Spectrochimica Acta Part B 66 (2011) 517–521

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Spectrochimica Acta Part B

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Determination of arsenic and selenium by hydride generation and headspace solidphase microextraction coupled with optical emission spectrometry

Anna Tyburska, Krzysztof Jankowski ⁎, Agnieszka RodzikWarsaw University of Technology, Faculty of Chemistry, Department of Analytical Chemistry, ul. Noakowskiego 3, 00-664 Warsaw, Poland

⁎ Corresponding author.E-mail address: kj@ch.pw.edu.pl (K. Jankowski).

0584-8547/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.sab.2011.03.010

a b s t r a c t

a r t i c l e i n f o

Available online 2 April 2011

Keywords:Hydride generationSolid phase extractionArsenicSeleniumOptical emission spectrometry

A hydride generation headspace solid phase microextraction technique has been developed in combinationwith optical emission spectrometry for determination of total arsenic and selenium. Hydrides were generatedin a 10 mL volume septum-sealed vial and subsequently collected onto a polydimethylsiloxane/Carboxensolid phase microextraction fiber from the headspace of sample solution. After completion of the sorption, thefiber was transferred into a thermal desorption unit and the analytes were vaporized and directly introducedinto argon inductively coupled plasma or helium microwave induced plasma radiation source. Experimentalconditions of hydride formation reaction as well as sorption and desorption of analytes have been optimizedshowing the significant effect of the type of the solid phase microextraction fiber coating, the sorption timeand hydrochloric acid concentration of the sample solution on analytical characteristics of the methoddeveloped. The limits of detection of arsenic and seleniumwere 0.1 and 0.8 ngmL−1, respectively. The limit ofdetection of selenium could be improved further using biosorption with baker's yeast Saccharomycescerevisiae for analyte preconcentration. The technique was applied for the determination of total As and Se inreal samples.

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1. Introduction

The determination of total amount of arsenic and selenium infood and beverages is of great importance due to its health effects.The allowed concentration for arsenic and selenium in drinkingwater imposed by the World Health Organization regulation is10 μg L−1.

Solid phase microextraction (SPME) is widely applied for theseparation and preconcentration of analytes prior to the determination,being a simple, effective and environmental friendly methodology. Themost important advantages include low sample and reagent consump-tion and long “lifetime” of the extraction fiber as compared to sorbentcartridges, and high preconcentration rate capability. The SPME iscommonly used in combination with various chromatographic tech-niques in order to determine simultaneously a number of analytespresent in the sample using various detection techniques [1–8]. It can beused also for determination of total content of an element after suitablederivatization procedure without chromatographic separation [9–12].However, the direct coupling of SPMEwith a detection system receivedlittle attention.

Non-volatile analyte species can be collected from the sampleliquid phase and separated by liquid chromatography that has been

exemplary done for As [3] and Hg [4], using inductively coupledplasma mass spectrometry (ICP-MS) and ultraviolet detection,respectively. Volatile compounds can be collected by SPME fromthe sample headspace or liquid phase, directly or after derivatization.Currently, most SPME applications in the field ofmetal determinationconsist in the analyte alkylation and the headspace extraction, followedby the gas chromatography separation of generated species. A variety ofspecies of tin, lead and mercury [5,6] have been determined using thismethod. Additionally, 4,5-dichloro-1,2-phenylodiamine has been uti-lized as a derivatization reagent for the determination of seleniumspecies [13] as well as thioglycol methylate for arsenic [14]. Hydridegeneration (HG) for analyte derivatization prior to the SPME sam-pling has been used for the determination of total As and for thedetection of Se, Sb, Sn [9] and Ge [10] by ICP-MS as well as organo-mercury by gas chromatography atomic absorption spectrometry(GC-AAS) [11,12] or arsenic and selenium species by GC-AAS and gaschromatography optical emission or mass spectrometry (GC-OES/MS) [15,16].

In this work we combine the HG reaction with the headspace solidphase microextraction (HSSPME) technique for separation and pre-concentration of As and Se followed by the optical emission spectro-metric determination without chromatographic separation. Tracearsenic and selenium were determined in beer, wort, brewer yeastand the Selm-1 selenium yeast certified reference material (CRM), andin theharddrinkingwaterCRM(ERM-CA011a). TheuseofHG-HSSPME-ICP-OES for determination of arsenic and selenium in real samples hasnot been reported previously.

