- Email: [email protected]
Chapter 56 Tellurium
Chapter56
Tellurium
Tellurium (Te, atomic weight 127.6, melting point 450°C, d = 6.24 g cm -3) is a metalloid which exists in two allotropic forms: amorphous black powder and silver-white shiny crystals. It occurs in the earth's crust with an average abundance of 0.01 ppm, primarily in pyrite ores. Tellurium dissolves in dilute HNO 3 to give tellurous acid, H2 TeO3, in oleum to give a red solution of Te4+ and in a polysulphide solution to form tetrathiotellurate, TeS 2-. The element is commonly found in oxidation states -II, IV and VI, Te(IV) being the most stable. The amphoteric Te(IV) is precipitated by NH 3(aq) and alkalis. Elemental Te is precipitated from acidic or ammonia TeO- solutions by SO 2, SnCl2 and NH2 OH or NH 4, respectively. Strong oxidants (e.g., MnO-) oxidize tellurite (TeO3-) to tellurate (TeO4-). The need for trace analysis for Te results from occupational exposure concerns (Te compounds are toxic) and its applications in electronics and metallurgy (microalloying). 56.1 SEPARATION AND PRECONCENTRATION Volatilization
Tellurium hydride TeH 2 is obtained is acidic media by reduction of Te(IV) with NaBH 4 . To form the hydride Te(VI) must be reduced to Te(IV), usually by boiling with HCI [1-3]. The hydride formation is interfered by transition metals, e.g. Cu and Ni, and hydride-forming elements (As, Bi, Se, Sn and Pb) [2,4-6]. Iron(III) was reported to alleviate the Cu and Ni interference [4]. Lead matrix was removed by precipitation [5]. Tellurium hydride was preconcentrated by trapping in a graphite furnace at 150500°C [1,31. Formation of a volatile Te compound by reaction with (4fluorophenyl)magnesium bromide has been reported [7]. 707
Extraction
Extraction of Te(IV) from concentrated HCl with MIBK has been proposed [8]. Elements forming oxychlorocomplexes (Fe(III), Cr(VI), Sn(IV)] should be removed, e.g. by extraction from an HCl medium into ethylacetate, after oxidation of Te(IV) to Te(VI) with Cr20O7. Tellurium was separated from excess of Cu, Fe(III), Pb and Zn by extraction of its xanthate complex from 9.5 M HC1 into cyclohexane in the presence of thiosemicarbazide to mask Cu [9]. Tellurium can be stripped from the extract with 16 M HNO 3 [9]. For samples rich in Cu the preliminary separation of Te by coprecipitation with Fe2 0 3(aq) is necessary [9]. Co-extraction of As is avoided by AsBr3 during the decomposition step [9]. Extraction of the Te-bis(trifluroethyl)dithiocarbamate complex into toluene has been reported [7]. Coprecipitation
Coprecipitation of Te(IV) with Fe(OH) 3 at pH 8-9 (followed by flotation) [10] and with Mg(OH) 2 [31 has been reported. Elemental Te was coprecipitated with As and Pd as collector on reduction with hypophosphorous acid [51 and ascorbic acid [11], respectively. Cation exchange
Cation exchange has been applied to the separation of Te(VI) (anionic) and Te(IV) (retained as TeO(OH) + or Te(OH)') complexes [12,13]. Cation exchange separation was used to retain the interfering Cu, Hg, Ni while Te was eluted with dilute HCl [13]. 56.2 DETERMINATION TECHNIQUES Atomic absorptionspectrometry
Flame AAS offers a sensitivity of ca 0.5 gg ml-l in the recommended air-C 2H 2 oxidizing (lean, blue) flame at the most sensitive 214.3 nm line. The signal is suppressed by large excess of Ca, Cu, Si, Na, Zn and Zr and matrix matching is required. An EDL is available but offers hardly any gain in sensitivity. Quartz furnace AAS offers a DL of 0.02 ng ml-l on atomization of TeH 2 [5]. Dimethyl- and diethyltellurium were determined by GC QF AAS, with ADLs in the low nanogram range [14]. Graphite furnace AAS offers an ADL of 1 pg. Tellurium losses in the drying-ashing step are common unless a suitable matrix modifier, e.g. Pd [8,11], Pd-Mg(NO3) 2 [15], H 2PtC16 [16] or Ni [9], is used. Inter708
ferences can be eliminated by trapping TeH 2 in the GF [3] or extraction [8]. A silver-coated graphite atomizer has been recommended for the determination of Te in organic matrices [17]. Plasma source atomic emission spectrometry
