Atomic Absorption Spectrometry

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Current Status of Direct Solid Sampling for Electrothermal AtomicAbsorption Spectrometry—A Critical Review of the Development between1995 and 2005Maria Goreti R. Valea; Nédio Oleszczuka; Walter N. L. dos Santosb

a Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil b CampusProf. Soane Nazaré de Andrade, Universidade Estadual de Santa Cruz, Rodovia Ilhéus-Itabuna, BA,Brazil

To cite this Article Vale, Maria Goreti R. , Oleszczuk, Nédio and Santos, Walter N. L. dos(2006) 'Current Status of DirectSolid Sampling for Electrothermal Atomic Absorption Spectrometry—A Critical Review of the Development between1995 and 2005', Applied Spectroscopy Reviews, 41: 4, 377 — 400To link to this Article: DOI: 10.1080/05704920600726167URL: http://dx.doi.org/10.1080/05704920600726167

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Current Status of Direct Solid Samplingfor Electrothermal Atomic AbsorptionSpectrometry—A Critical Review of theDevelopment between 1995 and 2005

Maria Goreti R. Vale and Nedio Oleszczuk

Instituto de Quımica, Universidade Federal do Rio Grande do Sul,

Porto Alegre, RS, Brazil

Walter N. L. dos SantosUniversidade Estadual de Santa Cruz, Campus Prof. Soane Nazare de

Andrade, Rodovia Ilheus-Itabuna, BA, Brazil

Abstract: The literature about direct solid sampling (SS) and slurry sampling atomic

absorption spectrometry (AAS) over the past decade has been surveyed critically. It

became apparent that a very significant change had occurred, particularly in the

relation between the two major techniques used for that purpose. In the 1990s, slurry

sampling was typically considered the technique of choice, combining the significant

advantages of the solid and the liquid sampling methods, at least in part because of

the availability of a commercial accessory for automatic slurry sampling. The

situation is completely inverted now, as the above accessory has been discontinued

and rugged and reliable accessories for direct SS became available. Direct SS electro-

thermal (ET) AAS has been shown to provide the best limits of detection because of the

absence of any dilution and a minimal risk of contamination. Calibration against

aqueous standards appears to be feasible after careful program optimization. The

absence of any significant sample handling makes SS ET AAS ideally suited for fast

screening analyses. The introduction of high-resolution continuum source AAS

appears to open additional attractive features for SS ET AAS because of the signifi-

cantly simplified optimization of furnace programs and the visibility of the spectral

environment, which makes it easy to avoid spectral interferences. New calibration

Received 12 December 2005, Accepted 1 February 2006

Address correspondence to Maria Goreti R. Vale, Instituto de Quımica, Universi-

dade Federal do Rio Grande do Sul, Av. Bento Goncalves, 9500, 91501-970,

Porto Alegre, RS, Brazil. E-mail: [email protected]

Applied Spectroscopy Reviews, 41: 377–400, 2006

Copyright # Taylor & Francis Group, LLC

ISSN 0570-4928 print/1520-569X online

DOI: 10.1080/05704920600726167

377

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strategies make a “dilution” of samples unnecessary, which used to be one of the major

limitations of SS ET AAS. Finally, direct SS analysis is an important contribution to

clean chemistry, as practically no reagents are used.

Keywords: Solid sampling, slurry sampling, electrothermal atomic absorption spec-

trometry, high-resolution continuum source atomic absorption spectrometry, clean

chemistry

INTRODUCTION

The atomic absorption spectrometry (AAS), as it has been proposed for chemical

analysis by Walsh in 1955 (1) was intended for solution analysis, using a

pneumatic nebulizer; a spray chamber, where the sample aerosol was mixed

with the oxidant and fuel gas; and a laminar flame for atomization. The use of

solutions for chemical analysis undoubtedly has a number of distinct advantages.

Solutions are easy to handle with conventional laboratory equipment, they can

readily be diluted to bring the analyte concentration within the optimum

working range, and the addition of buffers, releasers, and other reagents is

straightforward. The introduction of solutions into all kinds of atomizers can

easily be automated, and to bring a sample into solution is obviously the most

efficient way of homogenization, resulting in excellent repeatability and

precision. However, sample preparation for solution analysis is usually labor

intensive and time consuming, and it is the major source of errors (2), mostly

due to analyte loss and/or contamination. Sample digestion might also

include health hazards for the laboratory personnel, and it usually produces a

lot of corrosive and/or toxic waste. Last but not least, any digestion or dissol-

ution is inevitably associated with a dilution of the analyte content, making

solution analysis not necessarily suitable for determination of the lowest trace

concentrations. Direct solid sample analysis has therefore always been investi-

gated as an alternative in order to avoid the above limitations.