Table 1Experimental conditions for HG-HSSPME-ICP/MIP-OES.

ICP-OESIncident power (W) 1000Carrier gas flow rate (L min−1) 0.5Outer argon flow rate (L min−1) 12Intermediate argon flow rate (L min−1) 0.5Height above coil (mm) 4

MIP-OESMicrowave power (W) 150Helium flow rate (L min−1) 0.35

As Se

Wavelength (nm) 278.022228.812 203.985

HG acidic modeBorohydride concentration (%) 0.06 0.06HCl concentration (mol L−1) 2 3

HG alkaline modeNaOH concentration (mol L−1) 1 –

Borohydride concentration (%) 0.12 –

SMPEFiber coating PDMS/Carboxen Stable Flex

85 μmSorption time (min) 2 7Desorption time (s) 2 2Thermal desorption temperature (°C) 150

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2. Experimental

2.1. Reagents

High grade analytical reagent chemicals were employed for thepreparation of all solutions. Sodium borohydride solutions (Merck,Germany) in 0.5% (m/v) sodium hydroxide were prepared fresh dailyand filtered.

Standard solution of selenium (1 mg mL−1) was prepared bydissolution of suitable amount of sodium hydrogen selenite (POCH,Poland) in a 3 mol L−1 hydrochloric acid, followed by dilution withMILLI-Q purified water.

Standard solution of arsenic (1 mg mL−1) was prepared bydissolution of suitable amount of arsenic(III) oxide (POCH, Poland)in a small volume of 2 mol L−1 sodium hydroxide, then acidified withhydrochloric acid and diluted with MILLI-Q purified water. Thesestandard solutions were stable for at least one month.

Standard solution of arsenic in alkaline medium (100 μg mL−1)was prepared by dissolution of suitable amount of arsenic(III) oxide(POCH, Poland) in a 2 mol L−1 sodium hydroxide.

The baker's yeast Saccharomyces cerevisiae (Sigma-Aldrich, USA)were used for biosorption of trace selenium from solutions.

Selenium yeast (Selm-1) by NRC-CNRC, Canada and hard drinkingwater (ERM-CA011a) by LGC, United Kingdom, certified for arsenicand selenium concentration, were selected for analysis to assess theaccuracy of the technique.

Beer, wort and brewer yeast samples were purchased in a localsupermarket and brewery, respectively.

2.2. Apparatus

The analytical system consists in the laboratory-made thermaldesorption unit based on the construction of glass-lined splitless GCtype heated injector directly coupled to the ICP-OES spectrometer(Integra XL, GBC Australia) with radial viewing. Alternatively, thethermal desorption unit was directly coupled to the heliummicrowaveinduced plasma (MIP) system based on the TE101 rectangular cavity(Plazmatronika, Poland) [17]. The axially emitted radiation wastransferred to the Integra XL monochromator with the use of theoptical fiber (wavelength range 200–900 nm).

Three types of polydimethylsiloxane-based (PDMS) commercialSPME coatings were examined: PDMS/Carboxen Stable Flex 85 μm,PDMS/Carboxen 75 μm and PDMS 65 μm coating. The fibers wereconditioned and used at temperature specified by the manufacturer(Supelco, USA). The experimental conditions applied for ICP-OES,MIP-OES, HG and SPME are summarized in Table 1.

2.3. Analytical procedure

2.3.1. Acidic modeFive ml of a sample containing arsenic or selenium in hydrochloric

acid 2–3 mol L−1 was placed in a 10 mL vial together with a stirringbar, the vial was closed with an aluminium seal and a septum. Thenthe sodium borohydride solution was injected into the vial with theuse of microsyringe and hydride generation was carried out. After asuitable time a SPME fiber placed in a fiber holder was immersedthrough the septum for headspace sampling. The loaded fiber wastransferred to the desorption unit for measurement directly aftersampling.

2.3.2. Alkaline modeFive ml of the sample containing arsenic in 1 mol L−1 NaOH

solution was placed in a 10 mL vial as described above. Then theNaBH4 solution was injected and after 1 h the hydrochloric acid wasinjected to the sample solution to release the volatile hydride. Then

the analyte extraction and desorption procedure with a SPME fiberwas carried out.