Plasma source AES offers a DL of 50 ng ml-1 at the most intensive 214.28 and 238.58 nm ICP emission lines. The former is interfered with by Cu. Hydride generation is preferred for sample introduction, often in multielement mode (cf. Part II). The HG DCP AES determination of Te has been optimized [6]. Mass spectrometry
Natural Te consists of eight isotopes (the most important are 130Te, and 126Te) whereas 127 Te (0.87%) is unstable with a half-life of ca 1013 years. The sensitivity of ICP MS is poor; 130Te is the most sensitive. Tellurium isotopes l30 Te and 126Te have been determined by ETV ICP MS with a DL of 5-10 ng ml-l [18]. Trace amounts of Te were determined by the redox sub-superequivalence method of ID analysis with a DL of 2 pg ml-l [19]. On-line addition of enriched Te (12 5Te) as an internal standard for automated ID ICP MS determination of Te in waste waters has been developed [20]. Spark source ID MS has been used for the analysis of copper [21]. Isotope dilution GC MS after enrichment with 120 Te and derivatization with (4-fluorophenyl)magnesium bromide has been proposed [7]. 128Te,
Fluorescence techniques
A FI ND AFS using an Ar-H 2 flame was reported to give an ADL of 0.02 ng for TeH2 [221. Electrothermal LE AFS was reported to achieve an ADL of 20 fg; molecular backgrounds from nitric oxide and silicon monoxide have been discussed [23]. 56.3 ANALYSIS OF REAL SAMPLES Analytical procedures for the determination of Te are summarized in Table 56.1. Environmental materials
Elemental Te ° and gaseous forms of Te(IV) and Te(VI) can be adsorbed on gold-coated beads [12] and charcoal [27] but at higher flow 709
TABLE 56.1 Combined procedures for the determination of tellurium Sample (amount)
Sample decomposition
Separation
Air
none
sorption on Au-coated GF AAS beads, selective leaching, cation exchange to separate Te(VI) and Te(IV)
0.03
matrix removal by cation exchange
GF AAS
n.g.
none Sea, rain water (4 1)
copptn. with Mg(OH) 2, volatn. as TeH 2
GF AAS
60 pg/l
3
Seawater (0.05 1)
none
volatn. as TeH 2 and trapping in a GF
GF AAS
2-4 pg
1
Geochemical (1 g)
HNO 3-HC10
extrn. with xanthate (cyclohexane), backextrn. (HNO 3)
GFAAS
8 ng/g
9
Urine (1 ml)
HNO 3 -H2 02
extrn. with bis(triGCIDMS fluoroethyl)-DTC (toluene), derivatization with (4-fluorophenyl)MgBr
n.g.
7
Urine (50 ml) HNO3-HCl04
extrn. of TeCl 4 (MIBK)
GF AAS
0.1 ng/ml
8
Urine
HC104-HNO 3
volatn. as TeH2
QF AAS
1 ng
2, 25
Blood
HNO 3
extrn. into MIBK
GF AAS
1 ng/ml
26
Tissues
hyamine hydroxide
none
GF AAS
170 ng/g 16
volatn. as TeH 2, trapping in GF
GF AAS
2-4 pg
1
copptn. with Pd
GF AAS
10 ng
11
pptn.of Pb as PbCl 2; copptn. as Te ° with As
GF AAS
n.g.
5
pptn. of Pb as PbC1 2; volatn. as TeH 2
QF AAS
n.g.
5
Aerosols (0.lg)
HNO 3 -HCIO,4
Bio and HNO 3-HC10 aerosol (0.2 HCI-HF g)
4
4-
Iron Pb-based HNO3-HC10 alloys (2 g)
4
Pb-based HNO3-HCl0 4 alloys (10 g)
710
Detection
DL
Ref. 12 3
ng/m
13, 24
rates the efficiency of the gold-coated beads decreases, in contrast to charcoal [27]. The species can be thermally desorbed or selectively leached with water [Te(VI)], 1 M HCl [Te(IV)] and 3 M HNO3 (Te ° ) [27]. Graphite furnace AAS is the usual determination technique [12,27]. In natural waters Te needs to be preconcentrated, e.g. by coprecipitation with Mg(OH)2 [3] or volatilization followed by trapping of TeH 2 in a GF [1]. Adsorption and colloidal behaviour of Te(VI) in aquatic solutions has been discussed [28]. Preliminary results of Te speciation in wastewater by LC ICP MS have been presented [29]. Biological materials Clinical samples require digestion (usually by HN0 3) to enable extraction of TeC14 [8] or volatilization of TeH 2 [2]. Detection limits in direct analysis are a factor of 10 poorer [161. Tellurium has been determined by GF AAS in blood plasma of rabbits with a DL of 1 ng ml-' [30]. Graphite furnace AAS and HG AAS have been compared for the determination of Te in urine [31]. The use of stable and radioactive isotopes for the determination of biokinetic parameters of Te in rabbits has been compared; tracer solutions enriched in stable (124Te or 126Te) and radioactive (12lmTe or 123mTe) isotopes were administered to animals followed by the analysis of the blood samples by SIMS and y-ray spectrometry [26].
