and germanium, which are capable of forminggaseous hydrides at room temperature by reaction with sodium tetrahydrobo-rate, and elemental mercury, it has been reported that several other analytes,including Ag, Au, Cd, Co, Cu, Ni, Sn and Zn, have also been determined byvapor generation [6]. There are also alternative reagents that may be used toform volatile vapors, including various salts of tetraethylborates. Mercury maybe reduced to its elemental state by stannous chloride. There are severaladvantages of introducing analytes as a gas rather than as a liquid. The first isthat the analyte is separated from the bulk of the matrix. Together with highersample uptake rates this means that spectroscopic interferences are minimal.Also, gases are more easily transported than liquids, and hence the transportefficiency of the vapors to the atom cell is closer to 100 than 10–15% obtainedfor liquids. This will obviously lead to a sensitivity improvement by a factor of30–50.

Frequently, the atom cell is a quartz T-piece placed on top of the burnerhead that is heated by the flame and the light beam from the HCL passes

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through it. As the gaseous analytes enter the heated T-piece, the compoundsdissociate, forming analyte atoms. The advantage of using the T-piece ratherthan allowing the analyte vapors to simply enter the flame is that it acts as asort of trap, increasing the analyte residence time in the optical path. Further,the T-piece may also provide a longer absorption path length (15 cm).

There are disadvantages associated with vapor generation techniques.Only relatively few analytes form gaseous compounds at room temperature andnot all of the oxidation states react with the same efficiency. Arsenic in its þ3state forms a hydride far more efficiently and with a different sensitivity thandoes As(V). Similarly Se(IV) forms a hydride with relative ease whereas Se(VI)does not form a hydride at all. In addition, when the analyte is an integralpart of an organic molecule, e.g., selenium in the form of selenomethionine orarsenic as arsenobetaine (AsB), a hydride is not formed. As such, anunderestimate of the total concentration of the element of interest will beobtained unless steps are taken to transform all species of the analyte into astate that will form a hydride. There have been numerous chemical andphysical methods used to accomplish this, including the use of L-cysteine toreduce As(V) and monomethylarsonic acid (MMAA) and dimethylarsinic acid(DMAA) to As(III) [8], alkaline persulfate to oxidize arsenobetaine [9], iodide/iodate reactions [10], etc. Included in the physical methods used are photolysis[9] and the use of microwave energy to accelerate the action of acids [11].

Another potential problem with the technique is the presence in thesamples of transition metals such as zinc, copper and iron and of precious groupmetals such as gold, palladium and platinum. These elements interfere withthe hydride formation process and often result in an underestimate of theanalyte’s concentration. These potential interferences may be overcome bythe addition of a chelating agent, such as 1,10-phenanthroline [12], 8hydroxyquinoline [13] or picolinic acid [14].

Limits of detection for the vapor generating analytes can be improved by afactor of over 100 compared with their conventional nebulization, with LODsfor many of the analytes being at the low ng ml21 level. Precision should againbe at the 0.5–3% RSD level. Sample consumption will depend on the mode ofvapor generation. In the continuous mode, a typical analysis is likely to use 10–12 ml of sample for a measurement time of approximately 30 s. This type ofoperation is therefore slightly more wasteful of sample than conventionalnebulization, but sensitivity is improved still further. The other mode ofhydride generation is the “batch” mode. Here, a discrete volume of sample isused and the signal will appear as a transient, i.e., a peak. This method uses farless sample, although several injections will have to be performed so that anestimate of precision can be made. The other drawback is that it may benecessary to have a chart recorder or integrator output to the spectrometer sothat measurement of the peak height or area may be performed moreaccurately. Sample throughput for hydride generation introduction to flame

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spectrometry is less than for conventional nebulization, but it should bepossible to analyze 20–30 replicates in an hour.

5.3.6.2 Sampling cups and flow injectionIf a very limited sample volume is available, e.g., ,2 ml, it will probably not bepossible to determine any more than one analyte if conventional nebulization isused. Also, if a sample contains a very high concentration of dissolved solids,there is a chance of both nebulizer and/or burner head blockage. Sampling cupsand flow injection (FI) methods are both means of introducing discrete volumesof sample, thereby decreasing the volume of sample introduced and hence theamount of dissolved solid entering the instrumentation. As with the batchmode of vapor generation, a transient signal is obtained. For maximum signalto be obtained, an injection volume of approximately 0.5 ml is required, butsmaller volumes may be introduced with a concomitant drop in response. Thisoccurs because approximately 0.5 ml is the minimum volume required to obtaina signal equivalent to that generated with conventional nebulization. Theoverall result is that 0.5 ml injection volumes are likely to lead to improvedprecision when compared with smaller volumes.

The sampling cup is a device that has a small hole in the bottom of a cup ofvolume of approximately 1 ml. The nebulizer uptake tube is inserted into thehole, so when sample is dispensed into the cup via a high accuracy and highprecision micropipette, it is immediately aspirated into the flame. In betweensample replicates, several volumes of water or dilute acid may be injected toensure that no carry over effect occurs. The use of a micropipette to introducethe sample is a potential source of imprecision, since a worn seal will preventreproducible volumes from being taken up and dispensed. The method ofsample introduction using the sample cup is also called pulse or gulpnebulization.

Numerous FI methods have been reported and an overview of the relevantliterature from 1972 to 1995 has been presented by Fang et al. [15], with acurrent treatment available in this volume. The simplest of FI methodsrequires just a sample injection valve to be coupled to the nebulizer uptaketube, permitting discrete volumes of 0.01 ml upwards to be introduced via asample loop, although sample introduction via direct injection with a syringe isalso possible, but this leads to poorer precision. Flow injection frequentlymakes use of mini- or micro-columns of an ion exchange or chelating resin toretain the analytes of interest and eliminate or minimize concomitant elementinterference effects. Pre-concentration may also be readily achieved using FItechniques and will improve the LOD for flame detection (and any othertechnique) considerably. The pre-concentration factor achievable by FItechniques will depend on the analyte, the sample volume available, timeconstraints and, in some cases, by the purity of the chemicals used for buffers,etc. Time constraints must also be considered. If the sample is pumped throughthe column at 3–4 ml min21, it will still take 25–30 min to introduce 100 ml.

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The volume and concentration of eluent required will have to be optimized, buttypically 0.25–1.0 ml is used. Therefore, large pre-concentration factors arepossible theoretically, but it will be at the expense of time. Inevitably, a busylaboratory with 100 samples to analyze cannot afford to take 30 min per samplereplicate; especially when typically three replicates per sample are required sothat an estimation of precision can be made. In practice, pre-concentrationfactors of 10–40 are more common.

The precision of FI techniques that use a column of resin to afford matrixremoval/pre-concentration will depend on the reproducibility with which theanalyte is retained and then eluted from the column. Successful methods havea precision of typically ,5% RSD. Flow injection techniques that simply use avalve to introduce small volumes of sample into the spectrometer should have aprecision of 1–2% but, again, this will depend on the concentration of theanalyte within the sample. Sample throughput for sampling cups wouldtypically be 60 samples (assuming three or four replicates) per hour. Flowinjection methods tend to be slower, but simple FI methods may analyze 20–30samples per hour. Methods involving matrix separation/pre-concentration arethe slowest and will depend on the pre-concentration factor, but 5–10 samplesper hour is typical.

5.3.6.3 Slotted tube atom trap (STAT)The STAT acts in a very similar way to the quartz T-piece used for vaporgeneration. The tube is placed on the burner head, ensuring that a slot carvedinto the side of it is directly over the flame slot in the burner head; a smaller sloton the top and/or ends of the tube allow exit of flame gases from the tube. Theflame ensures the tube then acts as a heated atom trap. The analyte moleculesintroduced via conventional nebulization, or often by FI, then enter the tubethrough the slot, become thermally dissociated into atoms and atomicabsorption occurs. An increase in sensitivity by a factor of three- to fivefold isobtained. Precision has also been found to improve through the use of a STAT. Areview of the atom trapping procedures in flame spectrometry has beenpresented by Matusiewicz [16]. Closely related to the STAT is the application ofa water cooled atom trap (WCAT) that consists of a water cooled single or dualsilica tube suspended in the flame which serves as a condensation site foratoms/molecules introduced into the flame. Following a suitable collectionperiod, the water cooling is terminated by use of a pulse of gas through thetubing, which then rapidly heats to flame temperature and results involatilization of the collected analyte. Typical 2-minute collection periods canimprove the detection limit by an order of magnitude, but the technique isclearly most favorable for volatile elements such as Ag, Cd, Cu, Zn, Pb and Tl.Recently, the STAT and the WCAT have been combined in a synergisticarrangement and used for sample analysis [17]. In general, the samplethroughput and sample consumption will be governed by whatever sampleintroduction method is used.

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5.3.6.4 ChromatographySimply determining the “total” concentration of an analyte does not give anyinformation on the overall toxicity of the sample. Therefore, speciationanalysis, the determination of different forms of the analyte, is becomingincreasingly frequent. One of the most common methods of achievingspeciation analysis is to couple a separation technique, e.g., high performanceliquid chromatography (HPLC) or gas chromatography (GC), with an elementspecific detector. Flame spectrometry is one of the least sensitive methods ofatomic spectrometry and therefore the number of speciation analyzes that maybe performed with it is somewhat limited. However, despite this obviousdrawback, a large number of applications have been presented. Several reviewsof chromatography coupled with flame spectrometry have been published.These include those by Ebdon et al., who covered the earlier literature for liquid[18], and GC [19], and a more recent one by Szpunar Lobinska et al. [20].Several speciation approaches are discussed in detail in other chapters of thisbook.

Gas chromatography depends on the analyte being volatile. If the analytesare not naturally volatile, it may be necessary to resort to use of derivatizationreactions, such as use of a Grignard reagent [21]. For HPLC couplings, the endof the column may simply be attached to the nebulizer uptake tube. The flowrate through the chromatography column is typically 1–2 ml min21, which isless than the natural uptake rate for the nebulizer. It may therefore benecessary to insert a small air bleed to compensate for this mismatch [22]. ForGC couplings, it is usually necessary to utilize a heated transfer line from theGC oven to the atom cell. The end of the heated transfer line is usually placed inone of the slots of a STAT so that extra sensitivity is obtained.

Sample throughput will depend largely on the chromatographic stage. Bothliquid and gas chromatograms frequently take in excess of 10 min per sample,and so sample throughput is very limited. Instrumental precision will dependon the method of sample introduction. If a sample loop is used for HPLC, thenprecision should be less than 5% RSD. If a syringe is used to inject 1–10 ml intoa gas chromatograph, then precision can be .10%. This may be improvedsubstantially if an internal standard is used. For speciation techniques, this isnormally a compound that has similar properties to the analyte compounds,but is not found naturally in the sample. Ideally, the internal standard shouldelute in the middle of the chromatogram, and not co-elute with any of thespecies of interest. Another potential source of error, inaccuracy andimprecision, is the extraction technique used to remove the analyte species inan unchanged state from solid samples. This topic will be discussed in laterchapters, but it is worth noting here that an inadequate extraction method (onethat changes the speciation or that does not yield reproducible recovery, etc.)will render the entire analysis irrelevant.

One point that is worth noting for all speciation analyzes is that theconcentrations quoted should specify whether the values are related to the

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concentration of the species or of the analyte element. For instance, if duringthe analysis of fish extracts a concentration of 5 mg kg21 is quoted, the reportshould specify whether this is 5 mg of arsenic kg21 or 5 mg of arsenobetainekg21, etc. Failure to do this is likely to lead to confusion and error. Similarly,LODs should also be quoted with the same qualifications.

5.3.6.5 Multiple couplingsOccasionally, when extra sensitivity is required for speciation analysis andwhen instrumental costs preclude the purchase of a more sensitive detector, itis necessary to couple together several techniques. A technique that has becomerelatively common (in the research literature) is the coupling of HPLC withHG-AAS [23,24]. After the species have been separated using HPLC, either achemical or physical process is used on-line to convert each of the species to astate that will form a hydride and may require a chemical oxidation usingalkaline persulfate or photolysis. In the example given in Ref. [23], on-linemicrowave assisted oxidation yielded LODs of 2.5, 5.3, 3.3 and 5.9 ng ml21 AsB,DMAA, MMAA and As(V), respectively.

5.3.6.6 SlurriesAs discussed earlier, a slurry is a suspension of solid sample in a liquid medium.The advantages of slurry sample introduction include the ease of preparation,the non-requirement of powerful reagents such as hydrofluoric acid, nopossibility of losing volatile elements and, for most analytes, minimalcontamination. Fuller et al. [25] reported very early on the relative merits offlame, electrothermal and ICP atomization techniques for the direct analysis ofslurries. Several slurry preparation techniques have been reported, but thefundamental necessity is that it be representative of the sample, i.e., behomogeneous. This usually means that the powdered sample must be groundusing either the bottle and bead method or in a micronizer. In the bottle andbead method, a sub-sample is placed in a small plastic bottle, a small volume(e.g., 5 ml) of aqueous dispersant are added together with 10 g of zirconia beads(2 mm diameter) and then the bottle is sealed and placed on a mechanical flaskshaker for a period of time that is dependent on the sample type. Blanks areprepared in the same way, but omitting the sample. The drawback with thetechnique is that the blanks tend to give a “worst case scenario”, because thebeads have no sample to cushion the impact of the collisions between themduring the grinding process. This means that the beads will grind themselvesto a greater extent than when the sample is present. The concentration of thecontaminants in the blank is therefore often slightly larger than that found inthe samples. The process is, however, suitable for the determination of a greatnumber of analytes, with the obvious exceptions being zirconium and hafnium(which is often a substantial contaminant in the beads). The micronizer usesagate rods to grind the sample instead of zirconia beads. This will give rise to adifferent set of contaminants (i.e., Na, Mn, Si, etc.). The choice of which

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grinding procedure to use will therefore, depend on the analytes to bedetermined and on the nature of the sample. The zirconia beads are fairly hard[measure of hardness (MOH) ¼ 8 þ ], whereas the agate rods are softerðMOH ¼ 7Þ: If a particularly hard sample is to be slurried, then the zirconiabeads are a more suitable grinding medium because otherwise the agate rodsmay end up actually being ground by the sample. For exceptionally hardsamples, it may be necessary to use a tungsten carbide swing mill to affectgrinding. After grinding is complete, the beads (or agate rods) may be removedby simple filtration through a coarse Buchner funnel, without the presence of afilter paper. The beads may then be washed with more dispersant and thewashings collected and combined with the sample. Apart from the problemsassociated with insufficient grinding and contamination, another problem isthat some samples are very soft and have a tendency to become squashed orflattened during the grinding process rather than being smashed into smallerfragments. The overall effect, therefore, is that particle size is not reducedsufficiently. This problem is more common with organic based samples such asplants, etc.

The dispersant used will depend on the nature of the sample. For inorganicmatrices such as soils, rocks, ceramics and other refractory materials, sodiumhexametaphosphate or sodium pyrophosphate is suitable. For more organicbased samples such as plant material, blood, food samples, etc., then TritonX-100 or aerosol OT are more appropriate. In either case, it is necessary toinspect the ground sample under a microscope to ensure that the particles aresufficiently dispersed, i.e., they have not flocculated together. If sampleparticles do flocculate together to form an agglomeration, then they will actas a much larger particle and the slurry will no longer be homogeneous.

Slurries may be aspirated into either flame [26] or plasma-basedinstruments, introduced to ET-AAS instruments or they may even be analyzedusing a hydride generation technique. If the slurry is to be aspirated into aflame or a plasma via a conventional nebulizer/spray chamber assembly, it isnecessary to ensure that the particle size is extremely small and that, ideally,the particle size distribution covers only a small range. If this is the case, thenthe sample particles will act in a similar manner to aerosol droplets, enablingcalibration against standards prepared in the aqueous dispersant. If the slurryparticles are too large, the nebulizer and spray chamber select against themand they will preferentially be passed to waste so that the sample that reachesthe atom cell is not representative of the whole, leading to inaccuracy and poorprecision. In addition, the larger the particle size, the more difficult it will be toensure homogeneity, i.e., even if the slurry is stirred, the larger droplets willsink to the bottom of the container at a faster rate than the smaller ones. Thefundamental parameters required for slurry nebulization into plasmas werediscussed in a paper by Goodall et al. [27]. These authors determined that anupper particle size diameter of 2.0–2.5 mm was necessary for accurate resultsto be obtained, but the maximum particle size that yielded accurate results was

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also dependent upon the sample density. For a sample having a density of1 g cm23, a particle size of 2.9 mm could still be transported efficiently to theplasma; however, for a sample with a density of 7 g cm23, the size had todecrease to 1.5 mm.