Two of the three sample introduction and atomization techniques

routinely used in AAS, i.e., the flame technique and chemical vapor generation

(CVG) are not particularly suited for solid sample analysis for different

reasons. Conventional nebulizers can handle suspended solids only in excep-

tional cases without a high risk of clogging, and the flames used in AAS are

only capable of vaporizing extremely small particles, as the time available

for atomization is of the order of a few milliseconds, and the temperatures

are only between 2250 and 27008C. The few successful attempts for the

analysis of slurries (3–5) or the direct analysis of solid samples (6–9) using

flame atomization require specially designed accessories that are not commer-

cially available, limiting these approaches to research.

The application of slurry sampling has been tried for both CVG AAS

techniques, the cold vapor (CV) generation of mercury (10, 11) and hydride

generation (HG) AAS (12–14), and more recently also with inductively

M. G. R. Vale, N. Oleszczuk, and W. N. L. dos Santos378


coupled plasma optical emission spectrometric (ICP OES) and mass spectro-

metric (ICP-MS) detection (15–19). However, at least for mercury, it has been

shown that complete extraction of the analyte into the liquid phase is a pre-

requirement for CV AAS from slurries; i.e., this cannot really be considered

solid sample analysis. An additional problem arises for HG AAS, as it is

well known that for Group V elements (As, Sb, Bi) the oxidation state þ5

reacts much slower and results in lower sensitivity than the oxidation state

þ3, and for the Group VI elements (Se, Te) only the oxidation state þ4 is

forming a hydride. In addition, most of the hydride forming elements are

often present in biological and environmental materials as stable organic

compounds, which do not form hydrides, and require harsh digestion con-

ditions to be converted to the inorganic form. All this is obviously a source

of error in slurry sampling CVG techniques, as no pre-reduction or harsh

digestion is involved in solid sample analysis. These techniques will

therefore no longer be considered here.

This means that there remains electrothermal (ET) AAS with graphite

furnace (GF) atomization as the only technique to be considered for solid

sample analysis with AAS. Direct solid sample analysis was actually the

first application reported for ET AAS (20) back in 1957, when L’vov put a

“pinch of NaCl” into a graphite furnace that was standing unused in a corner

of his laboratory and watched how the radiation of a sodium lamp, passing

through the furnace, disappeared upon heating the furnace. Solid sampling

(SS) analysis has been used ever since in ET AAS (21), as graphite furnaces

allow a relatively easy introduction of solid samples, and the atomization

times are some three orders of magnitude longer than in typical flames,

making possible complete vaporization and atomization of even relatively

large amounts of sample. However, in spite of the obvious advantages of

direct SS analysis, which are a significantly reduced sample preparation time

and hence a faster analysis; higher accuracy, as errors due to analyte loss

and/or contamination are dramatically reduced; higher sensitivity due to the

absence of any dilution; and the absence of any corrosive or toxic waste, the

method has not been really accepted until the end of the 20th century. The

major arguments against direct SS analysis using ET AAS have been (a) the

difficulty of handling and introducing the small sample mass, although a

wide variety of tools has been developed for that purpose (22); (b) the difficulty

of calibrating, which usually requires solid standards of similar composition

and analyte content compared to the samples to be analyzed; (c) the limited

linear working range of AAS and the difficulty of diluting solid samples;

and (d) the high imprecision of the results due to the inhomogeneity of

natural samples, which requires a large number of repetitive determinations.

Slurry sampling ET AAS, in contrast to direct solid sampling, has been

accepted much earlier as an alternative technique for sample introduction,

mostly because of the availability of a commercial autosampler, the USS-

100 (Perkin Elmer) for that technique (23). It therefore appears worthwhile

to compare these two approaches regarding their pros and cons.

Direct Solid Sampling—A Review 379


SLURRY VS. SOLID SAMPLING ET AAS

The book of Kurfurst (22) summarizes the literature about solid and slurry

sampling up to the mid 90s, and Cal-Prieto et al. (24) published a review of

the literature about slurry sampling for ET AAS from 1990 to 2000, so that

only the most important aspects and recent development have to be considered

here. The breakthrough of slurry sampling ET AAS clearly came with the

introduction of the above-mentioned commercial equipment (23) that was

based on extensive research work done by Miller-Ihli (25), and which

consisted of a modified furnace autosampler, in which the slurries were hom-

ogenized by an ultrasonic probe immediately before their transfer to the

graphite tube. In a review article about solid sampling in ET AAS, which

covers the literature up to 1990, Bendicho and de Loos-Vollebregt (26)

came to the conclusion that “the slurry technique gives better analytical per-

formance than direct solid sampling”. This judgment was based on the facts

that (a) the slurry concentration can be easily changed so that the analyte con-

centration falls into the analytically useful range of the calibration graph; (b)

chemical modification is easier to perform in the slurry technique, because of

more effective contact between the sample particles and the modifier, and (c)

slurry introduction can be easily automated, hence combining the significant

advantages of the solid and liquid sampling methods. This latter sentence

has ever since been repeated in almost all publications about slurry

sampling without considering potential progress in the field of SS that

might relativize this statement.