REFERENCES 1 2 3 4 5 6 7
B.M. Yoon, S.C. Shim, H.C. Pyun and D.S. Lee, Anal. Sci., 6 (1990) 561. R. Kobayashi and K. Imaizumi,Anal. Sci., 7 (1991) 447. M.O. Andreae,Anal. Chem., 56 (1984) 2064. T. Wickstr0m, W. Lund and R. Bye, Anal. Chim. Acta, 208 (1988) 347. G.J. Fox, At. Spectrosc., 11 (1990) 13. H. Hayrynen, L.H.J. Lajunen and P. Perimiki, At. Spectrosc., 6 (1985) 88. S.K. Aggarwal, M. Kinter, J. Nicholson and D.A. Herold, Anal. Chem., 66 (1994) 1316. 8 R. Kobayashi and K. Imaizumi, Anal. Sci., 6 (1990) 83. 9 E.M. Donaldson and M.E. Leaer, Talanta, 37 (1990) 173. 10 S. Nakashima, Anal. Chim. Acta, 157 (1984) 187. 11 T. Ashino, K. Takada and K. Hirokawa, Anal. Chim. Acta, 297 (1994) 443. 12 S. Muangnoicharoen, K.Y. Chiou and O.K. Manuel, Talanta, 35 (1988) 679. 13 K.Y. Chiou and O.K. Manuel, Anal. Chem., 54 (1984) 2721. 14 V.T. Demarin, E. Portnova, N.K. Rudnevskii, E.N. Karataev, V.I. Telnoi, 711
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
712
M.Y. Gatilov and I.A. Freshchenko, Zh. Anal. Khim., 46 (1991) 188. B. Welz, G. Schlemmer and R. Mudakavi, J. Anal. At. Spectrom., 7 (1992) 1257. Z.H. Siddik and R.A. Newman, Anal. Biochem., 172 (1988) 190. Z. Ni, B. He and H. Han, J. Anal. At. Spectrom., 8 (1993) 995. R.A. Newman, S. Osborn and Z.H. Siddik, Clin. Chim. Acta, 179 (1989) 191. H. Yoshioka, Y. Miyaki and K. Hasegawa, Analyst, 116 (1991) 821. H. Klinkenberg, T. Beeren, W. Van Borm, F. van der Linden and M. Raets, Spectrochim. Acta, 48B (1993) 649. E.S. Beary, P.J. Paulsen and G.M. Lambert, Anal. Chem., 69 (1988) 733. T. Guo, M. Liu and W. Schrader, J. Anal. At. Spectrom., 7 (1992) 667. Z. Liang, R.F. Lonardo and R.G. Michel, Spectrochim. Acta, 48B (1993) 7. K.Y. Chiou and O.K. Manuel, Environ. Sci. Technol., 20 (1986) 987. R. Kobayashi and K. Imaizumi, Biomed. Res. Trace Elem., 1 (1990) 235. T. Kron, K. Wittmaack, C. Hansen and E. Werner, Anal. Chem., 63 (1991) 2603. S. Muangnoicharoen, K.Y. Chiou and O.K. Manuel, Anal. Chem., 58 (1986) 2811. Y. Yamaashi and Y. Maruyama, J. Radioanal. Nucl. Chem., 170 (1993) 347. H. Klinkerberg, S. Van der Wal, J. Frusch, L. Terwint and T. Beeren, At. Spectrosc., 11 (1990) 198. T. Kron, Ch. Hansen and E. Werner, Colloq. Atomspektrom. Spurenanal., 5 (1989) 809. R. Kobayashi and K. Imaizumi, Anal. Sci., 7 (1991) 841.
