CHAPTER 12 CHEMICAL VAPOR GENERATION WITH SLURRY SAMPLING:

A REVIEW OF ATOMIC ABSORPTION APPLICATIONS

Henryk Matusiewicz Department of Analytical Chemistry,Poznan University of Technology,

00-965 Poznań, Poland

ABSTRACT This review summarizes and discusses the analytical methods and techniques described in the literature for hydride generation as a mode of slurry sample introduction into atomization cells. A brief comparison of detection limits for analytical atomic absorption methods and practical applications to analytical samples that utilize slurry hydride generation is discussed. The current state-of-the-art, including advantages and limitations of this approach is discussed.

Chapter 12

1 INTRODUCTION Chemical vapor generation combined with atomic absorption spectrometric detection in the forms of cold vapor generation (CVAAS) for the determination of Hg and Cd and hydride generation (HGAAS) for elements forming gaseous covalent hydrides (As, Bi, Ge, Sb, Se, Sn, Te and even In, Pb and Tl) has become one of the most powerful analytical tools for the determination of these elements. The generation of gaseous analytes and their introduction into atomization cells offers several significant advantages over conventional solution phase pneumatic nebulization of samples. These include enhancement of analyte transport efficiency (approaching 100 %), elimination of the need for a nebulizer/spray chamber assembly, higher selectivity due to a significant reduction of interferences, better detection limits (at the µg l-1 level or lower), automation of methods and possibility of speciation studies and coupling with different techniques. The continuous interest in this technique is reflected in the number of recent reviews [1-4], a new book [5] and a book chapters [6,7]. However, as a rule, CVAAS and HGAAS requires complete decomposition and/or dissolution of the samples prior to analysis, and the quantitative production of a single labile analyte species, which increases both analysis time and the risk of sample contamination and/or losses of the analyte. In addition, the problem with using a large amount of reagents during pretreatment leads to increased blank values and higher detection limits. Pretreatment, by avoiding sample digestion by wet or dry oxidation methods, of solid samples by slurrying in liquid medium overcomes these problems and has the advantages of rapid analysis, reduction of blank levels and risk of analyte losses. In recent years, the slurry sampling approach coupled with AAS techniques with electrothermal and flame atomization has been extensively employed for the analysis of analytical samples, in order to simplify sample preparation procedures and to avoid some inconveniences related to wet digestion and dry ashing methods. Introduction of slurry samples combines the advantages of direct solid sampling (reduction of sample preparation time; in sample contamination; decrease in analyte losses through volatilization prior to analysis and/or associated with retention by insoluble residues) and liquid sampling (sampling dispensing by using a conventional liquid sample handling approaches; straightforward automation; flexibility in slurry preparation and the advantage that slurries may be prepared in advance). The extensive reviews published in the last decade [8-12], has confirmed the great applicability of this approach. The method of CV/HG-AAS directly from slurried samples has recently been proposed as an attractive alternative to avoid the need for intensive treatments (e.g., acid digestion). This analytical technique is based on efficient extraction of the analyte into the liquid phase. It is difficult to understand how the reducing agent, which is inside the solid particles, can act on the analyte. A possible explanation for this fact could be that analyte adsorbed onto solids can also take part in the hydride generation reaction. However, no review has dealt with the use of slurried samples coupled with HGAAS and CVAAS. This review is an attempt to fill this void and will consider the historical development of direct hydride generation of vapors from slurried samples coupled with batch and flow injection formats, and detection via AAS. Current state-of-the-art, including basic properties, advantages and limitations of this approach will be discussed.

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2 CHEMICAL VAPOR GENERATION WITH SLURRY SAMPLING Analyses involving the formation of a slurry (i.e., a solid suspension in a liquid medium) are much simpler than direct analyses on a solid. This method has been used for the determination of a variety of elements by flame and electrothermal AAS. Plasma emission analyses on slurries have reached the same extent of development. Slurries also allow the application of the hydride generation technique which offers the advantage of simpler and faster sample pretreatment than conventional methods, plus a detection technique (hydride generation) that is selective and sensitive enough to determine low analyte levels in complex matrices and is easy and cheap to operate. This combined system is a novelty as it has been used for this purpose in a several cases. 2.1 Hydride Generation Atomic Absorption Spectrometry (HGAAS) The concept of a method of hydride generation directly from slurried samples with subsequent atomization was first reported by Haswell et al. [13]. This approach has been applied for the determination of arsenic in complex matrices such as solid environmental samples. Samples were weighed directly into the glass hydride generation vessel and 10 ml of 4 M HCl added. Arsine was generated by adding 1% (m/v) sodium tetrahydroborate (III). The influence of particle size, homogeneity and matrix on the reproducibility and amount of analyte released was examined. The influence of interfering elements in the sample matrix has also been examined. The technique gave similar results to those obtained by the hot acid extraction with aqua regia which commonly requires lengthy handling procedure. The method developed shows promises as a rapid screening procedure for a range of environmental sample types (soil, sewage sludge, coal fly ash, incinerator ash) being able to achieve acceptable accuracy together with precision at the order of 7% RSD.