2.3.3. Determination of Se in yeast samplesA weight of yeast was placed into a 10 mL vial and 5 mL of

hydrochloric acid 3 mol L−1 was added for lysis of cells. The solutionwas stirred for 30 minand thenhydride generationprocedureaccordingto the acidic mode was conducted.

2.3.4. Selenium preconcentration with yeastA portion of 0.2 g of baker's yeast was introduced to the sample

solution (pHabout7) containing selenium.Next themixturewas stirredfor 1.5 h at temperature of 40 °C and then centrifuged. The yeast werewashed with water and again centrifuged. Then the sample was placedin the vial, and the lysis of cells and hydride generationwere carried out.

3. Results and discussion

3.1. Selection of the detection system

Initial studies were undertaken with the use of helium MIP-OES,because helium plasma is known as highly effective for excitation ofhard-to-ionize elements i.e. As and Se. The respective hydrides weregenerated in a batch process, sampled by SPME and subsequentlyreleased directly to the plasma gas by thermal desorption. The thermaldesorption unit was placed directly at the base of the discharge tube tominimize the length of the analyte transfer-line. Unfortunately, adramatic decrease of the signal measured at wavelengths below240 nm was observed for the optical fiber used. Nevertheless, theoptimization of experimental conditions for the determination of arsenicwas carried out for the less intense line at 278.022 nm. The conventionaloptimization of measurement conditions was done and working para-meters are presented in Table 1.

The experimental conditions for the determination of seleniumwere optimized with the use of the argon ICP-OES and the desorptionunit directly coupled to the base of the plasma torch. The selectedmeasurement conditions are summarized in Table 1.

Fig. 2. Transient signals arising from desorption of As (a) and Se (b) hydrides from thePDMS/Carboxen Stable Flex fiber based on signal from As(I) 228.812 nm and Se(I)203.985 nm.

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3.2. Optimization of the SPME procedure

TheHS-SPME by sampling of volatile hydrides from the vapour phaseoffers a reduction of the possible interferences from most concomitantelements during both the sorption and the desorption stage, includingexcess hydrogen as well as a better plasma stability due to the lowplasma loading. Additionally, thefiber is better protected fromexposureto aggressive HG reagents that prolongs the lifetime of the fiber.

Three different fibers PDMS/Carboxen Stable Flex 85 μm, PDMS/Carboxen 75 μm and PDMS 65 μm were tested in order to investigatetheir sorption capacity.

As expected from studies by Mester et al. [9], the PDSM/Carboxencoating was found to be better suited with the sampling and thedesorption of trace amounts of metal hydrides. The sharp transientsignal obtained enables the use of the peak height for the calibrationinstead of the peak area which simplifies the calibration procedure.The shape of the peak also indicates that desorption of the hydridesfrom the Carboxen coating was relatively fast. The PDMS/CarboxenStable Flex fiber with the 85 μm-thick coating exhibits higher sorptioncapacity in comparison with PDMS/Carboxen 75 μm resulting in higherpreconcentration rates and wider linear dynamic ranges. Therefore, forfurther experiments the PDMS/Carboxen Stable Flex fiber was selected.

An optimal time of the fiber exposure was examined to allow theanalytes to reach equilibrium between the headspace of the sampleand the fiber coating. Fig. 1 shows the effect of the fiber exposure timeon analyte signal intensity. The selected sorption time was 2 and 7 minfor As and Se, respectively. It is much longer than this obtained byMester et al. [9]. This could be explained by the slower releasing of thehydrides to the headspace due to the lower sodium borohydrideconcentrationused.Desorptionof analytes fromthefiberwas controlledat scan time lasting50 sas shown inFig. 2.Underexperimental conditions,the signals for both As and Se were observed after 2–3 s, Fig. 3.

3.3. Optimization of the hydride generation conditions

The optimization of hydride generation conditions for arsenic andselenium from the acidic medium was carried out by HG-HSSPME-MIP-OES and HG-HSSPME-ICP-OES, respectively. Substantial opti-mization was not undertaken, because this information is readilyavailable from the literature [9]. Fig. 4 shows the dependence of theconcentration of hydrochloric acid on the peak height for both As andSe. The optimum HCl concentration was 2 and 3 mol L−1 for As and

0

20 000

40 000

60 000

80 000

100 000

120 000

140 000

0 1 2 3

Desorption time (s)

Inte

nsity

(co

unts

)

As

Se

Fig. 3.Effect of desorption time of arsenic and seleniumhydrides from thePDMS/CarboxenStable Flex fiber on the signal measured.

