Flow-injection hydride generation atomic absorption spectrometric study of the automated on-line pre-reduction of arsenate, methylarsonate and dimethylarsinate and high-performance liquid chromatographic separation of their l-cysteine complexes

Abstract

An automated on-line pre-reduction of arsenate, monomethylarsonate (MMA) and dimethylarsinate (DMA) using flow injection hydride generation atomic absorption spectrometry (FI-HGAAS) is feasible. The kinetics of pre-reduction and complexation depend strongly on the concentration of l-cysteine and on the temperature in the following increasing order: inorganic As(V)<DMA<MMA. Arsenate is pre-reduced/complexed within less than 50 s at 70–100°C compared to 1 h at room temperature, while MMA and DMA require 1.5–2 min at 70–100°C and up to 1–2 h at room temperature. The characteristic masses and concentrations for 100 μl injections are 0.01 ng and 0.1 μg l −1 in integrated absorbance and 0.2 ng and 2 μg l −1 in peak height measurements, and the limits of detection are ca. 0.5 ng and 5 μg l −1, respectively. In a high-performance liquid chromatography (HPLC)–HGAAS system, the l-cysteine complexes of inorganic As(III), MMA and DMA are best separated within 7 min by HPLC on a strongly acidic cation exchange column such as Spherisorb S SCX 120×4 mm (5 μm) with a mobile phase containing 12 mmol l −1 phosphate buffer (KH 2PO 4/H 3PO 4)–2.5 mmol l −1 l-cysteine, pH 3.3–3.5. Upon dilution to l-cysteine levels below 10 mmol l −1, which are compatible with HPLC separations, the DMA–cysteine complex is unstable on storage. No baseline separations are possible with anion exchange and reverse phase C 18 HPLC columns. The limits of detection with 50 μl injections in peak area mode are ca. 0.5 ng and 10 μg l −1, respectively.