Subsequent to the above work, a major series of studies based on that concept have been carried out by Cámara and co-workers [14-18]. Madrid et al. [14] reported a simple and rapid method for the determination of lead in foodstuffs and biological samples that combines a slurry procedure with lead hydride generation. Powdered samples were suspended in Triton X-100 as dispersing agent and shaken with 10 g of blown zirconia spheres until the slurry was formed. A few drops of silicone antifoaming agent were added before the slurry was diluted to minimize the foam formed on addition of NaBH4 to the viscous slurry. This grinding procedure ensured that 90% of the particles had a diameter of less than 25 µm, a particle size small enough to permit lead determination by HGAAS. Three oxidant media, namely H2O2-HNO3, K2Cr2O7-lactic acid and (NH4)2S2O8-HNO3, were evaluated for the generation of lead from slurry samples and their application to the determination of lead in vegetables and fish by HGAAS was investigated [15-17]. HNO3-H2O2 medium was unsuitable for the generation of lead hydride from slurry samples because of decomposition of the hydrogen peroxide by the organic matrix. Further, the low sensitivity provided by this medium made it necessary to increase the concentration of the powdered sample in the slurry, resulting in higher errors owing to sampling difficulties and increased matrix effects. (NH4)2S2O8-HNO3 gave reliable results for the determination of lead in vegetables but only semi-quantitative results with fish slurry samples. K2Cr2O7-lactic acid provided the best results for the determination of lead in slurried vegetable and fish samples, and also resulted in lower detection limits owing to its high sensitivity and low blanks. When mussel was analyzed, however, this medium gave lower results than the (NH4)2S2O8-HNO3 and wet digestion procedures, perhaps because potassium dichromate was unable to remove the lead completely from this sample, unlike ammonium peroxodisulfate, which is a sufficiently strong oxidant to eliminate lead

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completely. A simple and rapid flow injection-lead hydride generation atomic absorption spectrometry (FI-HG-AAS) method was optimized for the determination of lead in fruit [18]. The lead hydride was generated from slurries of the fresh sample. No matrix effect was found in the determination of lead. The method enabled the direct determination of lead in untreated samples with use of an aqueous calibration graph. Procedure based on the use of slurries on lead hydride generation for determination of lead in commercial iron oxide pigments has been evaluated [19]. No pretreatment was required. The samples were suspended in water containing 0.01% hexametaphosphate, and lead hydride was generated from a 0.7 M HNO3 acid and 14% ammonium peroxodisulfate medium by addition of 10% tetrahydroborate solution. In this way, an improvement in reproducibility and sensitivity as well as a saving of time and effort was achieved. Therefore, the slurry hydride generation approach can be of use for routine quality-control analyses. de la Calle Guntiñas et al. [20] have also utilized the slurry hydride generation approach. A method was described for the determination of total antimony, extractable into 4 M HCl, which combines a slurry procedure with HGAAS. Further, when slurry is prepared with HCl instead of water, a recovery of 100% of the total Sb is found in the supernatant of the antimony is adsorbed on the surface of the sediment. This suggest that antimony can be removed from the sample with minimum sample pretreatment. The lack of a matrix effect allows the analytical results to be obtained with a simple aqueous working curve. The proposed method might be applied to the determination of antimony in complex samples such as biological materials and foodstuffs, which would be of interest if there were contamination problems.

Nerín et al. [21] and López García et al. [22] extended the work of Cámara et al. [14-18]. For the determination of As, analyte introduction was accomplished with the generation of arsenic hydride on a fly ash slurry from a thermal power plant burning lignite [21]. The most critical variable of the method is the particle size, as it has been shown in the article. For the method to be successful the particle size of the ash must be below 8.5 µm. In the next article [22], the conditions for the determination of As and Hg in coal fly ash and diatomaceous earth samples using the vapor generation-slurry methodology are discussed. Data for the fraction of the analyte extracted into the supernatant as a consequence of the slurry preparation are reported. Calibration is performed using aqueous standards. The results agree well with those obtained with procedures based on whole dissolution of the samples. Mierzwa et al. [23,24] have provided a further examples of the generation of arsine directly from a slurry samples. A batch procedure for the determination of As in cigarette tobacco in which a slurry of the sample was injected into the reactor has been developed [23]. Slurries were prepared with the aid of an ultrasonic bath and a microwave oven. Pre-treatment of samples slurried in nitric acid by ultrasonication permitted the extraction of about 90% of the total arsenic from tobacco samples. Further improvement in the recovery efficiency (up to 93-94%) was accomplished by the use of an additional step of short microwave-accelerated treatment. Ŀ-Cysteine was used as a pre-reduction agent. The results suggested that all of the analyte was extracted into the solution phase prior to injection. The main factors that influenced the reliability of the method were sample homogeneity, particle size and slurry concentration. A slurry sampling procedure has been used [24] for the analysis of sediments. The slurry was first treated by microwave heating (total time 2 min) and then sonicated (12 min). Brindle’s reagent (Ŀ-Cysteine) was added to reduce As(V) to As(III), followed by Triton X-100. Copper, Fe and Ni did not interfere at concentrations up to 10 mg l-1. Recently, a flow injection procedure has been developed for the determination of acid-extractable arsenic in soils by HGAAS [25]. Several parameters, including acid and borohydride concentrations, exposure time to

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microwave energy, and the microwave power applied, were optimized. The on-line microwave extraction increased the recovery of the adsorbed arsenic significantly; whereas, preparation of the slurry in 10% HCl instead of water increased the recovery only when the microwave oven was off.

