On-line pre-reduction of pentavalent arsenicals by thioglycolic acid for speciation analysis by selective hydride generation–cryotrapping–atomic absorption spectrometry

An improvement of current method of selective hydride generation based on pre-reduction for differentiation of tri- and pentavalent arsenicals is described, applied for the oxidation state specific speciation analysis of inorganic, mono-, di- and trimethylated arsenicals with minimum sample pretreatment using atomic absorption spectrometry with the multiatomizer. The preconcentration and separation of arsine, methylarsine, dimethylarsine and trimethylarsine are then carried out by means of cryotrapping. The presented study shows that 2% (m/v) L-cysteine hydrochloride monohydrate (L-cys) currently used for off-line pre-reduction of pentavalent arsenicals can be substituted with 1% (m/v) thioglycolic acid (TGA). Much faster pre-reduction of pentavalent arsenicals at 25 °C with equal sensitivities as in the case of L-cys has been achieved with TGA. A setup for on-line pre-reduction by TGA has been optimized, with the application of segmented flow analysis for suppression of axial dispersion in the pre-reduction coil. Standard calibrations measured with or without on-line pre-reduction indicate uniform and equal sensitivities for all As forms. The possibility of standardization by water standards of single species (e.g. iAs(III)) for quantification of all other As forms in urine is demonstrated in the recovery study. Limits of detection were 100 ng l^− 1 for iAs(III), 135 ng l^− 1 for iAs(V) and 30 to 50 ng l^− 1 for methylated arsenicals.