One of the major problems of the slurry technique, which has already

been recognized by Bendicho and de Loos-Vollebregt (26), is the stabilization

of the slurries during the time until they are finally introduced into the graphite

atomizer. Although various techniques have been proposed for homogeniz-

ation of slurries, including the ultrasonic probe that is used in the USS-100,

particle size distribution, specific gravity of the sample, sampling depth, and

other factors were found to have an influence on the accuracy of the results

(27). Several authors have been investigating problems associated with

grinding time, particle size, and sedimentation errors (28–31), and a

particle size ,37mm appears to be necessary to obtain accurate results, at

least in the case of high-density materials. In contrast to sedimentation,

flotage of light and finely ground material, such as biological samples or

coal, on the surface has also been reported as a potential problem, which

requires the addition of a surfactant, such as Triton X-100 (30).

Another very important factor in slurry analysis is the quantity of analyte

that is extracted into the liquid phase, i.e., solubilized, during slurry prepara-

tion, which depends on the analyte, the matrix, the liquid phase, and the slurry

preparation conditions, and which can be between 0 and 100% (32–34).

Analytes are usually much more easily extracted from biological than

from inorganic materials, and 100% extraction of Cd, Cu, Mn, and Pb has

for example been reported using 1mol L21 nitric acid and ultrasonic



agitation (35). Usually, a high extraction rate is considered advantageous, so

that various authors have been using very high acid concentrations, including

concentrated hydrofluoric acid for slurry preparation of environmental

samples, such as fly ash, soil, and sediment (36–40), which obviously

results in a similar corrosive waste problem as with conventional sample

digestion. Cal-Prieto et al. (34) developed an analytical scheme for the deter-

mination of antimony in geological materials using slurry sampling ET AAS,

an element, for which the extraction rate to the liquid phase was negligible.

They found that the normalized absorbance remained constant up to a

sample mass of 50mg mL21, which corresponds to a sample mass of 1mg

in a 20-mL aliquot injected into the furnace. This corresponds to a typical

sample mass used also for direct SS, as will be discussed later. Normally,

however, significantly smaller sample aliquots are used for slurry sampling.

The same authors (34) also determined the minimum number of particles

that should be introduced into the graphite tube in order to ensure the repre-

sentativeness of the aliquot for the sample to be analyzed, and they

concluded that a minimum of 50 particles have to be introduced. This deter-

mines the limits of the dilution that can be applied in order to determine

higher analyte concentrations. This limit, similar to other problems such as

sedimentation, obviously depends on the quantity of analyte extracted to the

liquid phase, and disappears in the case of 100% extraction. Miller-Ihli (32)

has therefore introduced the term “representative sample mass”, which

takes analyte extraction into consideration. On the other hand, analyte extrac-

tion can also become a source of errors, as has been shown by Maia et al. (41)

for the determination of Tl in marine sediment samples. Due to the presence of

co-extracted chloride in the aqueous phase, extracted and added thallium was

lost at pyrolysis temperatures above 4008C as volatile TlCl, whereas the non-

extracted thallium was stable up to 9008C. Although the determination was

made with ICP-MS in this case, the reactions in the graphite furnace are

obviously independent of the detection system. This potentially different

behavior of the analyte in the liquid and in the solid phase has to be taken

into account as potential source of error particularly in cases where the

analyte addition technique is used to correct for differences in sensitivity

between the slurry and calibration solutions (34, 38). All these problems of

sedimentation, extraction and distribution of the analyte between two

phases are obviously non-existent in the case of direct SS analysis.