Tellurium
Tellurium (Te, atomic weight 127.6, melting point 450°C, d = 6.24 g cm -3) is a metalloid which exists in two allotropic forms: amorphous black powder and silver-white shiny crystals. It occurs in the earth's crust with an average abundance of 0.01 ppm, primarily in pyrite ores. Tellurium dissolves in dilute HNO 3 to give tellurous acid, H2 TeO3, in oleum to give a red solution of Te4+ and in a polysulphide solution to form tetrathiotellurate, TeS 2-. The element is commonly found in oxidation states -II, IV and VI, Te(IV) being the most stable. The amphoteric Te(IV) is precipitated by NH 3(aq) and alkalis. Elemental Te is precipitated from acidic or ammonia TeO- solutions by SO 2, SnCl2 and NH2 OH or NH 4, respectively. Strong oxidants (e.g., MnO-) oxidize tellurite (TeO3-) to tellurate (TeO4-). The need for trace analysis for Te results from occupational exposure concerns (Te compounds are toxic) and its applications in electronics and metallurgy (microalloying). 56.1 SEPARATION AND PRECONCENTRATION Volatilization
Tellurium hydride TeH 2 is obtained is acidic media by reduction of Te(IV) with NaBH 4 . To form the hydride Te(VI) must be reduced to Te(IV), usually by boiling with HCI [1-3]. The hydride formation is interfered by transition metals, e.g. Cu and Ni, and hydride-forming elements (As, Bi, Se, Sn and Pb) [2,4-6]. Iron(III) was reported to alleviate the Cu and Ni interference [4]. Lead matrix was removed by precipitation [5]. Tellurium hydride was preconcentrated by trapping in a graphite furnace at 150500°C [1,31. Formation of a volatile Te compound by reaction with (4fluorophenyl)magnesium bromide has been reported [7]. 707
Extraction
Extraction of Te(IV) from concentrated HCl with MIBK has been proposed [8]. Elements forming oxychlorocomplexes (Fe(III), Cr(VI), Sn(IV)] should be removed, e.g. by extraction from an HCl medium into ethylacetate, after oxidation of Te(IV) to Te(VI) with Cr20O7. Tellurium was separated from excess of Cu, Fe(III), Pb and Zn by extraction of its xanthate complex from 9.5 M HC1 into cyclohexane in the presence of thiosemicarbazide to mask Cu [9]. Tellurium can be stripped from the extract with 16 M HNO 3 [9]. For samples rich in Cu the preliminary separation of Te by coprecipitation with Fe2 0 3(aq) is necessary [9]. Co-extraction of As is avoided by AsBr3 during the decomposition step [9]. Extraction of the Te-bis(trifluroethyl)dithiocarbamate complex into toluene has been reported [7]. Coprecipitation
Coprecipitation of Te(IV) with Fe(OH) 3 at pH 8-9 (followed by flotation) [10] and with Mg(OH) 2 [31 has been reported. Elemental Te was coprecipitated with As and Pd as collector on reduction with hypophosphorous acid [51 and ascorbic acid [11], respectively. Cation exchange
Cation exchange has been applied to the separation of Te(VI) (anionic) and Te(IV) (retained as TeO(OH) + or Te(OH)') complexes [12,13]. Cation exchange separation was used to retain the interfering Cu, Hg, Ni while Te was eluted with dilute HCl [13]. 56.2 DETERMINATION TECHNIQUES Atomic absorptionspectrometry
Flame AAS offers a sensitivity of ca 0.5 gg ml-l in the recommended air-C 2H 2 oxidizing (lean, blue) flame at the most sensitive 214.3 nm line. The signal is suppressed by large excess of Ca, Cu, Si, Na, Zn and Zr and matrix matching is required. An EDL is available but offers hardly any gain in sensitivity. Quartz furnace AAS offers a DL of 0.02 ng ml-l on atomization of TeH 2 [5]. Dimethyl- and diethyltellurium were determined by GC QF AAS, with ADLs in the low nanogram range [14]. Graphite furnace AAS offers an ADL of 1 pg. Tellurium losses in the drying-ashing step are common unless a suitable matrix modifier, e.g. Pd [8,11], Pd-Mg(NO3) 2 [15], H 2PtC16 [16] or Ni [9], is used. Inter708
ferences can be eliminated by trapping TeH 2 in the GF [3] or extraction [8]. A silver-coated graphite atomizer has been recommended for the determination of Te in organic matrices [17]. Plasma source atomic emission spectrometry