For very refractory samples, even if the particle size of the slurry issufficiently small to pass through the nebulizer/spray chamber assembly andreach the plasma, complete dissociation may not occur and an inaccurateconcentration value will be determined. To overcome such problems, alterna-tive gases have been used. Ebdon and Goodall [28] introduced hydrogen to thenebulizer gas flow to yield more accurate results when slurries of refractorycertified reference materials (CRMs) were analyzed. This was attributed tothe increased thermal conductivity of the hydrogen improving the energytransfer from the toroidal part of the plasma to the annulus, thereby increasingthe rotational temperature and, hence, improved dissociation of the par-ticles. A review of slurry nebulization into plasmas has been prepared byEbdon et al. [29].

Even when the slurry is to be analyzed by ET-AAS, sample homogeneitymust be maintained. Since the particulate material of slurries will settle withtime, it is necessary to agitate the slurries vigorously to ensure completehomogeneity before the sample is introduced. Failure to do this will lead toexceptionally poor precision and accuracy. Therefore, hand-held pipettes areoften used so that sample introduction takes place immediately afterhomogenization. The introduction of slurries using an autosampler is apossibility provided that there is a mechanism by which homogeneity isensured. Miller-Ihli developed an ultrasonic probe that is inserted into theautosampler cups to mix the slurry and hence ensure homogeneity [30]. Usingsuch a device, the analysis of slurries can become completely automated.A review of the slurry sampling for ET-AAS applications between 1990 and2000 has been presented by Cal-Prieto et al. [31]. For many slurry types andsome biological liquids such as blood, it may be necessary to introduce air oroxygen during the pyrolysis stage to ensure a more efficient oxidativecombustion process that decomposes the organic material more efficientlyand hence helps reduce interferences arising from smoke. In addition, for theblood samples, it will also prevent the build-up of a carbonaceous residue thatwill, in time, start to obscure the light beam. If a reactive gas is introducedduring the pyrolysis stage, it is normally necessary to use a second pyrolysisstage with just an inert gas passing through the tube to remove all traces of theair before atomization. Failure to do this will lead to accelerated tube wear.Precision for slurry analysis by ET-AAS is dependent upon the homogeneity ofthe slurry, but could be as low as 3–5% RSD.

Occasionally, the slurry will be mixed with nitric acid or some otherreasonably strong reagent to help leach the analytes from the solid matrix intothe liquid phase. This will often have the effect of increasing accuracy of theanalysis, because some of the analyte is in solution and will therefore act in

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a similar manner to the standard. Additionally, the particle size is likely to bedecreased, enabling more efficient transport of these to the atom cell. Anexample of a procedure that has used acid leaching as an aid to slurrynebulization into inductively coupled plasma-mass spectrometry (ICP-MS) hasbeen published by Persaud et al. [32].

If the sample is biological in origin, the analytes may well be at a lowconcentration. It is sometimes possible to place the sample in a muffle furnaceand then char it at 4508C for several hours until only ash remains. This ashmay then be slurried in the normal way. Such a pre-treatment will enable aneffective pre-concentration to be achieved since, on ashing, many biologicalsamples will lose 90% of their mass and hence a larger quantity of sample maybe introduced before problems associated with excessive amounts of dis-solved/suspended solids occur. The usefulness of the dry-ashing pre-concent-ration technique is, however, analyte selective and will be inappropriate forvery volatile analytes such as mercury, cadmium and possibly lead and zinc.

5.4 ELECTROTHERMAL AAS

5.4.1 Introduction

Electrothermal AAS shares the same fundamental principles as flame AAS, themajor differences being the atom cell and the method of sample introduction. InET-AAS, the liquid sample is dispensed into a graphite tube, which is heatedresistively, undergoing a temperature programme that first dries the sample,pyrolyzes it so that as many matrix concomitants (i.e., potential interferences)are removed as possible, and then heats it to a temperature that is sufficientlyhigh to vaporize and atomize the analyte so that it can absorb the HCL lightbeam. There is then usually a cleaning stage to prevent analyte carry overbetween samples. The temperature of each of these stages is dependent uponthe analyte of interest. The drying temperature should be sufficient to ensuresmooth evaporation of the solvent. If it is too high, the sample may froth andspit out of the tube, decreasing precision. The pyrolysis temperature should behigh enough to remove as many interferences as possible, but not too high sothat the analyte is lost through volatilization. This temperature can rangebetween ,2508C for mercury through to 17008C for very refractory analytessuch as erbium. The atomization temperature should be sufficient to ensurecomplete atomization of the analyte whilst not being excessively high so as tocause accelerated tube wear. A temperature between 1200 and 28008C may beused, depending on the analyte and on the capability of the instrument.

The tube is protected from atmospheric oxidation by purging the entiresystem with argon, although nitrogen may be used for some analytes, exceptingthose that form a refractory nitride. The gas usually surrounds the graphitetube and flows at a rate of 1–3 l min21. Many modern instruments alsohave a flow of argon (200–300 ml min21) internally through the tube to aid

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the flushing of smoke and solvent vapor from it. This internal flow is normallyswitched off during atomization to prevent dilution of the analyte atoms andfrom flushing the atoms out of the light beam too rapidly. The speed of analysisis much poorer compared with flame AAS, as a typical ET-AAS temperatureprogramme can exceed 2 min and frequently three replicates will be analyzedper sample. It is therefore unlikely that many more than 10 samples can beanalyzed per hour.

The initial cost of the instrumentation is substantially higher than simpleflame spectrometers, with the least expensive electrothermal (also calledgraphite furnace) instruments being double the price. More complex instru-mentation will cost much more. The running costs also tend to be higher, withthe graphite tubes costing up to US $70 each, as well as the supply of argon gas.The lifetime of the tube depends largely on how corrosive the sample is andwhat analytes are of interest (i.e., how high the atomization temperature needsto be).

The advantages of using ET-AAS rather than many other detectiontechniques include the requirement of only a very small volume of sample.Typically, other instrumentation requires at least 0.5–1 ml of sample unlessdilution is performed (which may put the analyte below the LOD of thetechnique) or unless specialized sample introduction methods are used, e.g.,pulse nebulization, etc. The typical injection volume for ET-AAS is 10–30 ml, soeven if triplicate measurements are made, less than 100 ml would be sufficient.In addition, the sensitivity is 100–1000 times superior to flame AAS and formany elements it is also superior to inductively coupled plasma-atomic (optical)emission spectrometry (ICP-OES).

5.4.2 Conventional ET-AAS

There are several types of tube available commercially but most aremanufactured from some type of graphite (although there are a few applicationswhere metal atomizers have been used). Many of the applications of this lattertype of atomizer have been reviewed by Nobrega et al. [33]. Of the graphite-basedtubes, electrolytic (electro)graphite is the least expensive of the materials, but isvery porous and samples can soak into the graphite lattice leading to interactionsbetween the graphite and the analytes. For analytes such as chromium andother refractory carbide forming elements, this can be problematic. Pyrolyticgraphite is much less porous (more dense) and is far less reactive thanelectrographite. Therefore, there is less interaction between the tube and theanalytes and the lifetime is extended. The tube can either be coated withpyrolytic graphite or some may even be manufactured totally from it.

There are also several different styles of tube available commercially. Someare heated longitudinally from the ends, leading to a temperature gradientalong the tube with the middle being hottest. This is not a favorable scenario,since the analyte may be atomized from the hot central region of the tube and

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then condense at the cooler ends. Other tubes are heated transversely from thesides and do not exhibit a temperature gradient. Slightly lower atomizationtemperatures may be used without fear of condensation problems. Some tubeshave an in-built platform onto which the sample is introduced. The platformensures that the sample is heated both by the hot internal gas (convectively)and radiatively rather than by conduction from the tube walls, facilitating aprocess known as isothermal (or stabilized temperature) operation. This leadsto fewer interference problems (these will be discussed in a later section).

Sample is usually dispensed as a liquid into the graphite tube or ontothe platform. An autosampler can dispense the sample to the same part ofthe graphite tube in a more reproducible way than a hand-held pipette. If thesample is placed in the same place more reproducibly, better precision shouldbe obtained, especially for longitudinally heated tubes. Ideally, the sampleshould be dispensed from the same height each time, so that the sample drop isnot disturbed by the autosampler introduction arm. If it were disturbed, it mayspread over a larger area of the tube, again leading to impaired precision. Ingeneral, precision obtained with ET-AAS determinations are 1–3% RSD if anautosampler is used and 3–5% RSD if the sample is dispensed using a hand-held micropipette. Studies on the behaviour of various arsenic species inETAAS have ben reported [34].

Interferences are far more problematic for ET-AAS than for flamespectroscopy. Although true spectral interferences are equally as rare, chemicalinterferences and non-specific absorption (smoke) problems are exacerbated.The presence of some chemical species, e.g., chlorides, often increases thevolatility of the analyte and may lead to loss at a lower pyrolysis temperaturethan occurs for aqueous standards. If a temperature optimization experiment isperformed on standards and these “optimum” temperatures are used duringthe analysis, significant losses of the analyte may occur from the samples,leading to erroneous data. Other interferences have already been discussed,such as the formation of refractory carbides and nitrides (if nitrogen is used asthe inert gas). Careful optimization of the temperature programme canovercome some of these problems, especially if matrix modifiers (also knownas chemical modifiers) are used. Chemical modifiers are reagents that areintroduced with the sample that interact either with the analyte, stabilizing itthermally, or with the matrix, to make it relatively more volatile. This meansthat a higher pyrolysis temperature may be used before analyte loss occursthrough volatilization. If a higher pyrolysis temperature can be used, thenmore potential interferences may be removed. An assortment of chemicalmodifiers has been used; including phosphate based ones, magnesium nitrate(this helps occlude analytes within its matrix, hence preventing their loss) anda mix of magnesium nitrate and palladium nitrate. In general, their use canextend the temperature range used during the pyrolysis stage by 300–6008C.A review of many of the interferences encountered in ET-AAS was published bySlavin and Manning [35]. Slavin developed the stabilized temperature platform

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furnace (STPF) concept. This was a protocol based on recommended precau-tions to take to minimize the effects of interferences [36] and included the use ofan appropriate matrix modifier, integration of signals (area rather than peakheight measurements), isothermal operation, rapid heating during atomizationand the use of one of the more powerful background correction systems, such asthe Zeeman effect. Virtually all modern analyzes made by ET-AAS use most orall of the recommendations of the STPF concept. Modern autosamplers achievefar more than simply delivering the sample to the atom cell. They may be usedto perform on-line dilution, they mix the sample with appropriate matrixmodifiers and they may be programmed to run quality control standards,re-calibrate if necessary, etc.

As noted previously, isothermal operation using a platform usually helpsdecrease the extent of interferences because the analyte is vaporized from theplatform when the temperature of the tube wall is otherwise higher. There is,therefore, less chance that the analyte will re-condense on a cooler part ofthe tube or recombine with cooler gas phase species, hence becoming unavail-able for atomic absorption. An alternative to the use of a platform is probeatomization. This requires a specialized tube style that has a small slotmachined into its side. It is through this slot that a mechanically operatedgraphite probe is inserted. The sample is introduced onto the probe, which isthen inserted into the heated furnace through the slot wherein the sample isthen dried and pyrolyzed in the normal way. The probe is then withdrawn fromthe tube, which is then heated to the atomization temperature, and the probere-inserted. Again, the analytes will be radiatively and conductively heatedwithin the tube and will therefore be less vulnerable to interferences. Asdiscussed previously, the majority of samples introduced to ET-AAS are in theform of a liquid. However, the introduction of slurries is also possible. Therelative advantages and pitfalls of slurry atomization have been discussedearlier (Section 5.3.6.6).

Solids may be analyzed directly using ET-AAS if specialized tubes are used.The solid material (a few milligram) is usually weighed into a sample boat andthen this is placed through a slot into a specialized tube. The furnaceprogramme may then be run, although a drying stage may not be necessary.The technique is not common because precision can be poor. The technique isdependent upon the sample being completely homogeneous and, if only a fewmilligrams of sample are weighed into the boat, then homogeneity issues are ofparamount importance and these may directly influence precision. There isalso the possibility of some of the sample blowing from the boat prior toinsertion into the atom cell. Using this technique, a precision of 10% RSD wouldnormally be regarded as being good.

The limits of detection (quoted as a concentration) obtainable using ET-AASare relatively meaningless unless the injection volume is stated. Typically, 10–30 ml is injected, but inevitably, 30 ml will yield a better LOD than a smallervolume. Instead, the LOD quoted as an absolute mass is usually given. As an

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alternative, the characteristic concentration (or mass), i.e., the concentration (ormass) that gives rise to 0.0044 absorbance (absorbance-s for integrated measure-ments) is also often quoted. In general, it is possible to say that for most elements,the concentration LOD for ET-AAS is often approximately 2–3 orders ofmagnitude lower than flame AAS. The exceptions are for elements that formrefractory carbides. A list of approximate characteristic masses obtainable byET-AAS is given in Table 5.2. As well as many of the standard pre-concentrationtechniques discussed in Section 5.3.5 and in other chapters of the book, pre-concentration is also possible using ET-AAS. If a sample aliquot of 20 ml isintroduced and dried in the normal way, a second aliquot may then be introducedon top of it. If this sample introduction and drying cycle continues for four or fivealiquots and then the normal pyrolysis and atomize stages are performed, aneffective fivefold pre-concentration may be achieved. It should be noted, ofcourse, that there is also a fivefold increase in the amount of matrix present, sounless it is a very simple matrix, such as fresh water, or unless the matrix iseasily removed during the pyrolysis stage, then severe interferences may result.Although this is a very time-consuming process, the presence of an autosamplercan achieve this unattended and the analyst is free to perform other tasks.

It has been emphasized previously that a typical ET-AAS cycle can take inexcess of 2 min to achieve. Occasionally, this may be decreased if the method of“hot injection” is used. This is achieved when the sample is introduced at a slowrate into a furnace that has been pre-heated to 120–1308C, such that thesolvent evaporates as soon as it is introduced. This can reduce the overall timeof analysis dramatically. Occasionally, higher drying temperatures may beused, e.g., 4008C, although this is rare. Methods for minimizing the timerequired for ET-AAS determinations have been summarized by Halls [37].Using these accelerated programmes, the overall analysis time per replicatemay occasionally be decreased to 20–30 s.

5.4.3 Multi-element ET-AAS

The majority of ET-AAS instruments is capable of detecting only one analyte atany given time. However, instrumentation is now available that uses an echellespectrometer and a solid-state detector capable of multi-element determi-nations that offers considerable savings in terms of time, costs and sample andreagent consumption. The one drawback is that the analytes to be determinedmust have similar physico-chemical properties. This is a result of thetemperature programme used being a compromise, rather than an optimum,for any individual analyte. This means that analytes that are quite volatile andthat require the same chemical modifier, such as As, Se, Te and Ge, may bedetermined together. It would, however, be inappropriate to combine one ormore of these in the same determination as a much more refractory analytesuch as chromium, that requires a different chemical modifier and a verydifferent temperature programme for optimum sensitivity/reduction of

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interferences. At present, usually only three or four elements are determinedsimultaneously, although there are facilities for up to six.