DIRECT SOLID SAMPLING ANALYSIS USING LINE SOURCE

ET AAS

The situation regarding the commercial availability of equipment for slurry

and direct SS has changed completely within the last decade. Firstly, there

has been the ongoing effort of Krivan and co-workers at the Department of

Analytical Chemistry and Maximum Purification at the University of Ulm,



Germany, to improve existing equipment for direct SS analysis (42, 43). The

accessory he developed, which included a pair of tweezers for the reproduci-

ble introduction of a graphite platform containing the solid sample into the

graphite tube has been further tested (44, 45) and introduced as a commercial

device in the late 1990s. Nowadays, the second generation of this solid

sampling accessory is available both in a rugged and inexpensive manual,

as well as in a fully automatic version, as shown in Figures 1a and 1b. In

the automatic version, the graphite platform with the solid sample is trans-

ferred first to an integrated micro-balance, and then into the graphite tube,

and the software automatically calculates the integrated absorbance normal-

ized for a sample mass of 1mg. The second event was the discontinuation

of the USS-100, the only commercially available autosampler for slurry

sampling, which obviously completely inverted the situation comparing

slurry and direct SS ET AAS, and also the conclusion, at which Bendicho

and de Loos-Vollebregt (26) had arrived some 15 years ago. It is therefore

time to reconsider the relation between the two approaches based on recent

research and advances in that field.

Figure 1. Accessories for direct SS ET AAS; (a) manual system SSA 5, consisting of

a pre-adjusted pair of tweezers for reproducible insertion of the SS platform; (b) SS

autosampler SSA 62 for automatic transfer of up to 62 SS platforms to a microbalance

and into the graphite furnace (by courtesy of Analytik Jena AG, Jena, Germany).

(continued)



Table 1 gives an overview over a number of representative publications in

the field of direct SS ET AAS over the last decade, as the earlier literature is

fully covered in the book of Kurfurst (22). The survey shows that a great

number of elements have been determined in a wide variety of organic and

inorganic matrices, including high-purity metals and metal oxides and

plastic materials. Even natural waters have been analyzed, co-precipitating

Cr with Pd/8-quinolonol/tannic acid (46) or As with Ni–ammonium pyrroli-

dine dithiocarbamate complex (47), analyzing the precipitate directly in order

to avoid dilution and potential errors in the dissolution step. One of the

most important contributions in the field of direct SS analysis undoubtedly

came from Krivan and co-workers (42, 43, 48–52) who showed in an

impressive number of publications that SS ET AAS, at least in the analysis

of high-purity materials, always gave the best limits of detection (LOD),

even compared to much more sophisticated techniques, such as neutron

activation analysis (NAA), ICP-MS, isotope dilution mass spectrometry

(IDMS), and others.

The advantage of direct SS ET AAS is most obvious for materials that are

very difficult to be brought into solution and for elements that are kind of

omnipresent, and hence most susceptible to contamination. Friese and

Krivan (43) determined seven trace elements in high-purity tantalum

powders using a sample mass up to 60mg and aqueous standards for

Figure 1. Continued.



Table 1. Selected publications about direct solid sampling ET AAS using LS AAS, covering the period 1995–2005

Analyte Matrix

Background

correction Calibration Modifier LODa Remarks Reference

Bi, Pb, Se,

Te, Tl

Nickel-based alloys Zeeman-effect Aqueous

standards

Ni n.a.b Single-alloy chips (58)

Cr Biological samples n.a. Addition

calibration

None n.a. Comparison of

calibration

(64)

Cu, K, Mg, Mn,

Na, Zn

Molybdenum metal,

molybdenum silicide

Deuterium Aqueous

standards

None 0.06–0.5 ng g21

0.5–4.5 ng g21Up to 80-mg sample

introduced

(48)

Cd Kidney, liver Zeeman-effect Aqueous

standards

None n.a. Wet animal tissue (62)

Cd, Cr, Mn, Pb Biological samples Deuterium Aqueous

standards

Pd (Cd only) n.a. Trace element

distributions

(63)

Cu, Fe, Mn, Na, Zn High-purity tantalum

powder

Zeeman-effect Aqueous

standards

None 0.1–27 ng g21 Up to 40-mg sample

introduced

(42)

Cu, Fe, K, Mn, Na,

Zn

High-purity tantalum

powder

Deuterium Aqueous

standards

None 0.02–4 ng g21 Transversely heated

atomizer

(43)

Ca, Co, Cr, Cu, Fe,

K, Mg, Mn, Na,

Ni, Zn

High-purity tungsten Deuterium Aqueous

standards

None 0.01–4 ng g21 Up to 100-mg sample

introduced

(49)

Co, Cr, Cu, Fe, K,

Mg, Mn, Ni, Zn

High-purity aluminium

oxide

Deuterium Aqueous

standards

None 0.25–25 ng g21 (83)

Al, As, Ca, Co, Cr,

Cu, Fe, K, Mg,

Mn, Na, Ni, Pb,

Sn, Zn

High-purity titanium Deuterium Aqueous

standards

Carbon

powder

0.02–30 ng g21 Single pieces of

metal; up to 30-mg

sample introduced

(50)

Sb PVC Deuterium Aqueous

standards

Pd n.a. Screening method (59)