Another technique, which has been successfully used for increasing the sensitivity of vapor generation systems is the use of an in situ preconcentration (trapping) technique [2], which couples a hydride generation system with the graphite furnace. The subsequent atomization procedure is the same as in conventional electrothermal atomization systems [26-29]. The most attractive advantages of the in situ trapping HGAAS technique are the very low detection limits and the decrease of the kinetics interferences in HG and interferences in the atomizer. Moreda-Piñeiro et al. [26] have used Ir-treated graphite tubes for pre-concentration and atomization of the As, Bi and Se hydrides generated from acidified slurries of marine sediment, soil and coal samples. A batch mode generation system was used for the hydride generation. The variables affecting the acidified slurry preparation procedure (assisted by ultrasonic energy) and the hydride generation/trapping/atomization processes were studied by using a Plackett-Burman design. Subsequent work [27] reported different procedures of tin hydride generation from aqueous and acidified slurries of marine sediment, soil, coal fly ash and coal samples, coupled to ETAAS and were optimized by using factorial design. A batch mode generation system and Ir-treated graphite tubes were used for the hydride generation and atomization, respectively. Eight variables, affecting the hydride generation and hydride transport efficiency (HCl and NaBH4 concentrations, particle size, acid volume and argon flow), the hydride trapping efficiency (trapping temperature and trapping time) and the atomization efficiency (atomization temperature) were studied and optimized. In addition, acid pre-treatment procedures assisted by ultrasonic energy were used for soil and coal matrices, to obtain acidified slurries and acid leachates. The involved variables were hydrochloric and nitric acid concentrations, exposure time to ultrasound, particle size and leaching solution volume. By using acidified slurries, the hydride generation occurs from the acid liquid phase and also from solid particles. Thus, the use of acid increases the hydride generation efficiency from solid particles. Finally, a mean particle size lower than 50 µm is small enough to achieve adequate tin hydride generation efficiency from aqueous slurry samples. Very recently, Vieira et al. [28] also determined As in sediments, coal and fly ash by ETAAS after collecting the arsine, generated directly from the sample slurries by hydride generation, in an Ir-coated graphite tube, using the addition calibration technique. Matusiewicz and Mikołajczak [29] described a method for the HGAAS determination of As, Sb, Se, Sn and Hg in untreated samples of wort using a batch system and in situ preconcentration of the analytes onto the Pd- (for As, Sb, Se, Sn) or Au-pretreated (for Hg) interior was surfaces of a graphite furnace. Determination of the total concentration of these elements was obtained after a previous reduction with thiourea. The accuracy of the method was confirmed by comparing the results obtained with those found for wort using microwave-assisted digestion and by analyzing five certified reference materials. Calibration was achieved via the method of standard additions. 2.2 Cold Vapor Atomic Absorption Spectrometry (CVAAS) A number of papers appear recently devoted to the determination of As, Bi, Cd, Ge, Hg and Se by cold vapor generation techniques [30-35]. Slurry procedures avoid the problems of time consuming digestion procedures and the accompanying risks of contamination or analyte loss. This technique has been applied successfully to the determination of Hg by CVAAS in iron oxide-titanium oxide pigments [30]. The

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samples were suspended in water containing 0.02% (m/v) sodium hexametaphosphate and Hg generated from an HCl medium with NaBH4. Calibration was with aqueous solutions of Hg and there was excellent agreement between results from slurry and acid digestion procedures. Río-Segade and Bendicho [31] reported a method for the determination of total mercury in solid biological and environmental samples by slurry sampling combined with flow injection-cold vapor-atomic absorption spectrometry (FI-CV-AAS). Aqueous calibration was not considered adequate and calibration with the standard addition method was needed. Analytical results for total Hg determination in real samples of mussel tissue were in good agreement with those obtained by FI-CV-AAS after microwave-assisted digestion of samples. Ultrasonic pretreatment of slurries was proved to be necessary in order to improve the low mercury recoveries obtained. The ultrasonic pretreatment step caused an increase in the fraction of analyte extracted into the liquid phase of the slurry, thus facilitating the reduction to elemental mercury.

A method for the analysis of solids based on slurry formation with CV chemical vapor generation has been developed [32] in which the detection was also by AAS. Samples (sewage sludge, city waste incineration, Antarctic krill and human hair) were suspended in HCl and sonicated to reduce particle size and provide a homogeneous slurry. Potassium cyanide was added to overcome the interferences from Cu, Pb, Ni and Zn. Complete leaching from the environmental samples was obtained, but not from the biological materials, which were analyzed by the standard additions method. The main drawback of this technique is the non-homogeneity of the suspensions formed.

Flores et al. [33] developed a rugged and reliable method for the determination of mercury in coal without sample digestion, based on chemical vapor generation (cold vapor technique) from slurried coal samples. It involves collection of the mercury vapor in a graphite tube, treated with gold or rhodium as “permanent modifier”, and determination by ETAAS. Mercury quantitatively leached out of the investigated coal reference materials into 1 M HNO3 within 48 h when the coal was ground to a particle size of < 50 µm. No detectable quantity of mercury was generated directly from the slurry particles, but it was not necessary to filter the solution.

Iridium is both a well-recognized “permanent modifier” in ETAAS and an efficient collector for the trapping of volatile hydrides. Moreda-Piñeiro et al. [34,35] reported on the direct CV and hydride generation procedures for As, Bi, Ge, Hg and Se(IV) from aqueous slurries of environmental (marine sediment, soil, coal, coal fly ash) and biological (human hair, seafood) samples by using a batch mode generation system. Ir-treated graphite tubes have been used as a pre-concentration and atomization medium of the vapors generated. A Plackett-Burman experimental design has been used as a strategy for evaluation of the effects of several parameters affecting the vapor generation efficiency from solid particles, vapor trapping and atomization efficiency from Ir-treated graphite tubes. Optimum values of the parameters have been selected for the development of direct cold vapor/hydride generation methods from slurry particles. 2.3 Chemical Speciation Analysis Speciation of an element is the determination of the individual physico-chemical forms of the element, which together makes up its total concentration in sample. The hydride generation procedure coupled with AAS can afford several methods for inorganic and/or organic speciation of some hydride forming elements. In practice, the use of hydride generation for speciation analysis is dominant, when coupled to atomic absorption spectrometric system as element specific detectors. HGAAS is perhaps the most widely used technique for determination of volatile hydride forming elements.