5.4.4 Chemical vapor generation–ET-AAS

Chemical vapor generation has been coupled with ET-AAS. Generation phaseinterferences using this technique are basically the same as for any vapor

TABLE 5.2

Characteristic mass for ET-AAS under optimum conditions using the most sensitive line

Analyte Characteristic mass (pg) Analyte Characteristic mass (pg)

Ag 0.7 Mo 7

Al 5 Na 0.1

As 10 Nb

Ba 17 Nd

Be 0.5 Ni 5

Bi 9 P 2200

Ca 0.6 Pb 6

Cd 0.2 Pd 9

Co 4.2 Pr

Cr 1.5 Pt 70

Cs 11 Rb 1

Cu 6 Ru 15

Dy 45 Sb 10

Er 100 Sc

Eu 25 Se 14

Fe 2 Si 15

Ga 4.5 Sn 10

Gd Sr 2

Ge 9 Ta

Hf Tb 4

Hg 150 Te 9

Ho Th

In 7 Ti 50

Ir 135 Tl 15

K 0.4 Tm

La U

Li 4 V 22

Lu W

Mg 0.2 Y

Mn 0.6 Zn 0.15

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generation determination. If the tube of the ET-AAS system is treated with asemi-permanent modifier such as iridium, zirconium or tungsten, and heated to300–8008C, hydrides of selenium, arsenic and several other analytes may becollected quantitatively [38]. The technique is known as “in-atomizer trapping”or “in situ trapping”. The subject of in situ trapping has been reviewed byMatusiewicz and Sturgeon [39]. Since typically 5–12 ml of sample is consumedfor continuous vapor generation determinations, effectively the analyte from5 ml of sample rather than 20 ml is deposited in the tube and, hence, thesensitivity is greatly improved. For batch HG-ET-AAS determinations, asample loop of 500 ml has been used which yielded LODs of 0.82, 0.04, 0.26 and0.29 mg l21 for As, Bi, Sb and Se, respectively [38], representing an improve-ment of over 10-fold compared with conventional ET-AAS on the sameinstrument. Precision at the 5 mg l21 level was typically less than 3.5% RSD.The use of this method will limit the linear dynamic range accordingly. As wellas improving the limits of detection, this technique also separates the analytesfrom potential matrix interferences. This means that lengthy drying andpyrolysis stages are not required. As a result, the time required for HG-ET-AASis not dissimilar to conventional ET-AAS.

5.4.5 Speciation

On-line speciation analysis using liquid chromatography and ET-AAS as adetection system is relatively rare, because the atom cell is often required to beheated continuously at the atomization temperature. Since chromatogramsmay take several minutes to be complete, this leads to very rapid tube wear andgreat expense. Examples do exist, however, where very rapid temperatureprogrammes are used that have achieved on-line speciation [40]. The majorityof speciation analyzes undertaken using ET-AAS as a detector have thereforebeen off-line, wherein fractions (typically 0.5 ml aliquots) of the eluant arecollected at the end of the chromatographic column, which are then subjected tonormal ET-AAS temperature programmes so that any analytes present may bedetermined. The concentration of the analyte in each fraction is then plotted sothat a composite chromatogram is obtained. Since the separation and detectionstages are not coupled directly, there is a greater chance of contamination andmislabeling of a particular fraction. This could potentially lead to great error ifa fraction containing an analyte species is present, because the transient signalobtained from this species would be “moved” to a different retention time. Inaddition, closely eluting species may not be fully resolved, and hence wouldappear as only one peak. There are also major difficulties in optimizing thechromatography. Despite these drawbacks, numerous examples have appearedin the literature [41].

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5.5 INDUCTIVELY COUPLED PLASMA-ATOMIC EMISSIONSPECTROMETRY

5.5.1 Introduction

Inductively coupled plasma-atomic (optical) emission spectrometry is often themethod of choice of most laboratories when several analytes need to bedetermined in a batch of samples. This is because the technique can detectanalytes in either a rapid sequential manner or, for some instrumentation,detection of several analytes can be simultaneous. The cost of instrumentationvaries, but is typically in the range US $50,000–80,000.

5.5.2 Theory and interferences

5.5.2.1 TheoryThe basic theory of emission from an ICP is identical to that for flame emissionexcept that the ICP is an atom cell consisting of a very high temperature(6000–10,000 K) ionized gas. The theory behind the formation of the ICP isdiscussed in detail elsewhere [42,43]. Since the plasma is at such a hightemperature, any sample entering it will be desolvated; molecules will bedissociated forming atoms and, depending on the individual analyte’sionization energy, these will become (partially) ionized. The atoms and/orions then become thermally excited and emit light, the wavelengths of whichmay be separated from other wavelengths by an appropriate line isolationdevice and then detected.

5.5.2.2 InterferencesSince the ICP is such a good excitation source, there are many species, bothnaturally present in the plasma and those that are introduced to it with thesample that emit light. The resulting emission spectrum can be far morecomplex than that produced using flame techniques and the chances of lineco-incidence are much greater. The line isolation devices used in ICP-OES,therefore, tend to be more highly resolving than those required for the flametechniques. Despite the improved resolution, interferences are still common.The choice of which analytical line to use is therefore governed by both thepotential interferences and by the sensitivity required. A more comprehensivediscussion of interference effects in ICP-OES has been given in Ref. [42], andwill therefore, only be dealt with briefly here. Overlap from other spectral lines(atomic, ionic and molecular) is common. The high temperature of the plasmacauses species that are normally not problematic in flame spectroscopy to emitlight. The high temperature exacerbates the problem because it causes linebroadening, and broader lines are more likely to lead to spectral overlap thannarrow ones. The argon that forms the plasma emits at approximately 200different wavelengths and these emissions, together with the emission from

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assorted molecular species derived from water, entrained gases and the samplematrix, e.g., OH, N2

þ, NH and NO, which produce a series of molecular bandsthat are spread throughout the wavelength range, can clutter the emissionspectrum considerably. Line overlap by concomitant metallic species may alsoexist. This problem is especially severe when line rich elements such as iron,the lanthanides or uranium are present at an appreciable concentration in thesample. A background emission continuum is also present, the intensity andcharacteristics of which will vary, depending on the solvent loading, the solventtype and the matrix elements. Stray light, i.e., light that unintentionallyreaches the detector, may also be a problem. This often arises fromimperfections in the dispersing device, but many modern instruments sufferfrom this problem far less because of the quality of the optics. It should still benoted, however, that an analyte line that has a low intensity may be interferedwith by a nearby very strongly emitting species.

Most modern instruments have a software library giving a list of therelevant lines of the analytes, together with the potential interferences thatmay be experienced at each line. Inspection of the line tables and priorknowledge of the sample chemistry usually enables an analyst to pick asuitable “interference free” wavelength. Many instruments also enable back-ground correction methods to be used in an attempt to compensate for anyinterference effects. There are a number of correction methods that may beused and these are discussed elsewhere [42].


5.5.3.1 RF generatorsThe radio frequency (RF) generator may be of several types, e.g., crystalcontrolled or free running, 27.12 or 40.68 MHz. A far more detailed discussionon RF generators has been given elsewhere [42]. Both 27.12 and 40.68 MHzgenerators are used commercially and both normally produce RF power at up to2000 W, although for normal usage, a power of between 1000 and 1500 W istypical. In general, the 40 MHz generators are regarded as being more stable,to couple more efficiently and to produce a lower background signal. Therefore,slightly improved LODs may be achieved for these instruments compared withthose obtained from an instrument equipped with a 27 MHz generator.

5.5.3.2 TorchesThe plasma is formed in a torch, which is a concentric arrangement of quartztubes that permits delivery of independently adjustable flow rates of argon toone end which is located in the RF load coil. The plasma is formed from argongas flowing at a rate typically between 11 and 15 l min21. This flow is called thecoolant or plasma gas flow. The auxiliary (intermediate) gas flow (typically1 l min21) prevents the plasma from sitting too low in the torch and melting theinnermost tube (the injector). The nebulizer (carrier) gas flow passes through

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the injector and punches a hole through the fireball, forming an annular,doughnut-shaped plasma. Technically, this is termed the annulus and thesurrounding fireball the torus. Several types of torch exist, but most nowconform to the basic Fassel style, which consumes substantially less gas (14–17 l min21) when compared with the much larger Greenfield style torch(typically 12–38 l min21 argon and 20–70 l min21 nitrogen). Similarly, thepowers required to operate the Fassel torch are 1.0–1.5 kW, compared withseveral kW for the Greenfield torch. The advantage of the Fassel style torch isthat it is less expensive to operate, but its drawback is that it is less robust thanthe Greenfield torch and is less tolerant of gases other than argon.

Many torches are demountable or partially demountable. This usuallymeans that the coolant and auxiliary tubes are fixed, but that a differentinjector may be introduced. The shape and bore of the injector of the torch mayhave a large effect on the stability of the signal. Wider bore injectors (e.g., 2 oreven 2.5 mm) are less likely to block than normal injectors (1.5 mm) if sampleswith a high dissolved salt content are introduced. If the injector is too wide,however, problems may be experienced in “punching” the plasma and theplasma may simply extinguish. Some injectors are made from ceramic oralumina and are therefore more resistant to hydrofluoric acid than quartz ones.Some injectors taper gently from wide to narrow bore and these are lesslikely to become blocked than injectors that have a step reduction in bore.A demountable or partially demountable torch therefore gives the analyst morefreedom of choice to use an appropriate injector type.

Several variations of the Fassel style torch exist. These include low flowtorches that are much smaller (i.d. 13 mm compared with 18 mm for aconventional torch) and operate at a lower power (,1 kW) and with a lowerconsumption of gas (8 l min21) [44]. Micro-torches that operate at even lowerpower and gas flow also exist [45]. These torches are reported to offer similarsensitivity to their larger counterparts, but are more easily blocked because oftheir smaller diameter injectors.

5.5.3.3 Radial and axial plasmasThe majority of instruments use a radial configuration wherein the plasma isviewed from the side. Axial instruments have the torch turned at a right angleso that it lies horizontally and the plasma is viewed end-on. There is a greatdeal of discussion as to which orientation offers the best performance. Someworkers state that the axial instruments offer improved limits of detection by afactor of nearly 10, because the light from a much larger area may be detected.However, others have stated that they are prone to far more interferences,because they are less “optically thin” than the radial instruments. This meansthat the emitted light has to pass through a much larger distance, wherein itmay be absorbed by other analyte atoms/ions, or that effects from molecularinterferences are greater. A recent paper has addressed this problem and statedthat self-absorption effects in axially viewed plasmas are partially controllable

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by careful optimization of the operating conditions [46]. The relative merits ofboth axial and radial instruments have been discussed by Brenner and Zander[47]. For axially viewed plasmas, a shear gas is often required to preventthermal damage to the collection optics. It has the additional effect of removingthe ICP tail flame that, under normal circumstances, would be rich ininterfering species.

5.5.3.4 Wavelength isolation and detection systemsTraditionally, ICP spectrometers utilize a monochromator and photomultipliertube arrangement similar to AAS instrumentation. The monochromators,however, tend to have a much longer focal length and are of higher resolvingpower than those used for AAS. The higher resolution is required because thehigh temperature of the plasma excites many more species than does a flameand hence the emission from ion lines and some molecular species may becomeproblematic if low-resolution monochromators were to be used. As its namesuggests, instruments that utilize a monochromator can only interrogate onewavelength at any one time, and must scan over several wavelengthssequentially if more than one analyte is to be determined. The speed withwhich it can achieve this will govern the overall time required for analysis.Also, the accuracy and repeatability with which it finds each wavelength willhave a large effect on the accuracy and precision of the analysis. Fortunately,once optimized, most modern instruments tend not to drift significantly (unlessphysical parameters, such as the room temperature, change). Polychromatorshave been developed commercially that may determine several analytessimultaneously. Here, several PMTs are arranged at intervals around a circle(known as a Rowland Circle) and as the light emitted from the plasma isdiffracted from the grating, the wavelengths are separated and each PMT maydetect one particular wavelength. Since the PMTs are not easily moved, theinstrument is usually prepared in the factory to determine only specificwavelengths that the customer requires. Therefore, although simultaneousanalyte determinations are possible using such instrumentation, it iscumbersome and extremely inflexible.

Many modern instruments use an echelle-based spectrometer andspecialized charge transfer device detectors. These may be either chargecoupled devices, segmented charge coupled devices or charge injection devices.The theory behind their operation is beyond the scope of this chapter, but maybe found in Refs. [48,49]. These devices function as an “electronic photographicplate” and are therefore truly simultaneous and may be used to determineseveral analytes together with suitable background correction points at once.There are a few drawbacks associated with their use, including the possibilityof “blooming”, which occurs when an analyte may be so concentrated that theindividual pixels detecting that wavelength may become saturated so that thecharge spills over into adjacent pixels, thereby giving erroneously high signalsfor other analytes. Modern electronics have gone a long way in overcoming this

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problem. Also, these instruments do not have the flexibility of PMT basedspectrometers, because they may have only a limited number of lines that canbe detected. However, this number reaches into the hundreds and, therefore asuitable line should be available for most analytes.

5.5.3.5 Sample introduction systemsNebulizers and spray chambersThe nebulizer/spray chamber assembly performs the same functions as those inthe flame systems, i.e., to form an aerosol and then segregate large dropletsfrom smaller ones. There are, however, a very wide variety of nebulizers andspray chambers. A description of many types has been given recently byThomas [50]. Although this paper describes sample introduction for ICP-MSanalyzes, the principles and most of the instrumentation are identical. Theprocesses occurring within them have been discussed in two papers by Sharp[51,52]. Some nebulizers, such as the Meinhard style ones, are self-aspirating,i.e., they draw liquid samples up in a similar fashion to flame AAS nebulizers,and others require the sample to be pumped to them. Some nebulizers areeasily blocked by the presence of dissolved or suspended solids, whereas othersare far more tolerant. Examples of dissolved solids tolerant nebulizers includethe crossflow, the Ebdon, the Burgener, the Hildebrand grid and assortedspecialized pneumatic ones. The Ebdon, crossflow and some of the pneumaticones are also tolerant of suspended solids such as those found in slurries. Thechoice of nebulizer will depend largely on the application. Some nebulizers aremanufactured from inert polymers and are therefore more resistant tocorrosive samples, such as those containing hydrofluoric acid. As well asacting as a droplet size filter, for those nebulizers that require sample to bepumped to them, the spray chamber acts as a pump noise dampener. A typicalICP nebulizer/spray chamber assembly will have a transport efficiency of1–2%. Much more than this is likely to lead to severe plasma perturbation andits possible extinction. The problem is exacerbated by the aspiration of organicsolvents. As discussed previously, these tend to have lower viscosity, lowersurface tension and higher volatility (higher vapor pressure), leading totransport efficiencies substantially higher than 1–2%. Many of the moremodern generators (especially the more robust 40 MHz ones) may be able tocope with the increased solvent loading, however, many of the older ones cannotand plasma extinction occurs. Boorn and Browner discussed introduction oforganic solvents to ICPs [53]. If larger volumes of solvent are likely to reach theplasma, a desolvation device should be used. These come in several differentforms, including membrane drier tubes [54], desolvation devices made in-house[55] and commercial equipment. Many of these devices decrease the amount ofsolvent reaching the plasma, whilst not significantly decreasing the analytetransport efficiency. The reduction in the solvent loading often leads to greaterstability and, hence, improved LODs.

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An ultrasonic nebulizer increases transport efficiency to approximately25–30%. These usually have built in heating and cooling devices to desolvatethe aerosol and prevent plasma perturbation. Such a device, by increasinganalyte transport and decreasing the solvent loading, improves LODs bytypically 10-fold. The disadvantages of the nebulizer include its cost(approximately US $10,000) and the need to optimize the operating conditionscarefully. Failure to optimize the heating and chilling temperatures is likely tolead to inconsistent nebulization and a noisy signal, which degrades the limitsof detection. The whole device may be used without the need of a spraychamber.

Spray chambers are available in assorted shapes and sizes. Their functionis to separate large aerosol droplets from smaller ones and to act as a noisedampener. The efficiency with which the spray chamber achieves the latterfunction is often dependent on its internal volume. Larger spray chambers,such as the Scott double pass, dampen the noise quite effectively, whereas thenumerous reduced volume ones are less effective. Conversely, the larger volumeScott style has a greater internal surface area and regions of dead volume, i.e.,areas where the nebulizer gas flow does not rapidly flush any sample entering itaway. Some analytes may exhibit a much longer memory effect or “washout”period in such a spray chamber, because it may become adsorbed to the glasswalls (e.g., for lead) or may simply become trapped in an area of dead volume.For routine analysis, this problem is little more than annoying, because themain result is that a longer washout period is required between samples, whichobviously extends the analysis time and increases the cost of analysis.However, when transient signals are obtained, especially those arising fromchromatography, the memory effects can broaden the analyte peaks to theextent where they may start to merge. This is obviously undesirable, asconfusion between different analyte species may result. Broader peaks alsolead to lower signal-to-noise ratios and inferior limits of detection. Thebroadening effects are reduced for low volume spray chambers such as thecyclone and single pass styles. If a high quality liquid chromatography pump isused, the pump noise should be minimal and so the reduced volume spraychambers often offer the best resolution and sensitivity with adequate noisecharacteristics. Some corrosion resistant spray chambers manufactured frompolymers (e.g., Ryton) are also available. Many spray chambers come with ajacket surrounding them, through which a cooling fluid is pumped to maintainthe spray chamber at a constant temperature and improve stability. Thecooling fluid is usually water, although anti-freeze may be used at atemperature of 25 to 2108C to decrease the vapor pressure of organicsolvents. This may help to decrease plasma perturbation by minimizing plasmaloading.