Plasma source AES offers a DL of 50 ng ml-1 at the most intensive 214.28 and 238.58 nm ICP emission lines. The former is interfered with by Cu. Hydride generation is preferred for sample introduction, often in multielement mode (cf. Part II). The HG DCP AES determination of Te has been optimized [6]. Mass spectrometry
Natural Te consists of eight isotopes (the most important are 130Te, and 126Te) whereas 127 Te (0.87%) is unstable with a half-life of ca 1013 years. The sensitivity of ICP MS is poor; 130Te is the most sensitive. Tellurium isotopes l30 Te and 126Te have been determined by ETV ICP MS with a DL of 5-10 ng ml-l [18]. Trace amounts of Te were determined by the redox sub-superequivalence method of ID analysis with a DL of 2 pg ml-l [19]. On-line addition of enriched Te (12 5Te) as an internal standard for automated ID ICP MS determination of Te in waste waters has been developed [20]. Spark source ID MS has been used for the analysis of copper [21]. Isotope dilution GC MS after enrichment with 120 Te and derivatization with (4-fluorophenyl)magnesium bromide has been proposed [7]. 128Te,
Fluorescence techniques
A FI ND AFS using an Ar-H 2 flame was reported to give an ADL of 0.02 ng for TeH2 [221. Electrothermal LE AFS was reported to achieve an ADL of 20 fg; molecular backgrounds from nitric oxide and silicon monoxide have been discussed [23]. 56.3 ANALYSIS OF REAL SAMPLES Analytical procedures for the determination of Te are summarized in Table 56.1. Environmental materials
Elemental Te ° and gaseous forms of Te(IV) and Te(VI) can be adsorbed on gold-coated beads [12] and charcoal [27] but at higher flow 709
TABLE 56.1 Combined procedures for the determination of tellurium Sample (amount)
Sample decomposition
Separation
Air
none
sorption on Au-coated GF AAS beads, selective leaching, cation exchange to separate Te(VI) and Te(IV)
0.03
matrix removal by cation exchange
GF AAS
n.g.
none Sea, rain water (4 1)
copptn. with Mg(OH) 2, volatn. as TeH 2
GF AAS
60 pg/l
3
Seawater (0.05 1)
none
volatn. as TeH 2 and trapping in a GF
GF AAS
2-4 pg
1
Geochemical (1 g)
HNO 3-HC10
extrn. with xanthate (cyclohexane), backextrn. (HNO 3)
GFAAS
8 ng/g
9
Urine (1 ml)
HNO 3 -H2 02
extrn. with bis(triGCIDMS fluoroethyl)-DTC (toluene), derivatization with (4-fluorophenyl)MgBr
n.g.
7
Urine (50 ml) HNO3-HCl04
extrn. of TeCl 4 (MIBK)
GF AAS
0.1 ng/ml
8
Urine
HC104-HNO 3
volatn. as TeH2
QF AAS
1 ng
2, 25
Blood
HNO 3
extrn. into MIBK
GF AAS
1 ng/ml
26
Tissues
hyamine hydroxide
none
GF AAS
170 ng/g 16
volatn. as TeH 2, trapping in GF
GF AAS
2-4 pg
1
copptn. with Pd
GF AAS
10 ng
11
pptn.of Pb as PbCl 2; copptn. as Te ° with As
GF AAS
n.g.
5
pptn. of Pb as PbC1 2; volatn. as TeH 2
QF AAS
n.g.
5
Aerosols (0.lg)
HNO 3 -HCIO,4
Bio and HNO 3-HC10 aerosol (0.2 HCI-HF g)
4
4-
Iron Pb-based HNO3-HC10 alloys (2 g)
4
Pb-based HNO3-HCl0 4 alloys (10 g)
710
Detection
DL
Ref. 12 3
ng/m
13, 24
rates the efficiency of the gold-coated beads decreases, in contrast to charcoal [27]. The species can be thermally desorbed or selectively leached with water [Te(VI)], 1 M HCl [Te(IV)] and 3 M HNO3 (Te ° ) [27]. Graphite furnace AAS is the usual determination technique [12,27]. In natural waters Te needs to be preconcentrated, e.g. by coprecipitation with Mg(OH)2 [3] or volatilization followed by trapping of TeH 2 in a GF [1]. Adsorption and colloidal behaviour of Te(VI) in aquatic solutions has been discussed [28]. Preliminary results of Te speciation in wastewater by LC ICP MS have been presented [29]. Biological materials Clinical samples require digestion (usually by HN0 3) to enable extraction of TeC14 [8] or volatilization of TeH 2 [2]. Detection limits in direct analysis are a factor of 10 poorer [161. Tellurium has been determined by GF AAS in blood plasma of rabbits with a DL of 1 ng ml-' [30]. Graphite furnace AAS and HG AAS have been compared for the determination of Te in urine [31]. The use of stable and radioactive isotopes for the determination of biokinetic parameters of Te in rabbits has been compared; tracer solutions enriched in stable (124Te or 126Te) and radioactive (12lmTe or 123mTe) isotopes were administered to animals followed by the analysis of the blood samples by SIMS and y-ray spectrometry [26].