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However, a modest amount of work has been published in this field, and there is still a low level of activity related to the determination of total analyte concentration.

Speciation of Sb(III) and Sb(V) in marine sediment slurry was carried out by determining total antimony and Sb(III) [20] by the method which combines a slurry procedure with HGAAS. It appears that the most of the Sb(III) is released from the sample during hydride generation and slurry formation is a promising method for determining Sb(III) and Sb(V) selectively. de la Calle Guntiñas et al. [36] showed it was possible to speciate Sb(III) and Sb(V) by slurry formation in soil and sediments. Although oxidation states are unchanged in slurry formation with water and in 4 M HCl it was shown that a low recovery is obtained. Clearly, however, there are some aspects of the speciation of antimony.

Rondón et al. [37] devised a method for the determination of Sb(III) and Sb(V) in liver tissue. For the determination of Sb(III) the slurried sample was dissolved with 1 M acetic acid, whereas for Sb(V) the carrier was a mixture of sulfuric acid, potassium iodide and ascorbic acid. It was evident that the addition of organic acids leads to a speciation.

A novel method for speciation of phenyl-mercury (PH-HG) in the biomass slurry at trace levels, based on the retention of analyte by a living Escherichia coli strain, has been developed [38]. The mercury vapor was generated from the Hg-biomass slurry by treating it with Sn(II) or NaBH4 as a reducing agent and the amount of Hg was determined by AAS. The use of living bacterial cells coupled with the specific CVAAS detection provides a reliable procedure for characterizing mercury species based on the differences in their relative sorption under non-equilibrium situations.

A slurry sampling hydride generation method for As(III) and total inorganic As, without total sample digestion, has been developed using a continuous flow mode generation system coupled with AAS determination from environmental and biological samples [39]. It involves trapping of the arsenic vapor in a pre-heated Ir-coated graphite tube, and determination by ETAAS. Pretreatment of samples (slurried in HCl with addition of ozone) by ultrasonic agitation enabled the extraction of more than 95% confidence level of the total arsenic from reference materials investigated. For the estimation of As(III) and As(V) concentrations in samples, the difference between the analytical sensitivities of the absorbance signals obtained for arsenic hydride, without and with previous treatment of samples with thiourea, can be used. The concentration of arsenate (As(V)) was calculated by the difference between total As and As(III). Calibration was achieved via the technique of standard additions.

Selenium determination by HG techniques requires its presence as Se(IV). Consequently, inorganic speciation by hydride generation techniques is done by first determining Se(IV) and then, after reduction of Se(VI) to Se(IV), the total selenium (40). For real samples (garlic, sediment) selenium was determined from slurry (without any pretreatment). It was demonstrated, that dimethylselenium and dimethyldiselenium (organic selenium species) are forming other volatile species by reaction with NaBH4, applying the same reduction conditions as for inorganic selenium. These species can be subsequently detected by AAS. The error that their presence can cause in determination of inorganic selenium has been evaluated. 3 SUMMARY OF INSTRUMENTATION Table 1 summarizes the instrumentation and methodology reported for the slurry sampling hydride generation for atomic absorption spectrometry.

208

TABLE 1 Operating parameters for slurry sampling-chemical vapor generation-AAS studies

Hydride generation

Slurry sample preparation Temperature/oC Atomic

absorption

spectrometer

Atomizer

Chemical

modifier

System Reaction Size/ml Dispersant

Homogenization Particlesize/µm

Trapping Atomization

Element

Detection

mode

Ref.

Thermo

Electron 951

Air-

acetylene

flame

AVA 440 1% NaBH4,

4M HCl

10 Magnetic stirrer <90 As HGAAS [13]

Perkin-Elmer

2380

Laboratory

constructed

8% NaBH4,

10% HNO3,

10%

ammonium

persulphate

5 1% Triton

X-100

Mechanical

shaker, zirconia

spheres

Pb HGAAS [14]

Perkin-Elmer

300

MHS-10 4% NaBH4,

2M HCl

5 0.02%

sodium

hexametap

hosphate

Ball mill <45 Hg CVAAS [30]

Perkin-Elmer

2380

Laboratory

constructed

3% NaBH4,

4M HCl,

2% KI

3 1% Triton

X-100

Ultrasonic bath <4 Sb HGAAS [20]

209

209

Perkin-Elmer

370A

Air-

acetylene

flame

MHS-10 3% NaBH4,

0.4 M HCl

1 Vibrating,

stirring

<8.5 As HGAAS [21]

Perkin-Elmer

300

Flame-

heated silica

tube

MHS-10 10%

NaBH4, 0.7

M HNO3,

14%

ammonium

peroxodisul

phate

1 Ball mill <400 mesh Pb HGAAS [19]

Perkin-Elmer

2380

Air-

acetylene

flame

FI manifold 6% NaBH4,

15% H2O2,

40% HNO3

0.1 1% Triton

X-100

Mechanical

shaker, zirconia

spheres

<25 Pb FI-HGAAS [18]

Varian-

Techtron AA-

1475

Air-

acetylene

flame

VGA-70 0.1%

NaBH4, 0.5

M H2SO4,

10% KI

10 Ultrasonic Sb HGAAS [37]

GBC 902 Electrically

heated quartz

T-tube

Laboratory

constructed

1.2%

NaBH4, 6.5

M HNO3

1 0.005%

Triton X-

100

Ultrasonic bath,

vortex mixing

<60 As HGAAS [23]

210

210

Perkin-Elmer

2380

Non-heated

quartz cell

FI system 0.5%

NaBH4,

15% HNO3,

15% HCl

0.02%

Triton X-

100

Ultrasonic <100 Hg FI-CVAAS [31]