Sample throughput will depend on the type of instrument used. If asequential spectrometer is used, then the determination of each analyte maytake 20 s and so if 10 analytes are to be determined, then the analysis time for

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one sample may be 3–5 min; leading to a sample throughput of approximately12–20 samples per hour. Inevitably, the sample throughput will be dependenton the number of analytes, with throughput increasing with a decreasingnumber of analytes. If a simultaneous spectrometer is used, then the same timewill be required for one analyte as for 10, and so sample throughput is likely tobe greatly increased. Assuming that the instrument has been set up properly,with the analyte lines “trimmed” so that measurement is made from the top ofthe peak rather than the rapidly sloping sides, then precision can be 1–5%RSD. The limits of detection for many analytes using optimal conditions aregiven in Table 5.3. It should be noted, however, that since the ICP has a definitestructure and each region has a different temperature, ionizing properties, etc.,then each analyte has an optimal set of conditions which yield the bestsensitivity. The most critical parameters that govern analyte sensitivity are theviewing height, the nebulizer (carrier) gas flow rate, the power and, to a lesserextent, the auxiliary (intermediate) gas flow rate. If several analytes are to bedetermined, then compromise conditions will probably have to be used. Sincecompromise conditions are the “best overall”, but may not actually be optimalfor any given analyte, the LODs obtained will be inferior to those shown in thetable.

Occasionally, other specialized nebulizers are used, e.g., the thermospray,the electrospray and the direct injection nebulizers. The theory of these isbeyond the scope of this chapter, but may be found in Refs. [56,57]. Each ofthese nebulizers produces a very fine aerosol and usually operates at low flowrate (typically 10–50 ml min21). They can therefore be placed at the base of theplasma torch, omitting the spray chamber. Transport efficiency to the plasma isvirtually 100%, but plasma extinction is prevented because only a similarabsolute volume of sample reaches the plasma in any given time period as for aconventional nebulizer.

Other sample introduction methodsThere is a plethora of alternative sample introduction methods for ICPspectrometry, including those described for flame spectrometry, i.e., FI andchromatography. A typical sample flow rate for HPLC is 1–2 ml min21, whichis compatible with the sample uptake of an ICP. However, when organicsolvents are to be introduced, a desolvation device may be required to preventplasma extinction. Similarly, for HPLC applications that use a mobile phasecontaining a high dissolved salt content, nebulizers and torch injectors that aretolerant of this must be used. The use of ICP-OES as a detector for elementalspeciation studies has been described recently [58]. Liquid chromatography isnormally coupled with ICP spectrometry via the nebulizer and spray chamberassembly, although the electrospray, thermospray and direct injectionnebulizers have also been used. Gas chromatography has occasionally beencoupled with ICP spectrometry, but few routine applications exist. The problem(as with GC–AAS coupling) is the transfer of the analyte to the atom cell in a

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sufficiently hot form to prevent condensation. Coupling of a heated transfer lineto the ICP torch can be problematic because any metal components within thetransfer line may act as an aerial for the RF power, leading to a potentialhazard. The transfer line must normally be placed as far up the torch aspossible to prevent analyte condensation, whilst ensuring that the end of it doesnot melt and that potential hazards are avoided. A further problem is that the

TABLE 5.3

Limits of detection for ICP-OES under optimum conditions with conventionalnebulization


Ag 3 Mo 4

Al 1.5 Na 1

As 12 Nb 4

Ba 0.07 Nd 2

Be 0.2 Ni 6

Bi 12 P 18

Ca 0.03 Pb 14

Cd 1.5 Pd 7

Co 5 Pr 0.8

Cr 4 Pt 20

Cs 3200 Rb 3

Cu 2 Ru 6

Dy 0.3 Sb 18

Er 0.7 Sc 0.4

Eu 0.3 Se 37

Fe 1.5 Si 5

Ga 6.5 Sn 15

Gd 3 Sr 0.02

Ge 13 Ta 9

Hf 4 Tb 5

Hg 8.5 Te 27

Ho 0.5 Th 17

In 18 Ti 0.6

Ir 4 Tl 16

K 10 Tm 1.5

La 0.02 U 18

Li 0.6 V 2

Lu 0.05 W 17

Mg 0.1 Y 0.2

Mn 0.3 Zn 0.9

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transport gas flow rate typical of GC separations is not sufficient to punch asample channel into the plasma. A make up gas is therefore usually required.This too often requires heating to prevent analyte condensation.

Sample throughput is obviously dependent upon the length of time requiredfor the chromatography, but for many HPLC and GC applications, only 3–8samples per hour may be analyzed. Precision is also dependent on the methodof sample introduction, but often lies in the region of 3–10% RSD, and may beimproved if an appropriate internal standard is used.

Chemical vapor generation is a popular method of sample introduction forICP spectrometry as well as flame spectrometry and the benefits of thisapproach are the same. The one problem that may be encountered is theproduction of excess hydrogen as a by-product of the hydride generationreaction. As discussed previously, some instruments are relatively intolerant ofgases other than argon and so, if large quantities of hydrogen enter the plasma,perturbation may occur. The use of an automated continuous hydride generatoris highly recommended, as is careful optimization of both the reagentconcentrations, to minimize the excess hydrogen produced, and the instru-mental operating conditions. A recent example of HG-ICP-OES is presented byOverduin and Brindle [59]. Some workers have coupled chromatography withHG prior to ICP-OES detection [60]. Again, this would improve the sensitivitywhen compared with HPLC–ICP-OES. Just as with HG-AAS, sample pre-treatment may be necessary to transform some species into a form thatproduces a hydride.

Electrothermal vaporization has also been used to introduce assortedsample types to ICP-OES instrumentation. The principles of ETV are the sameas those described previously. Again, it is a useful technique when only limitedsample volume is available. It may be used to analyze liquid samples, slurriesand solid samples directly. The same heating programs are used, i.e., a dry, apyrolysis and a vaporization stage followed by a high temperature cleanup step.For ETV–ICP-OES (and -MS), the vaporization stage does not have to atomizethe analyte. As long as the temperature is sufficiently high to vaporize theanalyte, either as an atom or as a compound, it may be transported in a flow ofinert gas to the plasma. Since this is at a temperature of 6000–10,000 K andthe sample arrives in a dry form, the plasma has more energy available foratomization and excitation, so it will dissociate the vast majority of analytecompounds. If adequate pyrolysis temperatures are used, the analyte isseparated from the majority of the matrix, thereby facilitating interference freedetermination. Two reviews of the process of ETV–ICP have been published; anearly one by Carey and Caruso [61] and a more recent one that compares ETVwith laser ablation (LA) [62]. Sample throughput is again dependent on thetype of instrumentation used and the number of analytes to be determined.Since the signal obtained is in the form of a transient, for sequentialinstruments, a number of replicate analyzes will have to be performed becausethe instrument will not have sufficient time to scan to more than one

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wavelength before the signal returns to the baseline. Since each replicateanalysis may take up to 2 min, and three replicates are normally required perelement, then if three elements are to be determined, each sample will take20 min. This may be longer if solids are weighed directly into the graphiteatomizer. Precision should be roughly equivalent to that obtained withconventional ET-AAS. Again, LODs should be measured as an absolute amountrather than as a concentration. In general, sensitivity is improved becausetransport efficiency of the analyte to the atom cell is substantially larger thanfor conventional solution sample nebulization, although condensation of theanalytes within the transfer line may occasionally decrease the transportefficiency.

Direct sample insertion (DSI) and in-torch vaporization (ITV) are off-shootsof ETV. Sample (either liquid or solid) is dispensed onto the tip of a probe,usually made of graphite or a refractory metal. The probe is then inserteddirectly up the injector of a specialized torch towards the plasma. Sampletransport efficiency is close to 100%, but the analyte is not separated from thematrix and determinations are thus more prone to interferences. The techniquewas reviewed in 1990 by Karanassios and Horlick [63] and again in 1999 bySing [64].

In LA, a laser beam is focused either onto, or just above, the surface of asample. The laser vaporizes a small area of the sample and the vapor istransported in a stream of inert gas to the plasma. The laser may be focusedonto extremely small areas (,0.1 mm) and hence may be used, for example, toanalyze fluid inclusions in geological materials. If the laser is aimed at the samespot on some types of sample, then depth-profiling is possible, i.e., the top0.1 mm of surface is analyzed, followed by 0.1 mm below that, etc. This may beof use for some sample types where the depth may be correlated directly withage. There are several problems associated with quantitative analysis usingLA–ICP-OES. Since only a very small area of sample is vaporized, if a bulkanalysis of a sample has to be performed, it is essential that it is homogeneous,otherwise accuracy and precision will be affected. Since the laser radiation willinteract with different types of sample to different extents, it is necessary forcalibration to be performed using materials that have an identical matrix.Failure to calibrate properly will lead to the results being, at best, semi-quantitative. Laser ablation may also be used to “map” the surface of a sample,i.e., to determine how an analyte concentration varies over the surface of thesample. Laser ablation produces transient signals so, as with ETV sampleintroduction, a sequential instrument will not be able to determine typicallymore than one analyte at any given sample site, whereas a simultaneousinstrument could potentially do several. A review of the interaction of laserradiation with samples has been given by Darke and Tyson [65]. A more recentreview comparing ETV and LA has been made by Kantor [62]. Accuracy andprecision of the technique will depend on the sample homogeneity and on how

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closely the standards are matrix matched to the samples. These issues arediscussed in more detail by Russo in a later chapter in this book.

Solid samples may be introduced directly to the plasma by severaltechniques, including slurry sampling, ETV, DSI and LA. In addition to thesemethods, some instruments come with a solid sampling (SS) accessory. Therequirement for this is that the sample must conduct electricity and thereforemetallurgical samples, including steels, brass, other alloys, wires and even coalfly ash, may be analyzed. An arc or a spark is used to ablate material from thesurface of the sample and the dry aerosol produced is transported to the plasmain a stream of argon. The sample may be in the form of rods, powders orbriquettes. The technique has a few variants but most deliver a precision of0.2–1% RSD for a concentration of 1%. Custom accessories have even beenproduced commercially that enable the determination of wear metals inlubricating oils [66]. A direct current arc has been reported as giving a precisionof 3–10% RSD at the 1% concentration level. One of the drawbacks with thismethod of sample introduction is the need for very closely matrix-matchedstandards.

5.5.4 Figures of merit

As noted throughout the text, sample throughput and precision will depend onthe instrumentation used, the number of analytes to be determined and thesample introduction method. Assuming a liquid sample (or a digested solidsample) is to be analyzed using conventional nebulization, then a modernsimultaneous instrument may analyze 25–30 samples per hour and use only1–5 ml of sample. A sequential instrument will analyze fewer than this andwill consume substantially more sample. Both types of instrumentation shouldprovide analytical results with a precision of 1–5% RSD. The linear rangeshould extend to at least five orders of magnitude for ICP-OES determinations,although it should be noted that for some applications, e.g., chromatography,the chromatographic column may become overloaded at higher concentrationsof analyte.

5.6 INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY

5.6.1 Introduction

Inductively coupled plasma-mass spectrometry was developed in the 1970s andcommercial instrumentation was available in the 1980s. It is a coupling of anICP with mass spectrometric detection. The principles behind the sampleintroduction and the processes of plasma formation and of desolvation,dissociation, atomization and ionization within the plasma are the same asthose described previously. It has become a very popular method of analysisbecause it has several advantages over other techniques, i.e., it is extremely

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rapid, with simultaneous and quasi-simultaneous instruments being available;it offers improved sensitivity over many of the other techniques for mostanalytes and the mass spectrum produced from any sample is far more simplethan that obtained from an emission instrument. There are several differenttypes of ICP-MS instrument. Each of these will be discussed in more detaillater. The one major drawback is the cost, ranging between US $80,000 and US$400,000, depending on the type. A schematic diagram of a general ICP-MSinstrument is shown in Fig. 5.1.

5.6.2 Theory

As discussed previously, the sample introduction systems used for ICP-MS canbe identical to those for ICP-OES. Similarly, the processes occurring within theplasma are also identical. It is worth noting, however, that since all ICP-MSinstruments detect the analytes according to a mass-to-charge ratio ðm=zÞ, forany signal to be detected, the analytes must become ionized within the plasma.The extent of ionization will depend on several factors, but most importantly onthe first IP of the analyte. The plasma consists of ionized argon that has a firstIP of 15.76 eV. Therefore, any element that has an IP less than this will be atleast partially ionized. Cesium, having a first IP of 3.89 eV, will be 100%ionized, but arsenic has a first IP of 9.81 eV and will be only 30–40% ionized.Fluorine, with a first IP of 17.42 eV, will not be ionized and therefore cannot bedetermined directly by ICP-MS (using an argon plasma).

Fig. 5.1. Schematic diagram of an ICP-MS instrument.

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Once the ions have been formed, they must pass from atmospheric pressurethrough several chambers of increasingly high vacuum to the mass separationand detection stages. Several different types of mass filters and detectors exist,which will be discussed in a later section (Section 5.6.3.1). A more detailedaccount of how the ions pass from the plasma through the expansion chamberand ion lens system to the mass filter and detector may be found elsewhere[2,67–69].


5.6.3.1 Mass filtrationA discussion of ICP-MS instrument types has been given in a recent bookchapter [70]. In addition, several tutorials have also been published recentlythat have discussed the various types of mass filters [71–73]. The most commonmass analyzer is the quadrupole. Four rods (often made from molybdenum) arearranged in a set of two pairs in a square orientation. Two rods have a DCvoltage on them and the other two have RF voltage. The magnitude of thevoltages will allow one m=z to pass through the rods towards the detector,whilst ensuring that all ions of other m=z collide with one of the rods, hencepreventing them from being detected. A short while later (often ,1 ms) themagnitude of the voltages changes and an ion of different m=z is allowed to passto detection. A quadrupole instrument is therefore not truly simultaneous, butinstead is so rapidly sequential as to be regarded as being quasi-simultaneous.This device is relatively inexpensive and robust, but has a relatively poorresolution, i.e., between 0.7 and 1.0 atomic mass units (AMU). This means thatit is more prone to spectral interferences than other mass analyzers. Theproblem of interferences is discussed in a later section (Section 5.6.5).

Other, more highly resolving ICP-MS instruments are available commer-cially, although at much greater cost. An example is double focussing magneticsector instrumentation. The principle of operation of this instrumentation hasbeen discussed by Thomas [72]. These instruments have a resolving power ofup to 10,000 compared with a quadrupole-based instrument that has aresolving power of only 300. This large improvement in resolution will enabledistinction to be made between some interfering polyatomic ions and analyteions (see Section 5.6.5). The resolution of the instrument can be set by theanalyst so that individual interferences can be overcome. The resolutionrequired obviously depends on how close the interfering species is in mass tothe analyte. A few examples include 34Sþ and 16O18Oþ that may be separatedusing a resolution of 1300 and 75Asþ and 40Ar35Clþ, which require a resolutionof 7725. It should be noted, however, that the higher the resolution required,the lower the sensitivity. It is therefore advisable to use the lowest resolutionnecessary to achieve interference free determination. As well as specializing inthe reduction of interferences, it can be used to gather extremely precise isotoperatio data. If used in low resolution and for interference free analytes, a

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precision of 0.01–0.05% is obtainable. In addition, the sensitivity at lowresolution can be at least an order of magnitude superior to quadrupoleinstruments. Some instruments of this type use several detectors. These multi-collector instruments have the capability of detecting and measuring multipleions simultaneously and are regarded as being capable of producing theultimate in precision for isotope ratio measurements.

The time-of-flight ICP-MS (ICP-TOF-MS) is the most recent development ofICP-MS instrumentation, being commercially available since 1998. Althoughstill relatively immature, it does have several potentially important advan-tages over other instrument types. This instrumentation permits trulysimultaneous detection and therefore has advantages when measuringtransient signals; it produces high precision isotope ratios and can decreasethe amount of time required to complete an analysis. A more completediscussion of the theory behind ICP-TOF-MS, the instrumentation, and itsrelative merits, can be found in the literature [73–75]. Examples of the use ofICP-TOF-MS instruments include the determination of isotope ratios [76] andthe detection of rare earth elements in seawater after FI matrix eliminationand pre-concentration [77].