0 2 4 6 8 10

Sorption time (min)

Inte

nsity

(co

unts

)

As

Se

0

2 000

4 000

6 000

8 000

10 000

12 000

14 000

16 000

18 000

Fig. 1. Effect of sorption time of arsenic and selenium hydrides on the PDMS/CarboxenStable Flex fiber on the signal measured.

0

500

1 000

1 500

2 000

2 500

3 000

3 500

0 1 2 3 4 5

HCl concentration (mol L-1)

Inte

nsity

(co

unts

)

Se

As

Fig. 4. Effect of HCl concentration on the signal measured for As and Se.

Table 2Analytical figures of merit for the proposed method by HG-HSSPME-ICP-OES.

Parameter Se As

Detection limit (ng mL−1) 0.8 0.1Precision (%) RSD 3.4 2.5Linear dynamic ranges (μg mL−1) 0.004–25 0.001–12Correlation coefficient 0.9992 0.9943

520 A. Tyburska et al. / Spectrochimica Acta Part B 66 (2011) 517–521

Se, respectively. The effect of NaBH4 concentration on signal measuredis shown in Fig. 5. This effect is not significant providing that theconcentration is higher than 0.06%. The concentration of 0.06% wasselected, which is lower than this reported in [9], to reduce the pressurerising in a vial due to the hydrogen released and as a result to minimizepossible analyte losses during decompression process as suggested byMester et al. It should be noted that the precipitation of elemental Seoccurred when the concentration of NaBH4 in the solution injected tothe vial exceeded 5%. For this reason a suitable volume of the 5% NaBH4

solution was used for the generation of selenium hydride.Hydride generation from an alkaline medium was also studied,

because the alkaline dissolution is sometimes useful for samplepreparation. The concentrations of NaOH and NaBH4 in the samplesolution and that of HCl added for hydride releasing were optimized(Table 1). However, the signals obtained for As and Seweremuch lowerthan those obtained for acidic mode, and the latter mode was used infurther studies.

3.4. Analytical performance

After optimization studies for arsenic by HG-HSSPME-MIP-OES thelimit of detection (LOD) of 2.9 ngmL−1 was determined at 278.022 nm.

0

500

1 000

1 500

2 000

2 500

3 000

3 500

0.0 0.1 0.2 0.3 0.4

NaBH4 concentration (%)

Inte

nsity

(co

unts

)

As

Se

Fig. 5. Effect of NaBH4 concentration on the signal measured for As and Se.

The respective LOD value by HG-HSSPME-ICP-OES was found to be0.9 ngmL−1. Additionally, the limit of detection (3σ) of 0.1 ngmL−1 forarsenic at 228.812 nm was determined. For this reason only ICP-OESwas applied for the detection of arsenic and selenium in the samplesexamined. Analytical figures of merit, based on the peak height, aresummarized in Table 2. When peak area was used for calibration, arelatively lower correlation coefficient of 0.9811 was calculated.Interestingly, the LOD for arsenic is comparable to the value reportedby Mester et al. [9] for HG-HSSPME-ICP-MS detection (0.07 ng mL−1)while the LOD for selenium is even 6.5 times lower (5.3 ngmL−1 byHG-HSSPME-ICP-MS). This can be explained by the occurrence of the wellknown polyatomic interferences limiting the detection of As and Se byICP-MS. The precision calculated from the relative standard deviation(RSD) of the mean of five replicate measurements of analyte standardsusing a concentration 100-fold above the LOD is better than thatreported in [9] probably due to the use of the lower borohydrideconcentration resulting in less vigorous HG reaction and lower analytelosses. The linear dynamic ranges cover more than four orders ofmagnitude.

