EFFECT OF CYSTEINE ON THE SPECIATION OF ARSENIC BY USING HYDRIDE GENERATION ATOMIC-ABSORPTION SPECTROMETRY

The use of hydride generation techniques for arsenic species is pH dependent. Identical responses can not be obtained from arsenite, arsenate, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) when the same acid concentration is used. Thus some methods commonly used for the direct determination of total arsenic under compromised conditions are subject to error. This error is eliminated by the addition of 2% cysteine to samples prior to hydride generation. In the presence of cysteine the optimum condition for the determination of these arsenic species is in the same range, and a single arsenic species can be used for calibration. This finding is applied to the determination of arsenic in human urine by using flow injection, hydride generation, and atomic absorption spectrometry methodology. The effect of cysteine and thioglycerol is reported in detail, and it is proposed that arsenate, MMAA and DMAA all in the As(V) state, are reduced to the As(III) state as organo-sulfur-arsenic(III) compounds through the reaction between the arsenic species and the thiol. These products, organosulfur derivatives of arsenic(III), easily react with tetrahydroborate(III) under similar conditions to afford the arsines without interference from cysteine. Non-thiol-containing amino acids such as methionine, glycine and histidine do not react with arsenic species in this way, and therefore they do not affect the pH dependence of the generation of arsenic hydride species.