REFERENCES 1 2 3 4 5 6 7
B.M. Yoon, S.C. Shim, H.C. Pyun and D.S. Lee, Anal. Sci., 6 (1990) 561. R. Kobayashi and K. Imaizumi,Anal. Sci., 7 (1991) 447. M.O. Andreae,Anal. Chem., 56 (1984) 2064. T. Wickstr0m, W. Lund and R. Bye, Anal. Chim. Acta, 208 (1988) 347. G.J. Fox, At. Spectrosc., 11 (1990) 13. H. Hayrynen, L.H.J. Lajunen and P. Perimiki, At. Spectrosc., 6 (1985) 88. S.K. Aggarwal, M. Kinter, J. Nicholson and D.A. Herold, Anal. Chem., 66 (1994) 1316. 8 R. Kobayashi and K. Imaizumi, Anal. Sci., 6 (1990) 83. 9 E.M. Donaldson and M.E. Leaer, Talanta, 37 (1990) 173. 10 S. Nakashima, Anal. Chim. Acta, 157 (1984) 187. 11 T. Ashino, K. Takada and K. Hirokawa, Anal. Chim. Acta, 297 (1994) 443. 12 S. Muangnoicharoen, K.Y. Chiou and O.K. Manuel, Talanta, 35 (1988) 679. 13 K.Y. Chiou and O.K. Manuel, Anal. Chem., 54 (1984) 2721. 14 V.T. Demarin, E. Portnova, N.K. Rudnevskii, E.N. Karataev, V.I. Telnoi, 711
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
712
M.Y. Gatilov and I.A. Freshchenko, Zh. Anal. Khim., 46 (1991) 188. B. Welz, G. Schlemmer and R. Mudakavi, J. Anal. At. Spectrom., 7 (1992) 1257. Z.H. Siddik and R.A. Newman, Anal. Biochem., 172 (1988) 190. Z. Ni, B. He and H. Han, J. Anal. At. Spectrom., 8 (1993) 995. R.A. Newman, S. Osborn and Z.H. Siddik, Clin. Chim. Acta, 179 (1989) 191. H. Yoshioka, Y. Miyaki and K. Hasegawa, Analyst, 116 (1991) 821. H. Klinkenberg, T. Beeren, W. Van Borm, F. van der Linden and M. Raets, Spectrochim. Acta, 48B (1993) 649. E.S. Beary, P.J. Paulsen and G.M. Lambert, Anal. Chem., 69 (1988) 733. T. Guo, M. Liu and W. Schrader, J. Anal. At. Spectrom., 7 (1992) 667. Z. Liang, R.F. Lonardo and R.G. Michel, Spectrochim. Acta, 48B (1993) 7. K.Y. Chiou and O.K. Manuel, Environ. Sci. Technol., 20 (1986) 987. R. Kobayashi and K. Imaizumi, Biomed. Res. Trace Elem., 1 (1990) 235. T. Kron, K. Wittmaack, C. Hansen and E. Werner, Anal. Chem., 63 (1991) 2603. S. Muangnoicharoen, K.Y. Chiou and O.K. Manuel, Anal. Chem., 58 (1986) 2811. Y. Yamaashi and Y. Maruyama, J. Radioanal. Nucl. Chem., 170 (1993) 347. H. Klinkerberg, S. Van der Wal, J. Frusch, L. Terwint and T. Beeren, At. Spectrosc., 11 (1990) 198. T. Kron, Ch. Hansen and E. Werner, Colloq. Atomspektrom. Spurenanal., 5 (1989) 809. R. Kobayashi and K. Imaizumi, Anal. Sci., 7 (1991) 841.