CHAPTER 18 • Metals Analysis by High Performance Liquid Chromatography
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(PC-88A). The main advantages of this approach, in addition to high selectivity, are
that sample solutions do not contain concentrated salts or chelating agents, and clogging of ICP torch and spectral interferences due to molecular bands are removed;
nebulization efficiency is also optimized in respect to more viscous eluents.
The molecular forms of arsenic subject to speciation analysis are anions, i.e. arsenite
As(III), arsenate As(V), monomethylarsonate (MMA) dimethylarsinate (DMA) or cations, e.g. arsenobetaine (AsB), arsenocholine (AsC) and tetramethylarsonium ions
(TMAs) or uncharged compounds at neutral pH, e.g. arsenous acid. An IC-HPLC procedure for As(III),As(V), MMA and DMA separation and ICP-AES determination have
been developed (Rauret et al. 1991) by coupling the systems with the hydride generation sample introduction technique. The procedure was improved (Rubio et al. 1992)
by checking two different kinds of columns (Nucleosil-5SB and Hamilton PRP X-lOO)
and by comparing isocratic and gradient elution. The peak profile was improved by
filtering the data corresponding to low concentration with the Fourier transform. With
such a procedure detection limits between 2.7 As(III) and 11.4 As(V) J..lg rl were obtained. Ion chromatographic methods for elemental speciation (As, Se and Cr) using
microbore columns with direct injection nebulization by ICPAES have been described
(Gjerde et al. 1993). Arsenite, arsenate, MMA, DMA, AsB and AsC have been separated
by an anion HPLC procedure with a phosphate eluent and the analytes were determined by hydride generation atomic absorption spectrometry (HG-AAS) (L6pezGonzalvez et al. 1996). The HPLC-UV-HG-AFS (AFS, atomic fluorescence spectrometry) method has also been applied to investigate the stability of arsenic species in
relation to food (seafoods and mushrooms) treatment procedures (van Elteren and
Slejkovec 1997). Reversed-phase microbore columns and eluents containing ion-pairing agents could be coupled with mass spectrometric detection for arsenic speciation
by the use of a direct injection nebulizer interface (Shum et al. 1992a,b). Good efficiency was also obtained using interfaces based on hydride generation manifolds.
Hydride generation was tried to avoid the poor efficiency of conventional pneumatic
nebulizers in LC-ICP-MS studies of arsenic speciation (Branch et al. 1991; Story et al.
1992), but even by using a membrane gas separator (Nakahara 1991) the determination was subject to interference by the ArCI+ molecular ion. The removal of 40Ar35CI+
interference was optimized (Sheppard et al.1992; Vela et al. 1993) by coupling ion chromatography and ICP-MS detectors and lowered detection limits for As(III),As(V), DMA
and MMA with the use of an He-Ar gas mixture as ionization source: they ranged between 0.032 and 0.080 ng for DMA and MMA respectively. More recently (Ding et al.
1995) a speciation of these compounds was obtained by micellar liquid chromatography coupled with ICP-MS detection. The method, based on micellar mobile phase
(CTAB, propanol and borate buffer) and on a PRP-1 separation column coupled with
the ICP-MS system, allowed linear dynamic ranges of three orders of magnitude and
detection limits in the picograms range (90-300) and overcame the problems of chloride since it is not coeluted with any of the four arsenic species. Finally a detailed study
must be mentioned on suitability of the ion-spray (IS) technique for arsenic speciation analysis in biological samples. A cation-exchange HPLC has been coupled with
IS-MS-MS detection for analysis of oragnoarsenic species. Dual mode, elemental and
molecular, analysis is presented using standard mixtures. Although detection limits
are not as low as those obtained by HPLC-ICP-MS, the results indicate IS-MS-MS as a
complementary technique to ICP-MS for speciation analysis (Corr and Larsen 1996).
351
(PC-88A). The main advantages of this approach, in addition to high selectivity, are
that sample solutions do not contain concentrated salts or chelating agents, and clogging of ICP torch and spectral interferences due to molecular bands are removed;
nebulization efficiency is also optimized in respect to more viscous eluents.
The molecular forms of arsenic subject to speciation analysis are anions, i.e. arsenite
As(III), arsenate As(V), monomethylarsonate (MMA) dimethylarsinate (DMA) or cations, e.g. arsenobetaine (AsB), arsenocholine (AsC) and tetramethylarsonium ions
(TMAs) or uncharged compounds at neutral pH, e.g. arsenous acid. An IC-HPLC procedure for As(III),As(V), MMA and DMA separation and ICP-AES determination have
been developed (Rauret et al. 1991) by coupling the systems with the hydride generation sample introduction technique. The procedure was improved (Rubio et al. 1992)
by checking two different kinds of columns (Nucleosil-5SB and Hamilton PRP X-lOO)
and by comparing isocratic and gradient elution. The peak profile was improved by
filtering the data corresponding to low concentration with the Fourier transform. With
such a procedure detection limits between 2.7 As(III) and 11.4 As(V) J..lg rl were obtained. Ion chromatographic methods for elemental speciation (As, Se and Cr) using
microbore columns with direct injection nebulization by ICPAES have been described
(Gjerde et al. 1993). Arsenite, arsenate, MMA, DMA, AsB and AsC have been separated
by an anion HPLC procedure with a phosphate eluent and the analytes were determined by hydride generation atomic absorption spectrometry (HG-AAS) (L6pezGonzalvez et al. 1996). The HPLC-UV-HG-AFS (AFS, atomic fluorescence spectrometry) method has also been applied to investigate the stability of arsenic species in
relation to food (seafoods and mushrooms) treatment procedures (van Elteren and
Slejkovec 1997). Reversed-phase microbore columns and eluents containing ion-pairing agents could be coupled with mass spectrometric detection for arsenic speciation
by the use of a direct injection nebulizer interface (Shum et al. 1992a,b). Good efficiency was also obtained using interfaces based on hydride generation manifolds.
Hydride generation was tried to avoid the poor efficiency of conventional pneumatic
nebulizers in LC-ICP-MS studies of arsenic speciation (Branch et al. 1991; Story et al.
1992), but even by using a membrane gas separator (Nakahara 1991) the determination was subject to interference by the ArCI+ molecular ion. The removal of 40Ar35CI+
interference was optimized (Sheppard et al.1992; Vela et al. 1993) by coupling ion chromatography and ICP-MS detectors and lowered detection limits for As(III),As(V), DMA
and MMA with the use of an He-Ar gas mixture as ionization source: they ranged between 0.032 and 0.080 ng for DMA and MMA respectively. More recently (Ding et al.
1995) a speciation of these compounds was obtained by micellar liquid chromatography coupled with ICP-MS detection. The method, based on micellar mobile phase
(CTAB, propanol and borate buffer) and on a PRP-1 separation column coupled with
the ICP-MS system, allowed linear dynamic ranges of three orders of magnitude and
detection limits in the picograms range (90-300) and overcame the problems of chloride since it is not coeluted with any of the four arsenic species. Finally a detailed study
must be mentioned on suitability of the ion-spray (IS) technique for arsenic speciation analysis in biological samples. A cation-exchange HPLC has been coupled with
IS-MS-MS detection for analysis of oragnoarsenic species. Dual mode, elemental and
molecular, analysis is presented using standard mixtures. Although detection limits
are not as low as those obtained by HPLC-ICP-MS, the results indicate IS-MS-MS as a
complementary technique to ICP-MS for speciation analysis (Corr and Larsen 1996).