Perkin-Elmer

3100

Air-

acetylene

flame

Peristaltic

pump

0.6%

NaBH4,

10% HCl

0.2 0.005%

Triton X-

100

Grinding <212 As HGAAS [25]

Perkin-Elmer

3100

T-quartz cell 4% NaBH4,

1 M HCl,

0.5% KCN

0.5 Ultrasonic bath,

magnetic stirrer

Cd FI-CVAAS [32]

Analytik

AAS5EA

THGA

graphite tube

Pd, Au Laboratory

constructed

1% NaBH4,

HCl,

thiourea

10 50 µl

decanol

Ultrasonic 110-400 1300-2600 As,Sb,S

e,Sn,Hg

HGAASa [29]

Analytik

AAS5HydrEA

THGA

graphite tube

Au, Rh HS5 3% NaBH4,

1 M HNO3

1 Manually

shaking

<50(30) 900-1500 1000-2200 Hg CVAASa [33]

Perkin-Elmer

Aanalyst 800

Graphite

tube

Ir MSH-10 2% NaBH4,

1 M HCl, 5

M HNO3

5 0.02%

glycerol

Ball mill,

zirconia balls,

ultrasonic bath

<50 200-1000 2000-2500 As,Bi,Se HGAASa [26]

Perkin-Elmer

Aanalyst 800

Graphite

tube

Ir MSH-10 1% NaBH4,

6 M HCl

2(0.1) 0.02%

glycerol

Ball mill,

zirconia balls,

ultrasonic bath

<10 75 2600 Hg HGAASa [35]

211

211

Perkin-Elmer

Aanalyst 800

Graphite

tube

Ir MSH-10 1% NaBH4,

6 M HCl

0.02-1 0.02%

glycerol

Ball mill,

zirconia balls,

ultrasonic bath

<10 100-1000 2000-2600 As,Bi,G

e,Hg,Se

CVAASa [34]

Analytik

AAS5EA

THGA

graphite tube

Ir HS5 3% NaBH4,

2% HCl

10 0.1%

Triton X-

100

Ultrasonic <20 300 2150 As HGAASa [39]

aIn situ trapping.

212

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4 ANALYTICAL FIGURES OF MERIT The analytical performance of slurry sampling hydride generation – atomic absorption spectrometry is characterized by figures of merit, such as detection limit and quantification limit, linear dynamic range, and precision and accuracy of measurements. It is usual practice to quote the detection limit pertaining to a particular technique or method, and to draw comparisons between the detection limits obtained using similar techniques. The detection limits for slurry sample introduction – hydride generation – atomic absorption spectrometric technique is summarized in Table 2. This approach was adopted because the range of reported values is a reflection of differences in instrumental applications and a variability in the slurry sampling – hydride generation spectrometric technique capability. Detection limits are presented in terms of both mass and concentration or characteristic mass to simplify comparison. The compiled data refer mainly to the determination of the elements in slurry solutions containing the analyte in question. Because of this simplification, any application of the data to practical trace analysis must be subject to some restrictions. The limit of detection is only one of several figures of merit characterizing a technique and should not be used alone as a criterion of choice. Nevertheless, the data compiled here may be useful as an initial survey of the effectiveness of slurry sampling hydride generation AAS technique with respect to the determination of trace levels of these analytes. In addition, because of the wide range of slurry sampling hydride generation technique and atomizers and the resulting differences in optimized experimental conditions, it is very difficult to accurately compare published data with regard to analytical performance of the slurry sampling – hydride generation – AAS. Moreover, there is much confusion over the definition of the term “detection limit”, so users of such data as shown in Table 2 and in the literature should always check the definition applied in the original papers. Some relative standard deviation (RSD) values reported for the analysis of analytical samples are summarized in Tables 3 and 4.

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TABLE 2 Analytical characteristics for volatile hydride-forming element determination by slurry sampling-AAS

Limit of detection (LOD)a Element/nm Detection

Mode ng/l ng/g ng

Limit of quantification (LOQ)a

ng/g

Characteristic

mass/pg

Ref.

As HGAAS 2.8 (2σ) 940 [21]As 193.7 HGAAS 2.6 (3σ)

[23]As 193.7 HGAAS 2.75 (3σ) [24]As 193.7 HGAAS 200 7 [25] As 193.7 HGAAS 108-1800 (3σ) 360-6000 (10σ)

65 [26]

As 193.7 HGAASb 540-700 (3σ) [28]As 193.7 HGAASb 28 (3σ) 93 (10σ) 28 [29]As 193.7 CVAASb 11.5 (3σ) 40 ng/l (10σ)

60 [34]