M.G.R.Vale,

N.Oleszczu

k,andW

.N.L.dosSantos

384


As, Cd, Co, Cr,

Ni, Pb, V

Barytes Deuterium Solid

standards

HNO3 (for

Cr, Ni, V)

n.a. (45)

Cd, Sn Sewage sludge PVC Deuterium Aqueous

standards

Pd or PO4 n.a. (60)

Ca, Co, Cr, Cu, Fe,

K, Mg, Mn, Na,

Ni, Zn

High-purity tungsten

trioxide and tungsten

blue oxide

Deuterium Aqueous

standards

H2 gas during

pyrolysis

0.07–2 ng g21

0.01–1.7 ng g21Up to 70mg sample

introduced

(51)

Cr Natural waters Deuterium Aqueous

standards

None 20 ng L21 Coprecipitation with

Pd/8-quinolinol/tannic acid

(46)

Cu, Pb Sewage sludge Deuterium Aqueous

standards

None n.a. Gas flow during

atomization

(54)

P Plastic materials Deuterium Aqueous

standards

Pdþ ascorbic

acid

2.5mg g21 (61)

Si Niobium, titanium,

zirconium oxides

Deuterium Aqueous

standards

CH4 gas

during

pyrolysis

10 ng g21 3–15mg sample

introduced

(52)

Cd, Cu, Pb Mineral coal Deuterium Solid

standards

Ru

permanent

3–40 ng g21 Permanent modifier

for solid sampling

(71)

Tl River and marine

sediments

Zeeman-effect Aqueous

standards

Ru perm.þ

NH4NO3

20 ng g21 Permanent modifier

for solid sampling

(69)

Hg Environmental

materials

Deuterium Aqueous

standards

Pd permanent 200 ng g21 KMnO4 added to

stabilize Hg in

aqueous solutions

(72)

As Sediment Deuterium Aqueous

standards

None 440 ng g21 Residues from leach-

ing processes

(84)

(continued )

Direct

Solid

Samplin

g—

AReview

385


Table 1. Continued

Analyte Matrix

Background

correction Calibration Modifier LODa Remarks Reference

Fe Rice plants Deuterium Aqueous

standards

None 0.14 ng at

302.1 nm

Micro samples (85)

Co, Zn Vitamin complex,

agricultural soil

Deuterium Aqueous

standards

None Co: 0.5mg g21

Zn: 4mg g21Gas flow during ato-

mization 307.6 nm

line for Zn

(55)

As(III), As(V) Sea water Deuterium Aqueous

standards

None 20 ng L21 Co-precipitation with

Ni-PDC complex

(47)

Si Polyamide Deuterium Aqueous

standards

Pt 0.1mg g21 (75)

Cr Milk powder, lobster

hepatopancreas,

polyethylene,

sewage sludge

n.a. Aqueous

standards

None 2 ng g21 LOD better than with

SS-ETV-ICP-MS

(86)

Mn Vitamin-mineral

tablets

Deuterium Aqueous

standards

None n.a. Comparison of cali-

bration techniques

(65)

P Food samples Deuterium Aqueous

standards

Pdþ Ca 18 ng (87)

Cu, Zn Bovine liver Zeeman-effect Aqueous

standards

Zn: Wþ Rh Cu: 1.6 ng Zn:

1.3 ng

Cu: 216.5 nm Zn:

307.6 nm Micro-

homogeneity study

(56)

aLOD ¼ limit of detection.bn.a. ¼ not available.

M.G.R.Vale,

N.Oleszczu

k,andW

.N.L.dosSantos

386


calibration with detection limits of 4.0, 0.16, and 0.02 ng g21 for Fe, Na, and

Zn; i.e., elements that are known to be extremely sensitive to contamination.

Docekal and Krivan (48) reported similar detection limits for the same

elements in high-purity molybdenum metal and molybdenum silicide

powders. Huang and Krivan (52) determined silicon, another element that is

extremely susceptible to contamination, in niobium, titanium, and

zirconium oxides with a detection limit around 10 ng g21, again using

aqueous standards for calibration. The most decisive contribution to this

success obviously came from the fact that no reagents at all were used, or

reagents that did not contribute any blank signal. Hornung and Krivan

(49, 51) for example used hydrogen as alternate gas in the pyrolysis stage

for the analysis of high-purity tungsten, tungsten trioxide and tungsten blue

oxide, Huang and Krivan (52) used methane for the analysis of niobium

pentoxide, and Krivan and Huang (50) used high-purity carbon powder in

the determination of 15 elements in high-purity titanium.