3.5. Determination of selenium and arsenic in real samples

The proposed method was applied to the determination of totalselenium and arsenic in hard drinking water, beer, wort and yeastsamples. Despite that the analytes are present in widely consumedbeverages at low concentrations, they are essential or toxic in thehuman body and should be determined. The possibility of determiningAs and Se at ng mL−1 level in beer and wort samples by direct HG hasbeen proved previously [18,19]. The results are given in Table 3. Analyterecoveries were in the range 96–103%. Both arsenic and seleniumcontent in the CRM Selm-1 are in good agreement with the certifiedvalues. The same procedure was applied for the determination of totalselenium in yeast samples (Table 4). However, after placing a weight ofthe sample in the vial, the lysis of cells was first conducted with the useof 3 mol L−1 HCl during 30 min. The seleniumcontent in the CRMSelm-1 is in good agreement with the certified value. It is well known thatselenium is present in biologicalmatter in various organic and inorganicforms. However, the possibility of releasing volatile selenium com-pounds frombiologicalmatrices in the HG systemhas been proved [16].Obviously, in our experimental system the selenium species formedcould not be identified. Finally, brewer's yeast S. cerevisiaewere appliedfor selenium preconcentration from the samples because the seleniumcontent in beer was too low for the direct determination by HG-

Table 3Determination of As and Se by HG-HSSPME-ICP-OES in samples without preconcentra-tion and after preconcentration using baker yeasts.

Sample Determined content (ng mL−1) Certified value(ng mL−1)

withoutpreconcentration

afterpreconcentration

As Se Se As Se

ERM-CA011a 10.4±0.8a 10.4±0.7 10.8±0.8 10.1±0.6 10.7±0.7Wort“Ciechan”

– 15.6±0.9 15.2±0.8 – –

Beer “Lech” 8.3±0.8 b LOD 1.32±0.1 – –

a Mean value±standard deviation, for 5 determinations.

Table 4Determination of Se in yeast by HG-HSSPME-ICP-OES.

Sample Se found (ng mL−1) Certified value (ng mL−1) Recovery (%)

CRM Selm-1 2049±72a 2059±64 98Brewer yeast 61.1±1.5 – 99

a Mean value±standard deviation, for 5 determinations.

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HSSPME-ICP-OES method. The selenium uptake by yeast from watersamples was studied by Perez-Corona et al. [20] under slightly differentexperimental conditions. In our optimization studies the pH value of 7for the sample solution, the yeast weight of 0.2 g, the sorptiontemperature of 40 °C and the sorption time of 1.5 hour were selected.They are similar to those reported in [20] except the sorption time. Ayeast biomass was separated by centrifugation after the biosorption ofselenium and placed in the vial for the lysis of cells and subsequentdetermination byHG-HSSPME-ICP-OES. The selenium content in beer isin agreement with the previous results by hydride generationelectrothermal atomic absorption spectrometry with in situ preconcen-tration [18]while the selenium content in ERM-CA011a is in agreementwith the certified value.

4. Conclusion

The HG-HSSPME preconcentration technique has proved to beattractive in combination with ICP-OES for detection of arsenic andselenium and can compete with the ICP-MS detection system. Theproposed method allowed determination of total arsenic andselenium in various samples including drinking water, beer andyeast with a good linear response range and limit of detection. Forarsenic both alkaline and acidic mode of hydride generation can beused, but, in general, better signal characteristics were obtained forthe latter. For calibration both the peak height and the peak area of thetransient signal are useful however, the former was found moresuitable in this study. Additionally, selenium preconcentration bybiosorption using yeast can be applied for the extension of the analyteconcentration range available.

Acknowledgement

This work was financially supported by the Warsaw University ofTechnology.

References

[1] V. Kaur, A.K. Malik, N. Verma, Applications of solid phase microextraction for thedetermination ofmetallic and organometallic species, J. Sep. Sci. 29 (2006) 333–345.

[2] J.C. Wu, Z. Mester, J. Pawliszyn, Speciation of organoarsenic compounds bypolypyrrole-coated capillary in-tube solid phase microextraction coupled withliquid chromatography/electrospray ionization mass spectrometry, Anal. Chim.Acta 424 (2000) 211–222.

[3] C. Jia, Y. Luo, J. Pawliszyn, Solid phase microextraction combined with HPLC fordetermination of metal ions using crown ether as selective extracting reagent,J. Microcolumn Sep. 10 (1998) 167–173.

[4] H. Kataoka, H.L. Lord, J. Pawliszyn, Applications of solid-phase microextraction infood analysis, J. Chromatogr. A 880 (2000) 35–62.

[5] L. Moens, T. De Smaele, R. Dams, P. Van Der Broeck, P. Sandra, Sensitive,Simultaneous determination of organomercury, -lead, and -tin compounds withheadspace solid phase microextraction capillary gas chromatography combinedwith inductively coupled plasma mass spectrometry, Anal. Chem. 69 (1997)1604–1611.