As 193.7 HGAASb 1.2 (3σ) 1.5 4.8 (10σ) 15 [39]Bi 223.1 HGAASb 40-100 (3σ) 140-340 (10σ) 75 [26]Bi 223.1 CVAASb 48 (3σ) 160 ng/l (10σ) 80 [34]Cd 228.8 FI-CVAAS 50-200 125-600 [32] Ge 265.1 CVAASb 600 (3σ) 2000 ng/l (10σ) 220 [34]Hg 253.6 HGAAS 10 (3σ) [22]Hg 253.7 CVAAS 5 (2σ) [30]Hg 253.7 CVAASb 9 (3σ) 0.09 110 [33]Hg 253.7 CVAASb 55 (3σ) 180 ng/l (10σ) 370 [34]Hg 253.7 CVAASb 21-170 (3σ) 70-570 (10σ) 390 [35]Hg 253.7 FI-CVAAS 3-28 [31]Hg 253.7 HGAAS 50 (3σ) [38]Hg 253.6 HGAASb 90 (3σ) 320 (10σ) 92 [29]Pb 217.0 HGAAS 40-700 (3σ) 120-2100 [15]Pb 217.0 HGAAS 40-100 (3σ) 440 [16]Pb 217.0 FI-HGAAS 1 (3σ) 3.2 2000 [18]Pb 283.3 HGAAS 200 (3σ) 600 (10σ) [19]Sb 217.6 HGAAS 8.35 (3σ) [20]Sb 217.6 HGAASb 21 (3σ) 69 (10σ) 21 [29]Sb 217.6 HGAAS 100 (3σ) [37]Sb(III) 217.6 HGAAS 13.3 (3σ) [20]Sb(III) 217.6 HGAAS 150 [37] Sb(III) 217.6 HGAAS 2.97 3 [36] Se 196.0 HGAASb 20-50 (3σ) 70-170 (10σ)

100 [26]

Se 196.0 HGAASb 10 (3σ) 40 (10σ) 13 [29]Se(IV) 196.0 CVAASb 11 (3σ) 40 ng/l (10σ) 110 [34]Sn 286.3 HGAASb 36-80 (3σ) 120-267 (10σ)

160 [27]

Sn 286.3 HGAASb 50 (3σ) 180 (10σ) 50 [29]

214

aDetection and quantification limits were calculated according to IUPAC rules, based on a 3σblank criterion.bIn situ trapping.

214

TABLE 3 Applications of slurry sampling hydride generation atomic absorption spectrometric techniques to the analysis of environmental and mineral substances and technical products

Element

Matrix

Detection

mode

Amount

of samplea/g

Slurry-hydride generation approach

Ref.

As Soil BCR 142, sewage sludge BCR 144, fly ash BCR 38,

waste incineration ash BCR 176

HGAAS >0.025 Cold acid soluble technique, calibration with aqueous standards, 7% RSD [13]

Hg Iron(III) oxide and titanium oxide pigments CVAAS 1-5 Samples suspended in water containing 0.02% sodium hexametaphosphate (HMP)and

mercury vapor generated from HCl medium by adding NaBH4, 1-7% RSD

[30]

Sb Marine sediments, soils HGAAS 3 Slurry samples prepared by sonication. Antimony hydride was generated from HCl and KI

medium by adding NaBH4. Calibration by standard addition method

[20]

As Fly ash HGAAS 0.01 Slurry samples prepared by stirring for about 1 min. Aliquot (1 ml) of the fly ash slurry was

placed in a reaction vessel (0.4 M HCl, 3% NaBH4) to yield the As hydride. Calibration by

standard addition method. Method validated against BCR 38 fly ash, ca. 5% RSD

[21]

Sb Sediments, soils HGAAS 0.5 Slurries prepared by mixing 0.5 g of sample and 5 ml of 1% Triton X-100 and drops of

silicone, diluted to 25 ml with water or 4 M HCl. Antimony hydride was generated by

adding NaBH4, ca. 5% RSD

[36]

Pb Iron oxide pigments HGAAS 1 Slurries prepared by suspending 1g of sample in 50 ml of water containing 0.01% HMP.

Lead hydride was generated from 0.7 M HNO3 by addition of 10% NaBH4. Calibration by

standard addition method, 3.2% RSD

[19]

215

219

As, Hg Coal fly ash, diatomaceous earth HGAAS 0.1-1.5 Slurries prepared from the unsieved ground samples, suspended in 3 M HCl (As) or 5 M

HCl (Hg). Suspensions were sonicated, NaBH4 solution was used as the reducing agent.

Calibration was performed using aqueous standards

[22]

As Sediments HGAAS 0.05-0.35 Slurries pretreated by ultrasonic agitation and microwave-assisted extraction (0.75 M HNO3,

0.04% L-cysteine, 0.005% Triton X-100) and vortexed prior to each measurement for about

30s. Arsenic hydride was generated by 1.25% NaBH4. Calibration technique based on the

aqueous standard solutions was applied, 8.5% RSD

[24]

Hg Mussel tissues FI-CVAAS 0.4 Slurries suspended in 15% HNO3 containing 0.02% Triton X-100 subjected to ultrasonic

pretreatment for 2 min. Mercury vapor was generated by adding 0.25% NaBH4. Calibration

with the standard addition method was needed. Method validated against CRMs BCR 278

mussel tissue, BCR 60 aquatic plant, BCR 320 river sediment, BCR 145R sewage sludge,

4-7% RSD

[31]

As Soils HGAAS 0.6 25 ml slurries prepared by stirring and microwave-assisted acid-extractable arsenic in

samples was determined by hydride generation (NaBH4). Calibration by using aqueous

standards, 5% RSD

[25]

Hg Coal CVAASb 0.02-0.1 Slurries manually shaken, the mercury vapor transported from the reaction flask to, and

collected in the pre-heated graphite tube with a permanent modifier for 60s. Method

validated against coal reference materials: NIST SRM 1630a, 1632b, SACCRM SARM 19,

20, BCR CRM 40, 180, 181.

[33]

As, Bi,

Se

Marine sediment, soil, coal HGAASb 0.25 As, Bi and Se hydrides generated (batch mode) from acidified (HNO3, HCl) slurries. Vapors

transported to, and collected in the pre-heated graphite tube with a permanent modifier (Ir).