One of the arguments in favor of slurry sampling was that the slurry

concentration can easily be changed so that the analyte concentration falls

into the analytically useful range of the calibration graph (26). This is true

as long as the analyte is 100% extracted into the liquid phase; if this is not

the case, slurry sampling is subject to the same limitations as SS ET AAS

due to natural inhomogeneity of the small subsamples used, which is not

always improved by reducing particle size (53). It has been well established

that a minimum number of particles is required for a subsample to be

representative for the laboratory sample, independent of the technique used

for the analysis (34). Anyway, there are other means available to reduce

sensitivity in ET AAS that might be more appropriate than a dilution, such

as the use of a gas flow during atomization (54, 55), the use of alternate,

less sensitive analytical lines (55, 56), or the use of the three-field Zeeman-

effect (56), a technique that has recently become available commercially

(57). On the other hand, SS ET AAS is much less affected by the particle

size, as sedimentation, etc., obviously do not have to be considered, and

even relatively large pieces may be analyzed, such as metal chips (50, 58),

pieces of plastic materials (59–61), or wet tissue samples (62, 63). Finally,

while the maximum sample mass that can be introduced into the graphite

furnace by slurry sampling is around 1mg (34), an almost two orders of

magnitude greater sample mass has been introduced by Krivan et al.

(48, 49, 51) for SS ET AAS to improve sensitivity and LOD.

Another topic that has been discussed quite controversially in SS ET AAS

is calibration, and it has been stated repeatedly that accurate results could only

be obtained using solid standards, i.e., certified reference materials (CRM).

Kurfurst (22), for example, writes in his book: “If a CRM is available

whose matrix composition and analyte content match that of the sample,

ideal analytical conditions are fulfilled”—a situation that can rarely be met

in real life. Several authors have compared the three possible basic calibration

techniques for SS ET AAS, (a) calibration using solid CRM, (b) the analyte



addition technique, and (c) the standard calibration technique with aqueous

standards (64–67), and the errors associated with these calibration techniques.

Calibration using one or more solid CRM, besides being expensive and not

applicable for routine analysis, was found to contribute significantly to

imprecision, as the uncertainty of the certified value is fully introduced into

the calibration (66, 67). The analyte addition technique is particularly cumber-

some to apply in SS ET AAS, as it is impossible to reproducibly insert the

same sample mass into the graphite furnace. The analyte addition technique

hence requires a three-dimensional approach, the Generalized Standard

Additions Method (GSAM), which takes into consideration the added

analyte mass, the response and the sample mass (68). Besides being labor

intensive, as a large number of measurements is necessary to arrive at reason-

ably reliable results, the latter still exhibit rather high uncertainty. However,

this problem appears to be kind of academic when Table 1 is consulted, as

in essentially all of the manuscripts, with only two exceptions, the standard

calibration technique with aqueous standards has been used successfully.

This means that interference-free determination could be attained after

careful optimization of the analytical conditions. This might include a pre-

treatment of the sampling platform with the refractory matrix, as in the case

of the high-purity materials investigated by Krivan et al. (24, 43, 48–52)

and/or the use of a proper modifier (50–52, 61, 69).

Yet another argument that has been brought up in favor of slurry sampling

was that chemical modification is easier to perform, because of more effective

contact between the sample particles and the modifier (26). However, this

effective contact is not necessary for analyte stabilization, as the analyte

becomes mobile well before its atomization, and can be efficiently trapped

even if it is not been in contact with the modifier (see Maia et al. (70) and

the references therein). The fact that analytes can be stabilized by a

permanent modifier even when they are introduced as a solid sample was for

the first time demonstrated in 2001 for the determination of Cd and Pb in

mineral coal, using Ru as permanent modifier (71). This principle has

been confirmed later also for the determination of Tl in river and marine

sediments (69), for Hg in environmental reference materials (72), and for

Cd in coal using Ir as permanent modifier (73). In the case of mercury determi-

nation, it could actually be shown that analyte stabilization by a permanent

modifier is much more efficient for solid samples than for aqueous standards,

which are obviously in much closer contact with the modifier than a solid

sample (72).

Last but not least, it has to be mentioned that SS ET AAS is offering a few

unique possibilities that are not available with any solution or slurry

technique. Firstly, there is the possibility for a very fast screening, essentially

without any sample preparation, in cases where only outliers have to be

detected that will be investigated more carefully later using other techniques.

