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

An automated on-line pre-reduction of arsenate, monomethylarsonate (MMA) an d dimethylarsinate (DMA) using flow injection hydride generation atomic abs orption 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 degr ees C compared to 1 h at room temperature, while MMA and DMA require 1.5-2 min at 70-100 degrees C and up to 1-2 h at room temperature. The characteri stic masses and concentrations for 100 mu l injections are 0.01 ng and 0.1 mu g l(-1) in integrated absorbance and 0.2 ng and 2 mu g l(-1) in peak hei ght measurements, and the limits of detection are ca. 0.5 ng and 5 mu g 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 x 4 mm (5 mu m) with a mobile phase containin g 12 mmol l(-1) phosphate buffer (KH2PO4/H3PO4)-2,5 mmol l(-1) L-cysteine, pH 3.3-3.5. Upon dilution to L-cysteine levels below 10 mmol l(-1), which a re compatible with HPLC separations, the DMA-cysteine complex is unstable o n storage. No baseline separations are possible with anion exchange and rev erse phase C-18 HPLC columns. The limits of detection with 50 mu l injectio ns in peak area mode are ca. 0.5 ng and 10 mu g l(-1), respectively. (C) 20 00 Elsevier Science B.V. All rights reserved.