Standard additions method is required. Method validated against NRCC PACS-1 marine

sediment, GBW 07401 soil, NIST SRM 1632c coal, <10% RSD

[26]

216

219

Hg Marine sediment, coal, human hair, seafood CVAASb 0.25 Slurries magnetically stirred, the mercury vapor generated from aqueous slurry samples

(without acid-pretreatment) transported from the reaction flask to, and collected in the

graphite tube with a permanent modifier (Ir). Method validated against reference materials:

PACS-1 marine sediment, DORM-2 dogfish muscle, GBW-07401 soil, NIST-1632a coal,

BCR CRM-397 human hair, <15% RSD

[35]

Sn Marine sediment, soil, coal fly ash, coal HGAASb 0.25 Slurries magnetically stirred, the tin hydride generated (batch mode) from aqueous or

acidified slurries transported to, and collected in the pre-heated graphite tube with a

permanent modifier (Ir). Standard addition method is mandatory. Method validated against

reference materials: NRCC PACS-1 marine sediment, GBW-07401 soil, NIST-1633b coal

fly ash, NIST-1632c coal, <8% RSD

[27]

As, Bi,

Ge, Hg,

Se(IV)

Coal fly ash CVAASb 0.25 Slurries magnetically stirred, hydrides generated (batch mode) from aqueous slurries

transported to, and collected in the pre-heated graphite tube with a permanent modifier (Ir).

Method validated against NIST SRM-1633a coal fly ash, <8% RSD

[34]

As Sediments, coal, fly ash HGAASb 0.05 Slurries mixed with aqua regia and HF by ultrasonication. Arsenic hydride was generated

by adding NaBH4 and arsine vapor was transported to, and collected in the pre-heated

graphite tube, treated with iridium. Method validated against reference materials: NRCC

MESS-2, PACS-2, HISS-1 marine sediments, NIST SRM 2704 Buffalo River Sediment,

SRM 1646a Estuarine sediment, SRM 1632b coal bituminous, SRM 1633b coal fly ash,

BCR 181 coking coal, BCR 180 gas coal, SARM 19 coal, SARM 20 coal. Calibration was

performed using aqueous standard solutions (containing the same acid concentrations as the

slurries), <3.5% RSD

[28]

217

aAmount of sample refers to original test portion used in the analytical procedure. bIn situ trapping.

219

Chapter 12

TABLE 4 Applications of slurry sampling hydride generation atomic absorption spectrometric techniques to the analysis of biological and foodstuff materials

Element

Matrix

Detection

mode

Amount

of samplea/g

Slurry-hydride generation approach

Ref.

As Cigarette tobaccos HGAAS 0.05-0.25 Slurries prepared by mixing of sample with 0.005% Triton X-100 and then sonicated and

microwave treated. Arsenic determined by hydride generation (NaBH4). Method validated

against CRM CTA-OTL-1 oriental tobacco leaves. Calibration technique based on aqueous

standard solutions, <7.6% RSD

[23]

As Wort, gel, waste water HGAASb 10 ml Slurry sample solutions were subjected to sonication-ozonation procedure. As hydrides

generated (continuous flow mode) from acidified slurries. Vapors transported to, and

collected in the pre-heated graphite tube with a permanent modifier (Ir). Method validated

against nine certified reference materials. Calibration by standard addition method, 7.8%

RSD

[39]

Cd Sewage sludge, krill, human hair FI-CVAAS 0.25-0.50 Ultrasonic slurry formation. Volatile Cd species formation (HCl and NaBH4) by adding 4%

NaBH4 (0.5% KCN). Method validated against reference materials: CRM BCR 176 waste

incineration ash, MURST-ISS-A2 Antarctic krill, CRM BCR 397 human hair. Application

of external calibration method, 6-12% RSD

[32]

Hg Living bacterial cells CVAAS 0.006 Mercury vapor generated from Hg-biomass slurry by treating it with Sn(II) or NaBH4 as a

reducing agent. Calibration was achieved by treating standards in the same way as samples,

2.2-5.3% RSD

[38]

218

218

Pb Lettuce, mussel, tomato HGAAS 0.25-1.0 Powdered samples suspended in Triton X-100. Lead hydride generation carried out in an

ammonium persulphate-nitric acid medium by adding 8% NaBH4. Method validated against

IAEA H-9 whole total diet. Calibration by standard addition method

[14]

Pb Vegatables, fish HGAAS 0.25-1.0 Powdered samples suspended in Triton X-100. Lead hydride generation carried out in an

potassium dichromate-lactic acid medium by adding 4% NaBH4. Method validated against

IAEA V10 hay, CRM BCR 281 ryegrass. Calibration by standard adiition method, 5.1%

RSD

[15],[

16]

Pb Fruit FI-HGAAS 1.0 Lead hydride generated in HNO3-H2O2 medium using NaBH4 as reducing agent from slurries

of fresh sample

[18]

Sb Liver tissue HGAAS 0.5 Slurry prepared by sonication and microwave-assisted acid-extractable antimony. The

stibine generated from slurry sample by adding 0.1% NaBH4. Calibration by standard

addition method, ca. 3% RSD

[37]

Se Sediments, garlic HGAAS Selenium determined from slurry samples by hydride generation (HCl, NaBH4) [40]

As,Sb,Se,

Sn,Hg

Wort HGAASb 10 ml Wort slurry solutions were subjected to sonication by adding 50 µl of decanol. Hydride

forming elements determined by hydride generation (batch system) and in situ

preconcentration of the analytes onto the Pd-(for As,Sb,Se,Sn) or Au-pre-heated (for Hg) of

a graphite furnace. Method validated against reference materials: NRCC CASS-2 nearshore

seawater, NASS-2 open ocean seawater, TORT-1 lobster hepatopancreas, IAEA W4

simulated fresh water, Seronorm, urine. Calibration was achieved via the method of standard

addition, ca. 5% RSD

[29]

219

aAmount of sample refers to original test portion used in the analytical procedure. bIn situ trapping.