5.6.3.2 Reaction cellsThe use of reaction cells has been discussed recently by Thomas [78]. Thesedevices are placed between the ion lens system and the mass filter. As the ionsfrom the plasma enter, a quadrupole, hexapole or an octopole cell helps focusthem towards the reaction cell gas (usually helium or hydrogen). As the ionsand the reaction gas collide, the polyatomic ions fracture, leading to a decreasein the interference observed [79,80]. The use of reaction cell technology hasimproved the analytical capabilities of quadrupole based ICP-MS instrumentsin terms of both interference and, for some analytes, limits of detection. Evenfor analytes that are renowned for being difficult to determine using ICP-MS,such as Fe, LODs significantly below 1 ng ml21 may be obtained when areaction cell is used to overcome the interference caused by 40Ar16Oþ on 56Feþ.

5.6.3.3 DetectorsThere are several types of detector available for ICP-MS instruments. Thechannel electron multiplier is a horn shaped device that is coated with asemiconductor. As an analyte ion impinges on the surface, an electron is ejectedwhich is accelerated down towards the other end of the tube, but on its waydown, collides with the wall of the tube ejecting secondary electrons. Each ofthese is also accelerated towards the other end of the tube and they collide withthe wall producing further electrons. An avalanche effect is therefore built up.The number of electrons reaching the pre-amplifier at the far end of the tube isproportional to the number of analyte ions impinging on the detector, i.e., theconcentration of the analyte. The discrete dynode electron multiplier functionsin a very similar way, but instead of using a continuous tube like dynode, as in

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the channel multiplier, it uses a series of discrete dynodes. The Faraday cupmay be used when ultra-trace detection limits are not required. A more detaileddescription of the different detector types and how they may be used either incombination, or singly, to achieve a linear range spanning nine orders ofmagnitude, has been given by Thomas [81].

5.6.4 Different types of analysis

Analysis using ICP-MS offers a wide variety of options. It may be used in thenormal, fully quantitative analysis mode, or as a detector for chromatographicseparations where perhaps only one or two target elements may be determined.Other time-resolved functions include serving as a multi-element detector forETV, LA and FI analyzes. Semi-quantitative analysis is also achievablewherein a mass response curve is prepared using one mixed standard ofperhaps six elements, each at either 10 or 100 ng ml21. The response from eachof these elements is calculated and the line of best fit between each of the pointsplotted. The software will “assume” that for the same concentration, any otheranalyte will have a response on that line of best fit. The method is semi-quantitative, because only an estimate of the analyte’s concentration can bemade, although it is normally accurate to within a factor of two. A suite ofnearly 70 elements can have their concentrations estimated within 10–30 s.The method is especially useful when a sample of completely unknowncharacteristics must be analyzed. A semi-quantitative analysis will enable theanalyst to identify an appropriate concentration range for the standards priorto a fully quantitative analysis or it may be used to identify suitable internalstandards.

Isotope ratio measurements for isotope dilution (ID) is regarded as being adefinitive method of analysis. The subject of isotope ratio measurements hasbeen discussed in great detail in a recent book [82]. Although possessing anumber of advantages, the greatest drawback is cost; the price of pure isotopesor even isotopically enriched metals can be prohibitive. For species specific ID,the isotopically enriched compound will probably have to be prepared in-houseand the cost of enriched isotopes is high. Once prepared, the isotopicallyenriched compound should be analyzed using an assortment of instrumentalmethods, such as nuclear magnetic resonance, so that an estimate of its puritycan be made. A discussion of ID methods for trace metal speciation has beenpublished recently [83].

5.6.5 Interferences

There are several types of interference that may occur in ICP-MS analyzes.Those attributable to sample transport effects are the same as for ICP-OESanalyzes. The presence of 0.5% dissolved solid and high concentrations of acidin a sample will not nebulize with the same efficiency as a standard prepared in

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2% nitric acid, and hence a different signal may be obtained for the sameconcentration of analyte. Additionally, space charge interferences may arisewhen the ion current in the sampled analyte beam exceeds the capacity of theion lens systems to maintain focusing and the transmission efficiency ischanged. This usually occurs as a consequence of the presence of highconcentrations of concomitant elements and tends to favor the transmission ofhigher mass analytes from the beam. These effects may be partially overcomeby the use of at least one internal standard. As usual, the internal standardshould not be present naturally at a significant concentration in the sample andshould match, as closely as possible, the ionization energy and mass of theanalyte. If a range of analytes is to be determined, e.g., 65Cuþ and 66Znþ, 111Cdþ

and 208Pbþ, then it may be necessary to have up to three internal standards,one at the lower m=z range such as 59Coþ, one in the middle, such as 115Inþ andone at the higher m=z range, such as 205Tlþ. The use of more than one internalstandard may lead to greater long-term instrument stability. Since the massresponse curve (i.e., the signal obtained per unit concentration over the massrange) may change with time, the use of a single internal standard, such as115Inþ, may be insufficient. For example, if after 50 samples have beenanalyzed, a standard containing 100 ng ml21 of analytes is analyzed as a checkstandard, it may be found that the concentrations range from 70 ng ml21 at thelower mass range up to 130 ng ml21 at the higher end. If two or more internalstandards are used, instrumental drift can be diminished to ,10% over a wholeday’s work.

There are several types of spectroscopic interference. The most common isthat of polyatomic interferences. This occurs when two (or more) atoms form amolecule that has nominally the same mass as the analyte. The vast majority ofthis type of interference occurs below m=z 80 (the argon dimer) and these oftencontain argon (from the plasma) combined with an ion present in the matrix ofthe sample. Examples include the interference from 40Ar35Clþ on 75Asþ,32S16O2

þ on 64Znþ, 40Ar16Oþ on 56Feþ and 23Na40Arþ on 63Cuþ. A far morecomplete list is given in a review by Evans and Giglio [84]. In a paper by Nonoseand Kubota [85], the interferences observed in quadrupole and in highresolution ICP-MS instruments are compared.

Isobaric interferences occur when two analytes have isotopes of nominallythe same mass, such as for 113Cdþ and 113Inþ. However, most elements have atleast one isotope free from such interference. Doubly charged ion interferencesalso occur, but the only element that suffers from this to any significant degreeis barium, because it is the only commonly determined analyte that has asecond IP , 15:76 eV: The overall effect is that the signal for 138Baþ decreaseswhilst the signal for 138Ba2þ (which is equivalent to 69Gaþ) increases. Theextent of ionization may be different between samples and standards and soeither/or the Ba and Ga determination may be affected. Another type ofinterference arises due to the formation of metal oxides. This is similar topolyatomic interferences and occurs mainly for the rare earth elements, with

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the lower mass analytes, that are usually there at appreciably higherconcentration than the ones with higher m=z, combining with oxygen to givean elevated signal at M þ 16, e.g., 141Pr16Oþ interfering with 157Gdþ.

There are several means that may be used to overcome spectroscopicinterferences. The easiest is to use an alternative isotope that does not sufferfrom interferences, although some elements are mono-isotopic. Sometimes, it ispossible to perform some chemistry on the sample prior to analysis such thatthe interferences are separated from the analytes. An example has been the useof a FI technique with a micro-column of a chelating resin to retain theanalytes, whilst potential interferences were washed to waste by an appro-priate buffer [86]. Some of the other alternative sample introduction methodsalso succeed in separating the analyte from potential interferences, such aschemical vapor generation and ETV. The introduction of alternative gases hasalso been demonstrated to overcome some interferences. The introduction of4% v/v nitrogen to the nebulizer gas flow has been shown to markedly reducethe interference from 40Ar35Clþ on 75Asþ [87]. The mechanism by which thisworks is uncertain, but a concomitant increase in the signal at m=z 51(14N37Clþ) and at 49 (14N35Clþ) would appear to indicate that a favorablycompetitive reaction is occurring. Hydrocarbon gases have also been shown tobe beneficial for many analytes [88,89].

As well as chemical methods of interference removal, instrument manu-facturers have also produced several hardware and software methods. Thesoftware based methods are mathematical algorithms that rely on correctionfactors. For instance, the extent of 40Ar35Clþ interference on 75Asþ can beestimated by taking into account Se signals at m=z 77 and 82. The Se isotope atm=z 77 is also interfered with by chloride (40Ar37Clþ), but the isotope at m=z 82 isnot. Since each isotope’s theoretical relative abundance is known, any deviationfrom this known ratio can be measured and a correction made. Hardwaremodifications and accessories offer a more reliable method of overcominginterferences. The collision cell and the dynamic reaction cell have beensuccessfully used to overcome interferences (see Section 5.6.3.2). The use of highresolution mass analyzers also overcomes the vast majority of commoninterferences. Most polyatomic interferences exist only for quadrupole-basedinstrumentation. This arises because even though the interferences and analyteions do have a slightly different mass (e.g., 75Asþ actually has an m=z of 74.926whereas 40Ar35Clþ has m=z 74.932), as discussed previously, the quadrupoleonly has unit mass resolution and can therefore not distinguish between the two.Magnetic sector instruments are capable of much higher resolution and candistinguish between the two masses, hence eliminating the interference.

5.6.6 Sample introduction techniques

In general, the principles, advantages, drawbacks and applications of theassorted sample introduction techniques are the same for ICP-MS as for ICP-

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OES. As well as increased sensitivity, the one big advantage of ICP-MS oversome ICP-OES instrumentation is that it is simultaneous (or at least far morerapidly sequential). This means that for sample introduction techniques thatproduce transient signals, such as LA, ETV, FI and chromatography, more thanone element may be determined at any one time. For liquid chromatographyutilizing a mobile phase with a high dissolved salt content, coupling with ICP-MS may lead to additional problems. As well as potentially blocking thenebulizer (if an appropriate one is not used) and the injector of the torch,blocking of the orifice of the sampler cone may also occur. Inevitably, this willlead to significant signal drift, until blockage is complete, at which point nosignal will be obtained. A similar problem arises for the introduction of organicsolvents. The solvent will pyrolyze within the plasma and will produce largequantities of soot. Whereas in ICP-OES this soot will pass harmlessly to wastevia the fume extraction system, with ICP-MS instrumentation it may clog thesampler cone. In addition, the ion lens system also becomes dirty and theinstrument will have to be dismantled so that it can be cleaned. The problemsarising from soot deposition can be overcome by introducing oxygen (3–5% v/v)into the nebulizer gas flow. This turns the pyrolysis into an oxidativecombustion process, and so the soot is burned off as carbon dioxide, therebypreventing sampler cone blockage. It should be noted, however, that the amountof oxygen used is critical. If too much oxygen is introduced, then the nickelsampler cone itself becomes oxidized away. This process can occur rapidly and anew cone can become unusable within a few minutes. Platinum or platinumtipped sampler cones are also available, and these tend to be more robust andresilient to oxidative attack, but are obviously substantially more expensive.

Some workers have coupled GC with ICP-MS [90], but the coupling requiresa heated transfer line that has to be constructed to enable safe and simplecoupling. A commercial GC–ICP-MS instrument has now been produced, sothe overall coupling is more robust.

Some workers have also coupled capillary electrophoresis (CE) with ICP-MS. The flow rate through a CE instrument is typically at the low ml min21

level, or perhaps even nl min21. A specialized coupling is therefore required tomake sure that the flow rate of the CE and the uptake rate of the ICP-MS arecompatible, often achieved using a micro-flow nebulizer or a DIN [91].Occasionally, a gas inlet is used to prevent suction from the nebulizerdestroying the chromatographic separation by drawing the sample throughat an accelerated rate. Since the injection volume is exceptionally low (again atthe nl or ml range), the concentrations detected are normally at the mg ml21

range, although the absolute amount is at the pg or fg level. There are thereforevery few applications for this coupling and its use is far from routine.

Other sample introduction methods, including LA, ETVand chemical vaporgeneration, share the same advantages and disadvantages as discussed foranalyzes by ICP-OES.

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The limits of detection obtainable by conventional nebulization ICP-MS areusually at least two orders of magnitude lower than those obtainable by ICP-OES. The LOD will depend on a number of factors, including the ionizationenergy of the particular analyte, the number of isotopes the analyte has (if anelement has six or seven isotopes, the signal will be split between these andhence sensitivity will be less than for an element with only one isotope), theacquisition (integration) time and potential interferences. In addition, theoperating parameters and the type of instrumentation used will also have agreat effect. For a quadrupole based ICP-MS, approximate LODs are shown inTable 5.4. At a low-resolution setting, a magnetic sector instrument mayimprove these by at least an order of magnitude. Obviously, for sampleintroduction methods that give increased transport efficiency to the atom cell(ETV and chemical vapor generation), the LODs shown in the table can also beimproved by over an order of magnitude.

Under standard conditions, the linear dynamic range spans five or sixorders of magnitude. If, however, both pulse counting and analogue modes areused, then the linear range may be extended to eight or even nine orders ofmagnitude. It should be noted though that the standards still have to beprepared in an appropriate range for the individual analytes within thesample. Sample throughput will again depend on the method of sampleintroduction and the time of acquisition/number of replicate measurements,but for conventional nebulization and a 10 s acquisition (integration time) foreach of three replicates, potentially up to 50 or 60 samples may be analyzed inan hour. The number will also be affected by the speed of washout from thespray chamber, and so a fast clearing spray chamber will enable more samplesto be analyzed per hour than a slower one. Since the instruments aresimultaneous (or quasi-simultaneous), a large number of analytes may bedetermined simultaneously and an enormous amount of data may be collectedin a short period of time.

Precision will depend on the application. For conventional nebulization aprecision better than 1% RSD may be obtained. For isotope ratio and IDmeasurements, precision would normally be expected to be better than 0.1%RSD. For other sample introduction techniques, such as LA or ETV, precisionwill depend on the homogeneity of the sample rather than the detectiontechnique.

5.7 ATOMIC FLUORESCENCE SPECTROMETRY

5.7.1 Introduction

Atomic fluorescence spectrometry (AFS) is theoretically applicable to all of thecommonly determined analytes. Modifications to standard flame instruments

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may be used to obtain one capable of detecting atomic fluorescence. Despite itsgeneral applicability, in recent times AFS has been used almost entirely for thevapor generating elements. Indeed, commercial instrumentation has beenproduced that specializes in detecting As, Sb, Se and Te and another thatdetects Hg. The specialized commercial AFS detectors are relatively cheap,costing ,US $5000, although fully automated systems are also available at

TABLE 5.4

Limits of detection for quadrupole ICP-MS under optimum conditions and usingconventional nebulization


Ag 0.005 Mo 0.005

Al 0.05 Na 0.05

Au 0.005 Nb 0.005

Ba 0.001 Nd 0.001

Be 0.001 Ni 0.005

Bi 0.001 P 0.5

Ca 0.5 Pb 0.001

Cd 0.005 Pd 0.005

Co 0.001 Pr 0.001

Cr 0.005 Pt 0.005

Cs 0.001 Rb 0.001

Cu 0.005 Ru 0.005

Dy 0.01 Sb 0.005

Er 0.001 Sc 0.05

Eu 0.001 Se 0.05

Fe 0.05 Si 0.5

Ga 0.001 Sn 0.005

Gd 0.001 Sr 0.001

Ge 0.05 Ta 0.005

Hf 0.005 Tb 0.001

Hg 0.001 Te 0.05

Ho 0.001 Th 0.001

In 0.001 Ti 0.05

Ir 0.005 Tl 0.001

K 0.5 Tm 0.001

La 0.05 U 0.001

Li 0.001 V 0.005

Lu 0.001 W 0.005

Mg 0.05 Y 0.001

Mn 0.0004 Zn 0.005

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greater cost. Some research papers have been published concerning laserinduced fluorescence (LIF) or laser excited atomic fluorescence (LEAF). These,however, are research methods at present and are not used routinely. A reviewof LEAF spectrometry has been published recently [92].

