Determination of various arsenic species by flow injection hydride generation atomic absorption spectrometry: investigation of the effects of the acid concentration of different reaction media on the generation of arsines

An on-line flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) system was developed to study the effect of acid concentration on the arsine generation from arsenite (As(III)), arsenate (As(V)), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) pretreated in different reaction media. The signals for the arsines generated from MMAA and DMAA prepared in aqueous pure medium substantially decreased at hydrochloric acid concentrations higher than 2moll-1. Neutralization of the acid in the reaction mixture after the HG process did not produce any appreciable effect on the arsine signals. When the arsenic species were prereduced with L-cysteine or mercaptoacetic acid to yield thiolate complex of As(III), the arsines generated produced different response profiles compared to those obtained in pure aqueous medium. In this case, similar maximum signals were obtained within a narrow range at low acid concentrations (from 0.06 to 0.12moll-1); thereafter a sharp decrease of the signals at hydrochloric acid concentrations around 1moll-1, followed by a sharp increase to reach a constant value at higher and different acid concentrations. These results might suggest two mechanisms for the arsine generation: (i) at low acid concentrations, the thiolate complexes of As(III) are rapidly converted into the respective arsines by BH4- and (ii) at high acid concentrations, the arsines are generated by the action of nascent hydrogen produced by the decomposition of BH4-.