Use of perchloric acid as a reaction medium for speciation of arsenic by hydride generation–atomic absorption spectrometry

Abstract

Simple and inexpensive methods for the speciation of arsenite, arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in environmental water samples were developed. In these methods a hydride generation–atomic absorption spectrometry (HG-AAS) technique was employed and perchloric acid (as a reaction medium), L-cysteine (as a pre-reducing agent for a certain contact time between its addition and analysis) and sodium tetrahydroborate(III) (NaBH₄, as a reducing agent) were used. The use of L-cysteine greatly enhances the absorption signals of all four arsenic species at low acid concentration (0.001–0.04 M). The methods developed for the determination of total arsenic and total inorganic arsenic and speciation of the four arsenic species in environmental water samples are as follows. (i) DMA: 0.005 M acid and 0.04% NaBH₄ in the absence of L-cysteine. DMA can also be speciated in the presence of L-cysteine as follows: 2 M acid, 2.5% L-cysteine after a contact time of approximately 5 min and 0.6% NaBH₄. (ii) As(III): 5 M acid and 0.08% NaBH₄ in the absence of L-cysteine. (iii) Total inorganic arsenic [As(III) + As(V)]: 8 M acid and 0.6% NaBH₄ in the absence of L-cysteine. (iv) Total arsenic: 0.01 M acid, 5% L-cysteine after a contact time of 5 min and 2% NaBH₄. (v) MMA: 8 M acid, 3% L-cysteine after a contact time of 50 min and 0.6% NaBH₄. (vi) As(V): by difference. Detection limits and recoveries of added spikes for all analyses were found to be 0.5–1.7 ppb and 90–112% respectively.