5.7.2 Theory

The theory of AFS may be found elsewhere in the literature [93]. Briefly,radiation from an intense light source (line sources are used rather thancontinuum ones, although high intensity light emitting diodes that have abandwidth of 20–40 nm may also be used) is used to excite the analyte which,upon relaxation back to a lower energy state, emits light of discretewavelengths, depending on the transition involved. The intensity of the lightsource has a large impact on the sensitivity, as the fluorescence intensity isproportional to the intensity of the source. Standard HCLs may be used, butgreater sensitivity is obtained from boosted HCLs. A laser would provide themost intense source and a number of these have been used for this purpose,including standard YAGs, diodes, dye lasers and optical paramagneticoscillators but, apart from diode lasers, most are difficult to operate and costlyto maintain. Atomic fluorescence is exceptionally specific, ensuring thatspectral interferences are minimal.


The instrumentation used can be basically the same as for F-AAS, although thelight source must be positioned at a right angle to the detector so that emissionfrom the lamp is not detected as fluorescence. Since AFS is so specific, it doesnot require a complex line isolation device such as a monochromator. Instead,simple filters will suffice, although some high throughput multi-reflectance(interference) filters have also been used. These reportedly transmit 80% of thewavelengths of interest whilst virtually eliminating background noise.

The atom cell in commercial AFS detectors is usually an argon/hydrogendiffusion flame. This is a low temperature flame that is used to dissociate thehydrides of these analytes. Both argon and hydrogen have low quenching cross-sections for fluorescence. For the commercial Hg detector, a simple quartz cellor open argon sheathed chimney is used. Since atomic vapor is introduced,there is no need for a heat source. Detection is usually with a PMT.

It should also be noted that atomic fluorescence has also been achievedusing an ICP as an atom cell [94]. Although a commercial instrument wasmarketed briefly, this too has only really been of research interest. A review ofICP-AFS has been produced by Greenfield [95].

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5.7.4 Sample introduction

The majority of AFS techniques utilize chemical vapor generation to introducethe sample. The usual problems with chemical vapor generation are observedand therefore optimization of the generation chemistry to decrease interfer-ences and to transform non-vapor forming species into ones that can isrequired. Often, a membrane drier tube is used to prevent the ingress of watervapor into the atom cell, since its presence may lead to light diffraction,quenching of the fluorescence and possible interference. Chromatography,coupled with chemical vapor generation and AFS detection, has been usedfrequently as an alternative to ICP-MS detection, because the LOD iscomparable whilst the overall cost of the instrumentation is substantiallyless. An example of HPLC–HG-AFS that also incorporated an on-linemicrowave transformation of inert species into forms that can generate ahydride has been published by Gomez-Ariza et al. [96]. Several studies havecoupled chromatography directly with atomic fluorescence. As an example,Puskel et al. used a specialized type of nebulizer (a hydraulic high pressurenebulizer) to introduce assorted selenium species [97].

5.7.5 Interferences

Since the majority of applications of AFS utilize chemical vapor generationsample introduction, many of the interferences observed occur in the vaporgeneration step. Methods of overcoming these have been described previously.Once the vapor enters the atom cell, several types of interference may occur,including quenching by molecular gases (and other species), leading to adramatic reduction in sensitivity. To minimize or prevent this, commercial AFSdetectors use a gentle argon purge flow. Similarly, if water vapor enters theatom cell, quenching or diffraction/scattering of the light may occur. The idealflame is the argon/hydrogen diffusion flame, but this has a temperature that istoo low to prevent chemical interferences and is another reason why the vaporgeneration technique is the preferred method of sample introduction.


Atomic fluorescence, especially when the sample is introduced by a chemicalvapor technique, is exceptionally sensitive. For mercury, the detection limit isreported to be less than 1 ng l21 and for other analytes, such as As, Se, Sb andTe, the LOD is approximately 10 ng l21. The technique has a linear rangespanning five orders of magnitude. Sample consumption will depend on themode of chemical vapor generation used, i.e., batch or continuous; but is likelyto be several milliliter. Precision is comparable to other common detectiontechniques and is typically better than 5% RSD.

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5.8 OTHER ATOMIC ABSORPTION, EMISSION AND FLUORESCENCEMETHODS OF DETECTION

There are a number of other detection methods that have been used todetermine trace analyte concentrations, although many are either researchmethods or have fallen virtually into disuse.

5.8.1 Microwave induced plasma

The microwave induced plasma (MIP) is the most commonly used of theother methods. This plasma is formed from microwave radiation andusually helium as the support gas, although other gases have also beenused. The normal helium MIP is not a robust plasma and analytes mustusually be introduced in a gaseous form since the plasma will beextinguished by the presence of any solvent. Recently, however, a veryhigh power MIP (up to 1500 W) has been sustained while liquids wereaspirated [98]. Since helium has much higher ionization energy than argon,the MIP is capable of detecting several analytes with greater sensitivitythan argon based plasmas. Examples include the halogens (includingfluorine), sulfur and nitrogen. The MIP is used mainly in the atomicemission mode, although in the reference given above, a mass spectrometricdetection method was used. In this latter mode, a helium-based plasma isuseful because the argon polyatomic interferences observed in argonplasmas are largely eliminated. Detection limits were at the ng level. ForMIP-AES, a number of systems have been used. These include differenttypes of microwave cavity, e.g., Beenakker, slab-line and surfatron, anddifferent types of line isolation and detection devices, e.g., Czerny–Turnermonochromators, Rowland circle style polychromators and oscillatingbandpass filters. The development of a commercial instrument that hascoupled together GC and MIP-AES has ensured that this has become themost common method of sample introduction. The chromatography coupledwith the MIP detection means that the vast majority of analyzesperformed are speciation-based techniques. Speciation analyzes with MIP-AES detection, and many of the fundamentals of the technique, have beenreviewed recently [99]. The technique yields LODs in the range of0.1–5 pg s21 and linear ranges extend over four orders of magnitude.Sample throughput depends on the length of time required for thechromatography to be complete, but is unlikely to exceed 10 analyzes perhour. The sample throughput will also be dependent upon whether atemperature gradient was used to achieve the separation. If the chroma-tography is not isothermal, the GC oven will require time to cool to itsstarting temperature before another sample can be introduced. Precision istypically around 5% RSD.

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5.8.2 Direct current plasma

The direct current plasma (DCP) is an economical plasma to operate because ituses approximately 8 l min21 of argon and runs using a power of approximately1000 W. However, it suffers very badly from interferences caused by easilyionized elements. Most applications require blanks, samples and standards tobe spiked with high concentrations of lithium or barium to offset these effects.Although it is stable to the introduction of both aqueous and organic solvents,its use has declined almost to the point of non-existence. Commercialinstrumentation produced nearly two decades ago used a similar line isolationdevice as found in many ICP instruments (an echelle spectrometer) andtherefore had excellent resolution. Sample throughput and consumption issimilar to that with the ICP. Precision is similar to that obtained with F-AES,but LODs are usually superior, especially for the hard to excite elements. In onerelatively recent paper, the DCP was used to determine B in soils [100]. Thelinear dynamic range was reported as having five orders of magnitude and theLOD was 0.1 mg l21.

5.9 SECONDARY ION MASS SPECTROMETRY

5.9.1 Introduction

Secondary ion mass spectrometry (SIMS) is based on the mass spectrometry ofionized particles that are emitted when a surface, usually a solid, is bombardedby energetic primary particles, which may be electrons, ions, neutrals orphotons. The emitted or “secondary” particles will be electrons, neutral species,atoms or molecules or atomic and cluster ions. The large majority of speciesemitted are neutral, but it is the secondary ions that are detected and analyzedby the mass spectrometer. This is a process that provides a mass spectrum of asurface and enables a detailed chemical analysis of a surface or solid to beperformed.

The first mention of sputtered secondary ions in the literature was made in1910 by J.J. Thomson [101]. The first regular secondary ion mass spectrometerwas based on a patent by Herzog in 1942 [102,103], and the first successfulstudies of surface compositions using mass-analyzed sputtered ions were madeby several teams in the early 1950s [104,105]. An accelerated development ofthe field was stimulated by new efficient designs of narrow-beam primary ioncolumns [106] and of ion optics for “direct” imaging [107]. The late 1960s sawthe emergence of the first commercial instrumentation [108,109] and thecoining of the SIMS acronym [110].

Static SIMS emerged as a technique of potential importance in surfaceanalysis in the late 1960s and early 1970s as a consequence of the work ofBenninghoven and his group in Munster [110]. Whilst the SIMS technique isbasically destructive, the Munster group demonstrated that using a very low

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primary particle flux density (,1 nA cm22), spectral data could be generated ina very short time scale compared to the lifetime of the surface layer. Theinformation so derived would be characteristic of the chemistry of the surfacelayer because, statistically, no point on the surface would be impacted morethan once by a primary particle during the analysis.

Today, SIMS has an acknowledged place among the major techniques ofsurface analysis and microstructural characterization of solids. Profiling orother applications of SIMS that are not static are referred to as dynamic.Dynamic SIMS has found extensive application throughout the semiconductorindustry where the technique had a unique capability to identify chemically theultra-low levels of charge carriers in semiconductor materials and tocharacterize the layer structure of devices.

Secondary ion mass spectrometry is particularly noted for its outstandingsensitivity of chemical and isotopic detection. Quantitative or semi-quantitat-ive analysis can be performed for small concentrations of most elements in theperiodic table, including the lightest. However, the high versatility of SIMS ismainly due to the combination of high sensitivity with good topographicresolution, both in depth and (for imaging SIMS) laterally. Its generallysuperior trace element sensitivity, capability for spatial resolution in threedimensions and for isotope measurements, as well as potential for identifi-cation of chemical compounds in many cases, make SIMS the preferred methodfor the solution of an analytical problem. Deficiencies, however, still exist in thecapability of SIMS for quantitative elemental analysis compared to othersurface techniques (Auger, X-ray photoelectron spectroscopy, electron microp-robe techniques, etc.). These deficiencies can be traced to the extremedependence of relative and absolute secondary ion yields on severalparameters. Among these the following are the most important:

† matrix effects;† surface coverage of reactive elements;† angle of incidence of primary beam with respect to the sample surface;† angle of emission of detected ions;† mass-dependent transmission of the mass spectrometer;† energy band-pass of the mass spectrometer;† dependence of detector efficiency on element.

Quantitative elemental SIMS analysis poses a twofold problem. Firstly,spectral interpretation, namely, the extraction of total detected isotopic ioncurrents assignable to elemental and molecular ions from a complete SIMSspectrum of the sample; secondly, spectral quantification, namely the calcu-lation of elemental concentration from total isotopic elemental (and molecular)ion currents. Difficulties in spectral interpretation are considerably reduced ifhigh resolution mass analyzers ðM=DM . 3000Þ are used for mass analysis ofsecondary ions because most of the commonly occurring isotopic and molecularinterferences (e.g., hydrocarbons, oxides and hydrides) can be resolved.

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5.9.2 Practical principles

A diagram of a SIMS instrument with a double-focusing mass analyzer isrepresented in Fig. 5.2. Secondary ion mass spectrometry is based on:

† bombardment of the sample surface by focused primary ions, withsputtering of the outmost atomic layers;

† mass spectrometric separation of the ionized secondary species (sputteredatoms, molecules, clusters) according to their mass-to-charge ratios;

† collection of separated secondary ions as quantifiable mass spectra, as in-depth or along-surface profiles, or as distribution images of the sputteredsurface.

The primary ions are normally produced by a duoplasmatron type of gassource such as O2

þ, O2, N2þ, Arþ; by surface ionization as for Csþ and Rbþ; or by

liquid-metal field ion emission as Gaþ and Inþ. The most common primary ionsused are the oxygen ions, Csþ and Gaþ. The ions are accelerated and focused toa selected impact area on the specimen. The collision cascade following theincidence of a primary ion results in the implantation of the primary particle,reshuffling of some 50–500 matrix atoms, and emission of secondary particles,neutral or ionized. Secondary ions from the specimen are extracted into themass spectrometer, which can consist of electric (ESA)/magnetic deflectionfields or be of the quadrupole or time-of-flight design (see Section 5.6.3.1).Secondary ions with a given mass-to-charge ratio and within a certain interval

Fig. 5.2. Diagram of double focusing SIMS (adapted from Ref. [111]).

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of kinetic energy are collected for pulse or current measurement, ion-opticimaging and data processing.

The different ways of operating a SIMS instrument are presented inFig. 5.3. In the microscope mode, a defocused primary ion beam (5–300 mm) isused for investigating a large surface. In the microprobe mode, a focusedprimary ion beam (,10 mm) is used for investigating a very small portion of thesurface and detecting inclusions in bulk material. The lateral resolution isdefined by the primary ion beam size.

5.9.3 Sensitivity and quantification

Figure 5.4 shows schematically the types of analytical information that can beobtained by SIMS analysis. A SIMS spectrum normally shows mass peaks thatare characteristic of the sputtered solid but affected by experimental factors.For instance, among these factors the following should be mentioned: type,intensity, energy and incidence angle of the primary ions; the transmission ofthe secondary ions and the selectivity for them in the mass analyzer; the type ofdetector. There are, effectively, two spectra: that of the matrix and that of theimpurities.

The task of analytical SIMS is to quantify the secondary ion currents, thatis to convert the intensity of one or several peaks characteristic of an element to

Fig. 5.3. Operating modes of SIMS (adapted from Ref. [112]).

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its corresponding concentration ce. Assuming that a primary ion beam with acurrent density, ip; strikes the sample; collision cascades are initiated, resultingin, among other things, the emission of secondary ions, which are partiallydetected with an instrument transmission, h, as a mass spectrum of ions froman analyzed area, A. The detected positive or negative current of an ionicspecies M at the mass number m will be:

IM ¼ IpSPMhMgMbMce ð5:1Þ

where Ip is equal to ipA and P is the probability that the particle (atomic ormolecular) will emerge as the last step of the sputtering and recombinationcascade. S is the sputtering yield (secondary particles per primary ions), gM isthe positive or negative ionizability of M (ions per atom or molecule), and bM isthe isotopic abundance of M in the element.

5.9.3.1 Absolute sensitivityIn a situation where the prime goal is to detect trace elements of as low aconcentration as possible, without consideration of sample consumption andanalytical volume (e.g., in bulk analysis), the suitable figure of merit is thedetected secondary ion current of an element E per unit of atomic concentrationcðAÞ; that is the absolute sensitivity SaðEÞ :

SaðEÞ ¼ NqðEÞ=cðEÞ ð5:2Þ

where Nq is the detected current (in counts per second) of element E in chargestate q.

Fig. 5.4. Analytical information obtainable from SIMS analysis (adapted from Ref. [113]).

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5.9.3.2 Practical sensitivityThe practical sensitivity, SpðEÞ; takes into account the fact that in differentanalytical situations different primary beam currents may be appropriate:

SpðEÞ ¼ NqðEÞ=IpcðEÞ ð5:3Þ

This definition of practical sensitivity does not provide a figure of meritindependent of material consumption. The same value would be obtained ondifferent samples for the element E if, at the same primary beam current, thesecondary ion currents of element E are identical, even if X sputters muchfaster than Y.

5.9.3.3 Useful yieldIf the amount of sample is limited or the sampling volume has to be small, theappropriate figure of merit is the useful yield, tu: It is defined as the number ofdetected secondary ions/s, NqðEÞ; of element E per number, NðEÞ; of sputtered E(atoms/s) from the same sampling volume:

tuðAÞ ¼ NqðEÞ=NðEÞ ¼ SpðEÞ=Ytot ð5:4Þ

Using the previously introduced figures of merit, the fundamental SIMSformula can be alternatively written as:

NqðEÞ ¼ SaðEÞcðEÞ ½cps� ¼ SpðEÞNpcðEÞ ½cps� ¼ tuðEÞNpYtotcðEÞ ½cps� ð5:5Þ

where Sa is measured in counts per second (cps) and dimensionless units haveto be chosen for Sp and tu: When Sa; Sp or tu are known, Eq. (5.5) provides asimple means for calculation of elemental concentration, cðEÞ; from themeasured secondary particle current, Nq:

5.9.3.4 Sensitivity factorsQuantitation in SIMS can be achieved by external standards or by utilizing theconcept of sensitivity factors. Under scrupulously reproducible conditions ofanalysis, and using external standards with composition and microstructuresnot too different from the analyzed samples, useful calibration factors may beobtained. However, long-term instabilities in analysis (instrumental drift,changes in primary beam conditions, vacuum effects, crystalline effects) makethe use of absolute sensitivity factors hazardous. It is generally found to be bothvery feasible and more reliable to utilize the simultaneously measured ioncurrent, IR; of a matrix reference element, R.