This possibility has been theoretically evaluated by Belarra et al. (74) using

information theory. These authors found that the median can be used to



better effect than the mean value as the former is less affected by the presence

of outliers. The minimum number of measurements that need to be carried out

to guarantee a recall of over 0.95 in a screening method ranges from 5 to 20,

involving between 15min and 1 h of work. Lucker and Schuierer (62) inves-

tigated the sources of error in direct SS ET AAS analysis of fresh animal tissue

for screening purposes and found that manual working time and the necessity

of operator presence were reduced by at least 80% compared to conventional

digestion procedures. Belarra and co-workers investigated the determination

of antimony in polyvinylchloride (59), of phosphorus in polyethyleneter-

ephthalate (PET) and polypropylene (61), and of silicon in polyamide (75)

also for screening purposes, and claimed a high sample throughput, a

precision better than 10%, and low detection limits as the main advantages.

Secondly, SS ET AAS can be used to determine the homogeneity and

micro-homogeneity of CRM and other materials (53, 56) and to detect

potential outliers (74). Nomura et al. (56) found for a bovine liver sample

that both the drying and the grinding process had an influence on micro-hom-

ogeneity, which in addition was different for different elements. Belarra et al.

(53) also found that lack of homogeneity of the small subsamples used in SS

ET AAS is the major source of high RSD values, which is not always

improved by reducing particle size. Thirdly, there is the possibility of deter-

mining trace element distributions in extremely little samples, such as hair

segments, tissue biopsy samples or plant segments, as has been shown by

Stupar and Dolinsek (63).

HIGH-RESOLUTION CONTINUUM SOURCE ET AAS

The state of the art of high-resolution continuum source AAS (HR-CS AAS) is

fully described in a recent book of Welz et al. (76). Although this technique is

currently commercially available only with flame atomization, it appears to be

more than appropriate to discuss its potential for SS ET AAS in this context, as

there are already a number of results available that have been obtained with

prototype equipment. The general layout of this system, which is based on

preliminary work of Heitmann et al. (77) is shown schematically in

Figure 2. It consists of a specially developed xenon short arc lamp,

operating in a hot-spot mode for maximum emission intensity in the far UV

range, a high-resolution double monochromator with a spectral bandwidth

per pixel of 1.6 pm at 200 nm, and a charge-coupled-device (CCD) array

detector with full vertical binning. As only an intermediate, but no exit slit

is used, a segment of about 0.4 nm of the highly resolved spectrum is

reaching the detector and registered simultaneously by 200 pixels of the

detector, all of which act as independent detectors.

One of the features of this new concept is that the spectral environment of

the analytical line becomes visible at high resolution, as is shown in Figure 3

for the example of Tl in marine sediment CRM.While continuous background



absorption is eliminated automatically, as any of the 200 pixels can be used for

correcting spectrally continuous events, spectrally discontinuous absorption,

such as atomic absorption of concomitants and molecular absorption

with rotational fine structure, become visible. Obviously, any atomic or

molecular absorption that is not within the very small spectral window used

to measure analyte absorption does not cause any spectral interference. ET

AAS in addition offers the possibility to separate analyte and molecular

absorption in time by optimizing the temperature program.

Using all these possibilities, Welz et al. (78) succeeded in determining Tl

in marine sediment samples without any modifier and calibration against

aqueous standards, just by optimizing the temperature program, as shown in

Table 2. Using conventional LS AAS with deuterium background correction

and calibration against solid CRM it was not possible to arrive at consistent

results, even with very sophisticated modifier combinations (69). When the

same determination was carried out using LS AAS with Zeeman-effect back-

ground correction, an extremely careful selection of chemical modifiers was

still necessary, and only a combination of Ru as permanent modifier and the

addition of a solution of ammonium nitrate on top of the solid sample

resulted in accurate values (69). This actually appeared like a clear indication

that the problem originally encountered with deuterium background correction

was a combination of spectral interference (which could be eliminated Zeeman-

effect background correction) and non-spectral interference (which could only

be overcome with a carefully selected combination of modifiers). However, the

fact that the same determination could finally be carried out without any

modifier and calibration against aqueous standards using HR-CS AAS (78)

clearly showed that the interference still encountered with Zeeman-effect back-

ground correction must obviously have been of spectral origin, because

otherwise it would have appeared as well with the former system.

Figure 2. Instrumental concept for HR-CS AAS (from Welz et al. (82)).



The concept was later extended to the determination of Tl in mineral coal

and the only condition that had to be met was a pyrolysis temperature higher

than 6008C in order to avoid excessive continuous background absorption

due to radiation scattering at volatilized particles. Using a pyrolysis tempera-

ture of 7008C, this problem was solved, and no modifier was required for

the determination of Tl in coal (79). Cadmium could be determined in coal

under the same conditions, i.e., using a pyrolysis temperature of 7008C,which, however, required a chemical modifier to stabilize Cd, and Ir as

permanent modifier was found optimum for that purpose (73). It is interesting

to note that the same determination of Cd in coal, using the same equipment,

but slurry instead of direct SS ET AAS could not be carried out satisfactorily,

as the maximum applicable pyrolysis temperature in this case was only 6008C(80). This appears to be the first case where under otherwise identical analytical

conditions, the direct SS ET AAS method succeeded, whereas the slurry

method failed. Another application has been the determination of Co in a

variety of biological materials, which also did not require any modifier and

could be carried out against aqueous standards after program optimization (81).