[6] S. Aguerre, G. Lespes, V. Desauziers, M. Potin-Gautier, Speciation of organotins inenvironmental samples by SPME-GC: comparison of four specific detectors: FPD,PFPD, MIP-AES and ICP-MS, J. Anal. At. Spectrom. 16 (2001) 263–269.

[7] N. Campillo, R. Peñalver, I. Lopez-Garcia, M. Hernandez-Cordoba, Headspace solid-phase microextraction for the determination of volatile organic sulphur andselenium compounds in beers, wines and spirits using gas chromatography andatomic emission detection, J. Chromatogr. A 1216 (2009) 6735–6740.

[8] C. Dietz, J. Sanz Landaluze, P. Ximenez-Embun, Y. Madrid-Albarran, C. Camara,Volatile organo-selenium speciation in biological matter by solid phase micro-extraction–moderate temperature multicapillary gas chromatography withmicrowave induced plasma atomic emission spectrometry detection, Anal.Chim. Acta 501 (2004) 157–167.

[9] Z. Mester, R.E. Sturgeon, J.W. Lam, Sampling and determination of metal hydridesby solid phase microextraction thermal desorption inductively coupled plasmamass spectrometry, J. Anal. At. Spectrom. 15 (2000) 1461–1465.

[10] X. Guo, Z. Mester, R.E. Sturgeon, Comparison of chloride- and hydride-generationfor quantitation of germanium by headspace solid-phase microextraction–inductively coupled plasma–mass spektrometry, Anal. Bioanal. Chem. 373(2002) 849–855.

[11] B. He, G.B. Jiang, Z.M. Ni, Determination of methylmercury in biological samplesand sediments by capillary gas chromatography coupled with atomic absorptionspectrometry after hydride derivatization and solid phasemicroextraction, J. Anal.At. Spectrom. 13 (1998) 1141–1144.

[12] B. He, G.B. Jiang, Analysis of organomercuric species in soils from orchards andwheat fields by capillary gas chromatography on-line coupled with atomicabsorption spectrometry after in situ hydride generation and headspace solidphase microextraction, Fresenius J. Anal. Chem. 365 (1999) 615–618.

[13] N. Campillo, R. Peñalver, M. Hernandez-Cordoba, C. Perez-Sirvent, Comparison oftwo derivatizing agents for the simultaneous determination of selenite andorganoselenium species by gas chromatography and atomic emission detectionafter preconcentration using solid-phase microextraction, J. Chromatogr. A 1165(2007) 191–199.

[14] Z. Mester, J. Pawliszyn, Speciation of dimethylarsinic acid andmonomethylarsonicacid by solid-phase microextraction–gas chromatography–ion trap mass spec-trometry, J. Chromatogr. A 873 (2000) 129–135.

[15] S.J. Santosa, Arsenic and selenium species behavior during microwave-assistedconversion to their volatile hydride forming species, Anal. Sci. 17 (2001)i125–i128.

[16] A. Chatterjee, Y. Shibata, M. Yoneda, R. Banerjee, M. Uchida, H. Kon, M. Morita,Identyfication of volatile selenium compounds produced in the hydridegeneration system from organoselenium compounds, Anal. Chem. 73 (2001)3181–3186.

[17] K. Jankowski, R. Parosa, A. Ramsza, E. Reszke, Vertically positioned axially viewedaerosol cooled plasma – a new design approach for microwave induced plasmaoptical spectrometry with solution nebulization, Spectrochim. Acta, Part B, AtomicSpectroscopy 54 (1999) 515–525.

[18] H. Matusiewicz, M. Mikołajczak, Determination of As, Sb, Se, Sn and Hg in beerand wort by direct hydride generation sample introduction – electrothermalAAS, J. Anal. At. Spectrom. 16 (2001) 652–657.

[19] M. Segura, Y. Madrid, C. Camara, Evaluation of atomic fluorescence and atomicabsorption spectrometric techniques for the determination of arsenic in wine andbeer by direct hydride generation sample introduction, J. Anal. At. Spectrom. 14(1999) 131–135.

[20] T. Perez-Corona, Y. Madrid, C. Camara, Evaluation of selective uptake of selenium(Se(IV) and Se(VI)) and antimony (Sb(III) and Sb(V)) species by baker's yeastcells (Saccharomyces cerevisiae), Anal. Chim. Acta 345 (1997) 249–255.