It has been found that relative sensitivity factors (RSFs) remain practicallyconstant within quite wide ranges of concentrations, i.e., the differences areonly weakly dependent on concentration. Excellent quantitation with RSFs hasbeen reported, for example, for steels, binary alloys, glasses and semiconduc-tors. The dominant sources of variation and irreproducibility in absolute andRSFs are connected with the ionizability of the elements.

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5.10 GLOW DISCHARGE MASS SPECTROMETRY

5.10.1 Introduction

Glow discharge mass spectrometry (GD-MS) consists of the coupling of a glowdischarge atomization/ionization source with a mass spectrometer. As notedearlier, the relative simplicity of mass spectra compared with opticalspectra makes mass spectrometry an attractive alternative to opticalspectrometry for trace element analysis. Moreover, mass spectrometry permitsthe coverage of essentially the entire periodic table and, since the spectralbackground can be very low, detection limits are usually 2–3 orders ofmagnitude better by mass spectrometry than for optical atomic emission usinga glow discharge.

For over 50 years glow discharges have been known as ion sources for massspectrometry. The capability of generating a stable analyte ion populationdirectly from a solid sample, thereby precluding the problems of dissolution,dilution and contamination that may arise for techniques requiring solutionsamples, makes the glow discharge an attractive ion source for elemental massspectrometry of solids. The ability to obtain isotopic information across theperiodic table down to ng g21 detection limits, along with the developments ofimproved mass spectrometers with more reliable data acquisition and controlsystems, has made GD-MS a powerful tool, not only for research laboratoriesbut also for routine applications.

A wide variety of analytical glow discharge geometries have beeninvestigated as ion sources. Most GD sources, particularly the commercialversions, have used a direct insertion probe that permits certain flexibility insample shape, although pins or discs are normally used. In this configuration,the sample serves as the cathode of the glow discharge system and the cellhousing as the anode. Ions are sampled from the negative glow region throughan exit orifice. In Table 5.5, a comparison of the different sources is given.Hollow cathode glow discharges were coupled with a magnetic sector analyzerin preliminary investigations of analytical GD-MS [114,115]. Commercialinstruments employ a modified coaxial cathode geometry [116,117]. This is alsothe most widely characterized glow discharge ion source.

Whereas different glow discharge ion sources have not exhibited anysignificant performance differences, different methods of powering the sourcesshow specific performance differences. DC-powered sources are the mostcommon, even though RF-powered sources have been studied [118] and appliedas well as the pulsed sources [119].

The most widely used commercial GD-MS instrument is the VG9000 thatconsists of a DC-powered source, a double-focusing mass analyzer of thereverse Nier-Johnson geometry and Daly and Faraday cup detectors. Its cost isaround US $600,000.

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TABLE 5.5

Comparison of glow discharge sources

Source Voltage(V)

Current(mA)

Pressure(Torr)

Cathode Advantages Disadvantages

Hollow cathode 250–500 10–100 0.1–1.0 23 mm deepcylinderwith 5 mm diameterbase

High sputterIntense ion beamUseable for powders

Charge exchangeComplicated samplegeometry

Grimm 500–1000 25–100 1–5 6.5 mm diametercircle

Depth profilingEasy for compactedpowders

Only flat samples

Jet-enhanced 900 28 2.5 12 mm diametercircle

High sputter rateEasy for compactedpowders

Only flat samplesHigher dischargegas flow rate

Coaxial cathode 800–1500 1–5 0.2–2.0 1.5–2.0 mm diameter£ 4–8 mm long rod

Useable for varioussample shapesIonization dominatedby Penning process

Powders need to beconverted into solidsamples

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5.10.2 Glow discharge processes

A glow discharge is a partially ionized gas consisting of approximately equalconcentrations of positive and negative charges plus a large number of neutralspecies. It consists of a cathode and anode immersed in a low-pressure (<0.1–10 Torr) gas medium. Application of an electric field across the electrodescauses breakdown of the gas (normally one of the rare gases is used, typicallyargon) and the acceleration of electrons and positive ions towards theoppositely charged electrodes. Detailed description of the phenomena can befound in Refs. [120–122]. As an ion source for elemental mass spectrometry, theglow discharge is characterized by two attractive attributes, cathodicsputtering and Penning ionization, that are inherent to its operation. Cathodicsputtering generates a representative atomic population directly from the solidsample. Penning ionization selectively ionizes these sputtered atoms, permit-ting detection on the basis of their characteristic mass-to-charge ratios by massspectrometry.

5.10.2.1 AtomizationCathodic sputtering is the phenomenon that makes a glow discharge usefulin analytical spectrometry, providing the means of obtaining directly froma solid sample an atomic population for subsequent excitation and ionization.The sputtering involves directing an energetic particle onto a surface where,after collision, it transfers its kinetic energy in a series of lattice collisions.Atoms near the surface can receive sufficient energy to overcome the latticebinding and be ejected, generally as neutral atoms with energies in the rangeof 5–15 eV. The bombarding particles are normally ions, easily acceleratedby electrical fields. The sputter yield, defined as the number of ejected atomsper bombarding ion, depends critically on the mass and energy of theincoming ions. Under the operating conditions of most analytical glowdischarges, the sputter yield can be described as a function of kinetic energyand mass of the bombarding atom as well as of the lattice binding energy andmass of the target atoms. A related value is the sputtering rate, namelythe number of target ions sputtered per unit time. This value is determined bythe discharge operating current as well as the factors affecting the sputteryield.

5.10.2.2 IonizationThe glow discharge sputtering can introduce into the plasma a representativepopulation of the sample (cathode) ions. A fraction of them needs to be ionizedfor further elemental analysis by mass spectrometry. The discharge mustthen act as ionizing medium and must, of course, sustain itself. The fact thatGD-MS does not utilize optical transitions of the analyte atoms, ratherthe mass-to-charge ratio of the atoms that have been ionized, shifts theemphasis from excitation mechanisms to ionization mechanisms, specifically

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simplifying, to some extent, the relationship between analyte signal andanalyte concentration in the sample. Figure 5.5 shows schematically theprocesses in a GD [123] and in Table 5.6 ionization processes in glow dischargesare summarized.

Whereas we can assume that atomization does not differ significantlybetween elements in a given matrix, we cannot assume the same to be true forionization. Therefore, RSFs used for quantitative analysis are most likelycontrolled by differences in the probability of ionization among the elements.The RSF of an analyte element, E, is the ratio of its sensitivity to the sensitivityof some reference element. Sensitivity is defined as the intensity (I) of thesignal per unit of concentration (C):

RSFE=R ¼ ðIE=CEÞ=ðIR=CRÞ ð5:6Þ

The RSFs consider the contributions arising from instrumental factors, such asion transmission and sensitivity, and glow discharge processes, suchas differential atomization and differential ionization. The dominant contributionis related to the glow discharge processes and varies from sample to sample.

5.10.2.3 QuantificationThe mass spectrum obtained by GD-MS can be used directly for asemiquantitative measurement of the sample composition. One method isbased on the ion-beam ratio (IBR) [124]. In this procedure, the ion signals for all

Fig. 5.5. Processes in a glow discharge (Reprinted from Spectroscopy Europe, 15(3)(2003) 15).

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ionized sputtered species are summed and then the ratio of the ion signalfor individual species is calculated, which corresponds to the concentration ofthe species in the bulk. Since the IBR depends on the sensitivity of the differentelements, which varies by less than a factor of 10, this method can only providereliable means for semiquantitative analysis.

The signal intensity of the plasma species is influenced by several factors.Among these are: sample composition, matrix type, discharge power, cathodegeometry, cooling effects, discharge gas, source pressure, ion transmission, thetype of mass spectrometer and the detection system. Because of all this, forquantitative analysis, the use of standards is required for calibration. This canbe performed in two ways. The first consists of the construction of a calibrationcurve, based on a set of similar standards [125,126]. The second possibility isbased on the analysis of a reference material as similar as possible incomposition and behavior to the unknown sample, which allows the calculationof the RSFs [127]. Since suitable certified standards are not always available,powdered samples may be doped with an element of known concentration to beused as an internal standard.

5.10.3 Applications to trace element analysis

GD-MS has taken the place of spark source mass spectrometry (SS-MS) for theanalysis of trace elements in solid samples. In comparison to SS-MS, GD-MSpresents many advantages, for instance, a simple source producing a stablesupply of low energy ions characteristic of the sample and minimal matrixeffects.

In a DC powered GD-MS instrument, the samples must be conductive;therefore, bulk metals are the most ideal samples even though non-conductivesamples can also be analyzed. In this case, the samples need to be mixed with abinder material [127] (Ag, Ti or Ta) or the technique of the secondary cathodecan be applied [128]. Sample spectra may be obtained in a short time (min)and rapid qualitative analysis can be performed by the examination of the

TABLE 5.6

Ionization processes in the glow discharge

Electron impact ionizationa A þ e2 ! Aþ þ 2e2

Penning ionizationb Arm þ X ! Ar þ Xþ þ e2

Associative ionizationb Arm þ X ! ArXþ þ e2

Symmetric charge exchangeb Aþ þ A ! AþAþ

Asymmetric charge exchangeb Aþ þ B ! AþBþ

aCollisions of the first kind.bCollisions of the second kind.

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isotopic lines. Quantitative analysis can also be achieved, as previouslyexplained.

Semiconductors are another important category of bulk samples that can beanalyzed by GD-MS. The electrical properties of these materials are criticallydependent on the intrinsic and doped levels of impurities, making it essential toknow, not only qualitatively their chemical composition, but also theconcentration level of each element. Even extremely low concentrations of aspecific element can alter semiconductor properties.

GD-MS is always a surface analysis technique, even though it permitsmeasurement of bulk concentrations. That is, the sputtering process central tothe glow discharge acts as an atomic mill that regularly erodes away thesurface of the bombarded sample. Whatever atoms are sputtered away from thesurface are measured and, because GD-MS consumes significant quantities ofmaterial (up to milligrams per minute), these sequential layer analyzescombine to yield an averaged composition that is typical of the bulkconcentration. By slowing down the ablation process limiting measurementto a shorter duration, data indicative of the surface concentrations can beobtained. GD-MS and its optical analogue have found considerable applicationfor the analysis of layered samples.

Environmental samples can also be analyzed by GD-MS. In these cases thesamples need to be compacted with or without conducting material [129].Where a binder of conducting material is not added during the compaction ofthe samples, a secondary cathode has been used for their analysis [126].

5.11 X-RAY FLUORESCENCE SPECTROMETRY

5.11.1 Introduction

X-ray spectrometric techniques have been very useful in providing importantinformation for theoretical physicists and have found increasing exploitation inthe field of material science characterization.

Today, most stand-alone X-ray spectrometers use X-ray excitation sourcesrather than electron excitation. X-ray fluorescence spectrometers typically usea polychromatic beam of short wavelength X-radiation to excite longerwavelengths, characteristic lines, from the sample to be analyzed. In modernX-ray spectrometers, single crystals are used to isolate a narrow energy bandfrom the polychromatic radiation excited from the sample. This method iscalled “wavelength” dispersive spectrometry. The other possibility is to use aproportional detector to isolate a narrow energy band from those obtained fromthe excited sample. This method is called “energy” dispersive spectrometry.Since the relationship between emission wavelength and atomic number isknown, isolation of individual characteristic lines permits the uniqueidentification of an element and elemental concentrations can be estimatedfrom characteristic line intensities [130].

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X-ray fluorescence spectrometry provides the means for the identification ofan element by measurement of its characteristic emission wavelength orenergy. The quantitative estimation of an element is possible by first measuringthe emitted characteristic line intensity and then relating this intensity to theelemental concentration. A benefit of using X-ray emission spectra forqualitative analysis is that, because the transitions arise from inner orbitals,the effect of chemical combination or valence state is almost negligible.


Several different types of sources have been employed for the excitation ofcharacteristic X-radiation, including those based on electrons, X-rays, g-rays,proton and synchrotron radiation. By far, the most common source today is anX-ray photon source. This source is used in primary mode in the wavelengthand primary energy dispersive system, and in secondary fluorescer mode insecondary target energy dispersive spectrometers.

An X-ray detector is a transducer for converting X-ray photon energy intovoltage pulses. Detectors work by a process of photoionization, in whichinteraction between the incoming X-ray photon and the active detector materialproduces a number of electrons. The current produced by these electrons isconverted to a voltage pulse by capacitors and resistors, such that one digitalvoltage pulse is produced for each entering X-ray photon. The most importantcharacteristics of the detector are proportionality and linearity [131].

In the case of wavelength dispersive spectrometers, a gas flow proportionalcounter is generally employed for the measurement of longer wavelengths, anda scintillation counter is generally used for shorter wavelengths. Neither ofthese two detectors has sufficient resolution to separate multiple wavelengthson its own, and has to be employed with an analyzing crystal. However, in thecase of energy dispersive spectrometry, where no dispersing crystal is used, adetector of higher resolution must be used, generally the Si(Li) detector [132].

5.11.3 Matrix effects

In the conversion of net line intensity to analyte concentration, it may benecessary to correct for absorption and/or enhancement effects. As forabsorption, primary and secondary absorption needs to be considered. Primaryabsorption occurs because all atoms of the sample matrix absorb photons fromthe primary source. Since there is a competition for these primary photons by theatoms making up the sample, the wavelength distribution of intensity of thephotons available for excitation of a given analyte element may be modified byother matrix elements. Secondary absorption refers to the absorption ofcharacteristic analyte radiation by the specimen matrix. As characteristicradiation is released from the sample in which it is generated, it is absorbed by allmatrix elements in amounts relative to their mass attenuation coefficient.

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Enhancement effects of the total absorption occur when a non-analyte matrixelement emits a characteristic line of energy just in excess of the absorption edgeof the analyte element. This means that the non-analyte element is able to excitethe analyte, giving characteristic photons in addition to those produced by theprimary continuum. This results in an enhanced signal from the analyte [133].

5.11.4 Quantitative and trace analysis

The simplest challenge for quantitative analysis is the determination of a singleelement in a known matrix. In this case, calibration curves can be obtained.When the matrix is unknown, quantitative analysis is based on the use ofinternal standards, addition of standards and use of a scattered line from the X-ray source.

The most complicated case is the analysis of all, or most, of the elements in asample whose matrix is unknown. In this case, a full qualitative analysis wouldbe required before any attempt is made to quantitate the matrix elements.Once the qualitative composition of the sample is known, one of the followingthree techniques can be applied: type standardization, influence coefficientmethods, fundamental parameter techniques. The last two methods require acomputer for their application [134,135].

One of the problems with any X-ray spectrometer system is that theabsolute sensitivity (i.e., the measured cps per% of analyte element) decreasessignificantly as the lower atomic number region is approached. This is, aboveall, due to the fact that the fluorescence yield decreases with the atomicnumber, the absolute number of useful long-wavelength X-ray photons fromthe radiation source decreases with increasing wavelength and absorptioneffects become more severe with increasing wavelength of the analyte.

The X-ray fluorescence method is particularly applicable to the qualitativeand quantitative analysis of low concentrations of elements in a wide range ofsamples as well as to the analysis of elements at higher concentration in a smallamount of sample. Moreover, X-ray fluorescence is often used as a non-destructive qualitative method for a multi-element content evaluation prior toquantitative analysis with another method.

X-ray spectrometric methods based on total reflection geometry [136] havegained widespread strength in the past decade, principally because of theirdetection power (92–10 pg, 100 pg ml21 relative), quasi matrix independentcalibration (internal standard), multi-element capability and non-destructivenature. For trace element analysis the sample is prepared on a totally reflectingsample carrier as a small quantity of residue from solutions or fine-grainedsuspensions from the evaporation of a solvent and forms a layer of a fewmicrons thick. It is thus a micro technique, achieving its performanceadvantage as a consequence of the attenuation of background due to the highangle reflection geometry employed.

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5.12 UV/VISIBLE SPECTROPHOTOMETRIC ANDCHEMILUMINESCENCE TECHNIQUES

5.12.1 UV/Visible spectrophotometric techniques

Many metals form compounds or complexes that give rise to an ultraviolet orvisible absorbance spectrum. It is therefore sometimes possible to react themetal ion within a liquid sample (or a solid sample that has previouslyundergone a digestion or dissolution stage) with a reagent that will selectivelychange the optical properties of the sample. Many of the basic componentsrequired for such an instrumental based technique are similar to those in anatomic absorption instrument. However, the light source is different. Ratherthan using a HCL that produces a spectrum containing a series of discretelines, a continuum source produced by a tungsten–halogen lamp, for example,is used that has a broad band of light output. For UV applications (,340 nm), adeuterium lamp is usually used as a source.