It has to be mentioned that HR-CS AAS, in addition to the above features,

has the advantage that, due to the use of a high-intensity continuum source, all

Figure 3. Atomic and molecular absorption signals observed during the atomization

of a marine sediment sample in the vicinity of the Tl resonance line at 276.787 nm

using SS HR-CS ET AAS (from Welz et al. (78)).



Table 2. Determination of thallium in marine sediment reference materials (CRM) using direct solid sampling ET AAS and three different

instrumental concepts: Deuterium background correction with calibration against a solid CRM (from Vale et al. (69)), Zeeman-effect background

correction with calibration against aqueous standards (from Welz et al. (78)) and HR-CS AAS with calibration against aqueous standards (from

Vale et al.(78))

CRM

Reference

valueaETV-ICP-

MSb

D2 correction Zeeman-effect background correction HR-CS AAS

Ru/NH4NO3

c Withoutd NH4NO3e

Ru/NH4NO3

c Withoutd

BCSS-1 (0.6) — 0.36+ 0.03 — — 0.59+ 0.04 0.51+ 0.04

HISS-1 (0.06) 0.055+ 0.004 0.04+ 0.01 0.02+ 0.002 0.03+ 0.003 0.05+ 0.003 0.054+ 0.01

MESS-1 (0.7) — 0.42+ 0.05 — — 0.70+ 0.02 0.57+ 0.01

MESS-2 (0.98) 0.99+ 0.04 0.87+ 0.03 0.19+ 0.01 0.72+ 0.01 1.07+ 0.03 0.98+ 0.02

MESS-3 0.90+ 0.06 — 0.85+ 0.10 — — 1.08+ 0.07 0.93+ 0.04

PACS-2 (0.6) 0.52+ 0.02 0.21+ 0.06 0.12+ 0.03 0.22+ 0.04 0.58+ 0.02 0.50+ 0.01

SRM 1646a (,0.5) – 0.10+ 0.02 — — 0.20+ 0.02 0.20+ 0.02

aValues in brackets are not certified.bValues taken from Maia et al. (41).c400mg Ru as permanent modifierþ 20mL of 5% w/v NH4NO3 added to the solid sample.dWithout use of a modifier.eTwenty microliters of 5% w/v NH4NO3 added to the solid sample.

All values are in mg g21, and average and standard deviation of n ¼ 3.

M.G.R.Vale,

N.Oleszczu

k,andW

.N.L.dosSantos

392


analytical lines are available without any compromise regarding the signal-to-

noise ratio; i.e., there are no more “weak lines.” Another possibility offered by

this technique is that in addition to the usual evaluation of the line center using

1–3 pixels, there is the possibility to evaluate the absorbance at the line wings

only in order to reduce the sensitivity and expand the linear dynamic range to

some five orders of magnitude. This appears to be particularly important for

SS ET AAS to reduce the sensitivity in cases where no secondary line of

appropriate sensitivity is available, such as in the case of Cd or Zn. It is of

particular importance that this extension of the working range does not

require a new measurement, as a whole set of calibration curves can be estab-

lished simultaneously. All these possibilities obviously have yet to be explored,

but the expectations for this excitingly new technique are high for good reasons.

CONCLUSION

Direct SS ET AAS has matured significantly during the past decade, and with

proper accessories for sample introduction being available it can be con-

sidered a rugged technique ready for routine application. A relative standard

deviation (RSD) of the order of 10%, as it is typical for this technique due

to the naturally inhomogeneous distribution of trace elements in the relatively

small subsamples, should not be considered a major limitation. In the opinion

of the present authors a 10% RSD of an accurate result is much more accep-

table than a 1% RSD of a result that is affected by systematic errors due to

analyte loss or contamination during sample preparation. Direct SS ET

AAS is not affected by problems such as sedimentation or partial leaching

of the analyte, and particle size is much less critical than in slurry sampling.

It is fast, offers the lowest LOD, and has a significant contribution to clean

chemistry. HR-CS AAS is expected to further enhance the attractiveness of

SS ET AAS, making it even simpler to use and more flexible in its application.

ACKNOWLEDGEMENT

The authors are grateful to Conselho Nacional de Desenvolvimento Cientıfico

e Tecnologico (CNPq) for financial support. M. G. R. Vale and W. N. L. dos

Santos have scholarships from CNP.

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