219

Chapter 12

Acceptable precisions in most instances, reported as percentage RSDs usually range from 1% to slightly higher than 10%, with the most frequent value about 5%. Thus, in general, slurry sampling hydride generating elements can be detected at concentrations below 500 ng/g, and concentrations than are 10 or more times the detection limit can be measured with precision less than 10% RSD.

The accuracy of the present method for the analysis of analytical samples has been checked by different approaches. These include: recovery test and standard addition, use of independent analytical methods of proved validity and verification of the method by means of standards or certified reference materials (CRMs), the latter two methods being mostly applied. In the specific case of biological and environmental samples, a great variety of CRMs such as those issued by the NIST, BCR and IAEA are available. In consequence, the accuracy of the present technique has been mainly checked against these standards. From the survey of the literature it is evident that the accuracy of the slurry sampling hydride generation technique compares favorable with the accuracy of other techniques for these kind of materials.

Linear dynamic ranges for the slurry sampling – hydride generation – AAS vary from two to four orders of magnitude, depending on the particular method used. Since the figures of merit for solution nebulization are comparable to those for slurry nebulization, and the operating procedure is simpler, slurry sample introduction combined with hydride generation technique is the present “method of choice” for sample introduction of the elements. 5 PRACTICAL APPLICATIONS Illustrative applications of slurry sampling hydride generation in AAS have been summarized in Tables 3 and 4. These applications are listed for sample type (matrix) and elements determined, the analytical AAS instrumental mode, the methodological approach are given. In addition, the tables include the reported standard deviation. The RSD is only an informative value and does not differentiate between in-slurry and between-slurry precision, because this is often not stated in the literature and because of very variable numbers of individual measurements. The aim of this section is to examine publications, not merely to present potential users with established methods, but rather to point to the reasons why slurry sampling hydride generation AAS has been used to solve particular problems and to stimulate further interest in its application. The references cited may contain additional determinations, or trials, for a particular sample type. A wide range of applications is clearly evident, showing that slurry-hydride generation approach is applicable widely throughout biological, environmental and foodstuff analysis. The variety and number of samples indicate that future studies involving slurry sample introduction combined with hydride generation technique would be readily applied to the analysis of more complex samples. 6 CONCLUSION AND FUTURE PROSPECTS It is evident from this review that there is considerable research being done, in the last decade, in the area of sampling of pretreated slurry combined with the hydride generation AAS method. The most advantageous designs are probably those which generation of vapor from slurried samples produces reliable analytical data and uses less time for analysis since the full sample digestion (decomposition/dissolution) step is avoided. Consequently, the sample pretreatment is reduced to a slurry preparation procedure. Thus, this combined sample introduction method should complement “conventional” hydride generation for pretreated (i.e., by hazardous acid mixtures) solid materials. The technique, though being inexpensive and having considerable promise; is

220

Chapter 12

in the author’s opinion yet to be firmly established. The ultrasonication of the samples was the most important pretreatment part, and the additional microwave-assisted extraction step was useful for the further improving of analytes extraction efficiency. Although sample homogeneity is a critical factor influencing precision, it is not a determining factor in achieving accurate results. Reliable procedures for homogenization such as magnetic and ultrasonic agitation, and vortex and gas mixing are available. It would be fair to say that most of the fundamental parameters and requirements of this slurry sampling hydride generation technique have been established and virtually all of the work examined in this review mainly concerns applications (reports are summarized in Tables 3 and 4).

Very few observations have been reported concerning the speciation of slurry samples, thus making it difficult to draw any conclusion on this matter, although encouraging preliminary results were obtained for the speciation of analytes in slurried samples.

The present technique may be subject to a number of positive and/or negative systematic errors, which depend on the element to be determined, the analytical AAS instrumental technique, the matrix composition and other factors. However, as shown in this review (Tables 3 and 4) there is tendency of using the method of standard additions to eliminate some possible matrix effects and ensure accuracy of results. Nevertheless, it appears from the survey of the literature that the slurry sampling hydride generation introduction technique compares favorably with the accuracy of the other AAS methods for the determination of trace elements in analytical samples.

The slurry analysis using hydride generation technique should encourage their adoption and be consistently useful in AAS. Continued study and research into improving the analytical performance such as detection limits, precision and accuracy is required.

Further area of growing interest is speciation of elements to supplement the total element figure. In order to obtain data relating to the speciation, in situ hydride generation pre-concentration procedures is especially suitable for speciation work, because this approach allows direct slurry sample analysis without sample preparation (destroying the matrix), and thus also the original speciation of the analyte of interest. Accordingly, it would be desirable that the slurry sampling hydride generation AAS methodology is accepted both as a regular quality control technique and/or as a screening approach in different processes (environmental, biological, foodstuff, etc., fields). In this respect, one can recall its reduced sample manipulation requirements, low turnaround time and relative low cost of implementation. On the other hand, hydride generation from slurried samples for analytical purposes deserves mention for, although not yet combined with atomic emission spectrometric techniques (ICP, MIP, DCP), implementation of this approach for hydride forming elements should be straightforward. REFERENCES [1] Yan X.-P., and Ni Z.-M., Anal. Chim. Acta, 291, 89 (1994) [2] Matusiewicz H., and Sturgeon R.E., Spectrochim. Acta, B51, 377 (1996) [3] Tsalev D.L., J. Anal. At. Spectrom., 14, 147 (1999) [4] Sturgeon R.E., and Mester Z., 56, 202A (2002) [5] Dĕdina J., and Tsalev D.L., Hydride Generation Atomic Absorption Spectrometry,

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