The sample cell is frequently a cuvette, manufactured from glass or plastic(for wavelengths above 340 nm) or quartz for the UV region. The sample maysimply be poured into the cuvette for absorbance readings to be made. Insteadof a cuvette, gas cells are also available. The monochromator is usually fairlybasic, but operates in the same way as the atomic absorption spectrometers.Again, detection is usually via a photomultiplier tube.

The theory behind molecular absorption is analogous to atomic absorption,i.e., analyte molecules absorb light and electronic transitions occur within themolecule. A greater emphasis on the theory may be found in the literature[137]. It should be noted, however, that molecular absorbance occurs over awide band of wavelengths rather than a very narrow line, as in atomicabsorption. This means that, in many cases, even if detection is not at exactlythe most intense wavelength, some results may be obtained, although thesystem will not give optimal sensitivity.

As with all absorbance techniques, the Beer–Lambert law (Eq. (5.7))applies and, since this law is true over only a relatively small concentrationrange, the calibration will be linear over a range of perhaps two orders ofmagnitude.

A ¼ 1CL ð5:7Þ

where A is absorbance, 1 is the molar extinction coefficient (also called themolar absorptivity constant), C is the concentration and L is the path length.

It can be seen from Eq. (5.7) that, in addition to concentration, theabsorbance is dependent on the path length and the molar extinctioncoefficient. The path length is usually 1 cm for most liquid cuvettes, but maybe substantially larger for gas cells. The molar extinction coefficient is anumber whose numerical value is different for each metal–complex system, butis regarded as being a constant for each. The higher the value of 1, the greater

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the absorption of light by the compound formed, and hence the better thesensitivity.

One of the major problems with UV/Visible spectrophotometric methods isthat of interferences. Often, more than one metal ion will combine with acomplexing agent and the absorption spectra from the two species may overlap.This is shown diagrammatically in Fig. 5.6. It can be seen that even a smallcontribution from one interfering species could potentially lead to inaccuracyand since the interferences are additive, the presence of several concomitantscould lead to large errors. Assuming that all of the potential interferenceswithin a sample are known, it is possible to use simultaneous equations ormathematical algorithms to correct for the relative contributions from each. Anexample is the simultaneous determination of chromium and manganese insteel samples [1]. It should also be noted that pH may affect the absorptionprofile of the analyte complex and it is thus often necessary to buffer thesamples and standards to the same pH. If the analyte forms a very stablecompound with another concomitant, then it may not be available to form thecomplex and a low analytical result would occur. It may occasionally benecessary to treat the sample in some way to selectively bind the interferingspecies or to destroy it.

The linear range for these methods usually covers no more than two ordersof magnitude. The LOD will depend on the molar extinction coefficient, but forsome of the more sensitive methods may be as low as 10–50 ng ml21. Samplethroughput can be very rapid, with up to 5–10 samples per minute beingdetermined. Although gaseous samples may be analyzed, the norm is for thesample to be in a liquid form. Since the method of detection is through lightabsorption, it is necessary for the samples to be free from particulate matter.This may be achieved either by centrifugation or by filtering. Since the lightbeam in most instruments will pass through the bottom third of the cuvette, avolume of 1 ml is usually sufficient for the measurement step. It is necessary to

Fig. 5.6. Overlap of absorption spectra.

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remember that the sample must often be combined with a known volume ofcomplexing reagent and buffer prior to the measurement step, and thereforethe actual amount of sample used may frequently be less than 1 ml.

The UV/Visible instrumentation described thus far has concerned stand-alone spectrometers that may also be used to scan over a wavelength range sothat absorption maxima may be identified. If a flow-through UV/Visibledetector is used, then transient signals may be detected. Such instrumentationmay be used as a detector for chromatography, but is predominantly used forautomated high throughput analyzes, e.g., with FI techniques. If the sample,compleximetric reagent and buffer are mixed on-line and allowed to passthrough the detector, a transient signal will be obtained which may be recordedusing either a chart recorder (if peak height is to be measured) or an integrator.The same problems arise with interferences, but the major advantage of thisapproach is that the instrumentation is more portable and may be taken intothe field (assuming an adequate power supply is available) or aboard a ship.Therefore, savings in time are made and the disadvantages associated with thetransport of samples back to a laboratory (with possible loss of analyte throughadsorption or breakage of storage vessels, contamination from preservingagents, etc.) may be overcome. An example of such a method has been publishedby Hernandez et al. [138]. If a micro-column of resin is used to entrap theanalyte prior to mixing with the compleximetric agent, then matrix removaland pre-concentration may be achieved. Sample consumption will be dependentupon the size of the sample loop, but is typically 100–200 ml per injection, butthis will increase markedly if a pre-concentration technique is used.

If several analytes require determination and they all form a complex with aparticular reagent, it may be possible to determine them using liquid chroma-tography coupled with UV/Visible detection. A recently published paper hasoutlined the determination of several analytes (Th, V, Bi, U, Hf and Zr) using a10 cm column of neutral polystyrene loaded with dipicolinic acid and 1 Mpotassium nitrate as an eluent [139]. This application yielded LODs ofsubstantially less than 1 mg l21 for several analytes. A similar study reportedthe determination of a selection of transition and heavy metal ions using apolystyrene–divinyl benzene column loaded with 4-chlorodipicolinic acid and aneluent of 0.25 mM chlorodipicolinic acid in 1 M potassium nitrate at pH 2.2 [140].

5.12.2 Molecular fluorescence and chemiluminescence detection

Molecular fluorescence detection is less commonly used because it is necessaryto combine the analyte with a suitable reagent capable of fluorescing, whilstensuring that other metal species present in the sample do not. Despite thesedisadvantages, the technique can give LODs in the ng ml21 range, is moreselective than absorption and, when a chromatographic technique is used toseparate potential interferences from the analyte, excellent selectivity may beachieved and sample consumption need only be 50–200 ml. A recent example of

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such an application compared ICP-MS and fluorescence detection for Al speciesthat had been separated using chromatography [141].

Chemiluminescence is a form of emission, but is produced from the energyoriginating from a chemical reaction rather than the absorption of light. Sinceit is related to fluorescence, it possesses the same analytical advantages,including long linear ranges and very good sensitivity. Most methodologies useeither FI or chromatography as a sample introduction method. As usual, FImethods tend to be very inexpensive and easily portable, although for this typeof work they are also usually designed to determine only single analytes. Again,the sample must be in a liquid form, so soils and other solid materials must firstbe digested. Typically, the analyte is isolated on a mini- or micro-column of aresin. This will often mean that the sample must be buffered to a specific pHand reagents added to ensure that other concomitant species are not retainedsince, on elution, these may react with the chemiluminescent reagents andpotentially interfere. An example of a FI method has been published byAchterberg et al. [142], who determined Cu in seawater. This application alsoemphasized a method for overcoming another common problem experiencedwith this type of methodology. The presence of humic acids or other chelatingcompounds will usually mean that the analyte will complex with these inpreference to the resin in the micro-column, resulting in poor analyte retentionand its passage directly through the system undetected. The use of UVphotolysis (possibly in the presence of hydrogen peroxide) prior to analysis canhelp destroy the organic material and will therefore render the Cu (or otheranalytes) available for detection. In this particular application, the UVdigestion was performed on-line and, although less efficient than batchirradiation, was sufficient to enable successful determination whilst beingsubstantially more rapid. The use of a UV digestion will inevitably increase thelength of time required for the analysis, but even with a digestion time of 5 or6 min, 5–10 samples per hour may be analyzed. As with all FI techniques, pre-concentration is a possibility, and the pre-concentration process may limit thesample throughput further. An example of simple FI–chemiluminescencedetection without the need for UV digestion has been reported by Bowie et al.[143]. These workers determined Fe in seawater by reducing Fe(III) to Fe(II)using sulphite, and then retaining/pre-concentrating the whole Fe content ofthe sample on a column of 8-hydroxyquinoline. On elution, the Fe was reactedwith luminol for the chemiluminescence detection. Limits of detection werefound to be sub-nM, precision was 3.2% RSD and the whole analytical cycletook 3 min, enabling a sample throughput of close to 20 samples per hour. Theportability of the technique was demonstrated by the shipboard determinationof Fe in the Atlantic.

Chemiluminescence may also be used as a means of detection for liquidchromatography. The relative advantages of this are the same as thosediscussed for UV/Visible detection, with the added bonus of the extrasensitivity. Several papers have reported the determination of trace metals

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using chemiluminescence as a method of detection. Included amongst these isthe determination of silver [144]. After elution from a cation exchange column,the silver was detected by a novel post-column reaction system involving theoxidation of luminol by peroxodisulfate. The system yielded a LOD of 0.5 mg l21.A similar paper by the same authors managed to speciate Cr(III) and Cr(VI)in fresh waters [145]. This system enabled LODs of 0.05 and 0.1 mg l21 to beobtained for Cr(III) and Cr(VI), respectively, whilst yielding a linear range of0.1–500 mg l21. Precision at the 10 mg l21 level was 5% RSD. Multi-elementdeterminations are also possible using such systems. An example that yieldedmg l21 LODs for several analytes in 15 min has been reported [146].

5.13 ELECTROCHEMICAL METHODS

5.13.1 Differential pulse anodic stripping voltammetry

A liquid sample is placed in a sample cell along with a suitable buffer, and thena hanging mercury drop electrode, a reference electrode (usually a saturatedcalomel electrode) and a platinum wire counter electrode are immersed in thesample. The sample is purged with an inert gas to de-oxygenate it becauseoxygen causes an interference effect. A negative voltage may then be applied tothe mercury drop. The sample is stirred magnetically, and some of thepositively charged metal ions in the sample will diffuse to the mercury drop andplate onto and diffuse into it. The period during which this occurs is termed theplating time. After this, a brief relaxation period occurs, followed by a period inwhich the potential applied to the mercury drop becomes increasingly lessnegative. Each analyte ion will be stripped or oxidized from the mercury drop atits own reduction potential and will re-enter the solution. As they re-enter theliquid phase, they are detected by the counter and standard electrodes and willappear as a series of peaks. The area under each of the peaks is proportional tothe concentration of that particular ion in the sample. It is the length of theplating time that determines the overall sensitivity of the analysis. Shortplating times will be insufficient for many of the analyte ions within the sampleto become significantly accumulated in the mercury drop, whilst longer timeswill enable greater sensitivity to be obtained, but at the expense of decreasedsample throughput. The overall sensitivity will be limited by the contaminationwithin the buffer system and by time constraints. Detection limits substantiallybelow 1 ng ml21 may readily be obtained for Cd, Cu, Pb and Zn using a platingtime of just a few minutes, but plating times of over an hour are known, whichyield exceptionally low LODs. The technique is multi-elemental, withapproximately 20 metallic ions being detected. Despite this, the fouraforementioned ions are most commonly determined by this method. Anotheradvantage of the technique is that it is capable of determining different speciesof an element. An example has been the determination of assorted tin species,

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although an ion exchange procedure was also required for full speciationbecause many of the tin species produced only one peak [147]. Since only a verysmall portion of the analyte ions in the sample plate onto the mercury drop, themethod is very vulnerable to errors caused by differences in plating times,stirring rates, temperatures and current density between samples andstandards. It is this last potential problem that often leads to the method ofstandard additions being used to calibrate the process. The plating time isobviously the limiting factor for sample throughput, but if three standardadditions are made to each sample, then rarely more than 5–10 samples maybe analyzed per hour. Sample consumption is typically 1–10 ml, depending onthe volume of the sample cell and on the volume of buffer/diluent added. Sincemany instruments possess a spiking port, the standard additions may be madeto the same sub-sample. Precision is typically ,5% RSD. Care must be takenwith the stirring of the sample, as the hanging mercury drop is easily dislodged.If stirring is too vigorous and the drop is dislodged it may be necessary(depending on whether plating has begun) to start the analysis again with anew sub-sample. For this reason, some workers prefer glassy carbon workingelectrodes and co-plating mercury from mercury (II) chloride added to thesample solution to produce the thin film mercury electrode. Another drawbackwith the technique is that the presence of organic matter, that may complexwith many of the ions, will lead to a decrease in the plating efficiency (i.e., theanalytes are kept in solution as a complex and are not available for analysis).This will potentially lead to an underestimate of the true analyte concen-tration. It is possible to differentiate between the free ions and those complexedwith organic matter if an analysis is first made on an untreated sub-sample toyield the concentration of the free ions. A second analysis made on another sub-sample that has been treated with UV radiation, destroying the organic matterwill yield the “total” concentration of the analytes. The presence of highconcentrations of some metal ions may also lead to overlap of the peaks or to theformation of intermetallic compounds.1

5.13.2 Cathodic and adsorptive stripping voltammetry

This is analogous to ASV, and may usually be achieved using the sameinstrumentation. Cathodic stripping voltammetry (CSV) is used less for metalion determinations than ASV simply because there are fewer negativelycharged metal ions, but metals may be adsorbed as their complexes (e.g., Niwith dimethylglyoxime). The obvious exceptions are the metalloids, e.g.,arsenic, selenium, etc., and some of the transition elements (i.e., those that

1 Methods for the determination of the metals aluminum, cadmium, chromium, cobalt,iron, lead, nickel, uranium, vanadium and zinc in marine, estuarine and other waters bystripping voltammetry or concentration and atomic absorption spectrophotometry,HMSO, London, 1987.

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form negatively charged complexes). This technique shares many of therelative advantages and disadvantages of ASV. Again, speciation is possiblewith As(III) and As(V) species having been determined [148].

5.13.3 Ion selective electrodes

Ion selective electrodes (ISE) are so termed because they are selective for aspecific ion. Some texts describe them as ion specific electrodes, but this ismisleading because they are not specific. Most suffer from interferences in thatthey respond to the presence of other species in solution, although to a lesserextent than to the ion they are designed to detect. Examples of theseinterferences include Hþ, Csþ, Liþ and Kþ on the Na ISE and Zn2þ, Pb2þandMg2þ on the Ca ISE. Additionally, the presence of organic matter, such ashumic acids that may complex with the analytes of interest, may prevent themfrom being detected and hence, an underestimate of the true concentration willbe made, although it should be noted that the electrode is truly responding toactivity.

Several of these electrodes exist, but they do not exist for every metallicelement. Their response is based on the Nernst equation (Eq. (5.8)).

E ¼ Eu þ2:303 RT

zFlog ½ion� ð5:8Þ

where R is the gas constant, T is temperature, z is the charge of the species ofinterest, F is the Faraday constant and [ion] is the concentration of the analyte.

As can be seen from Eq. (5.8), the response is dependent upon temperatureand on the charge of the species under investigation, although at 258C and for asingly charged analyte, a change of 59 mV per decade of concentration isobtained. For a doubly charged analyte, e.g., Ca2þ at the same temperature, achange of 29.5 mV per decade should be generated if the electrode is Nernstianin response. Calibration curves can be plotted on semi-log paper, and can coverfive or six orders of magnitude. Detection limits will depend on the individualISE, but are often at the ng ml21 range; with copper, for example, the ISE islinear over the range 1028–0.1 M (i.e., 6.4 £ 1024–6350 mg l21).

The electrodes are readily portable and may easily be used in conjunctionwith a data logger for unattended operation in the field. Assuming thetemperature of the water body is monitored simultaneously, a simple algorithmcan be used to correct the data. It should be noted however, that any changes inionic strength of the sample may lead to interferences (depending on the ISE).Therefore, for some ISEs, most samples (and standards) are mixed with totalionic strength adjustment buffer (TISAB), a pH buffer that also contains aninert salt to fix the ionic strength.

Normally, liquid samples are required and so solid samples have to beextracted, digested or dissolved such that the analytes are present in a solution.The volume of sample can be low (5 ml or less), but one of the advantages of the

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technique is that it does not consume any of the sample. Assuming thatmeasures are taken to prevent contamination, the sub-sample used for thisanalysis may be used for other analyzes. The technique is also relatively rapid;once immersed in the sample, a brief equilibration time is allowed so that astable signal is obtained, but this still allows two determinations to be made perminute. An example in which ISEs have been used to determine metal ions hasbeen published by Vazquez et al. [149]. In this paper, Na, K and Ca weredetermined in wood pulp suspensions and the results compared with thoseobtained by ICP-OES and XRF.

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