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 how 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)), monomet hylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) pretreated in differ ent reaction media. The signals for the arsines generated from MMAA and DMA A prepared in aqueous pure medium substantially decreased at hydrochloric a cid concentrations higher than 2 mol l(-1). Neutralization of the acid in t he reaction mixture after the HG process did not produce any appreciable ef fect 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 ob tained in pure aqueous medium. In this case, similar maximum signals were o btained within a narrow range at low acid concentrations (from 0.06 to 0.12 mol l(-1)); thereafter a sharp decrease of the signals at hydrochloric aci d concentrations around 1 mol l(-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 co ncentrations, the thiolate complexes of As(m) are rapidly converted into th e respective arsines by BH4- and (ii) at high acid concentrations, the arsi nes are generated by the action of nascent hydrogen produced by the decompo sition of BH4-. (C) 2001 Elsevier Science B.V. All rights reserved.