Determination of four arsenic species in soil by sequential extraction and high performance liquid chromatography with post-column hydride generation and inductively coupled plasma optical emission spectrometry detection

Several procedures, involving various solvents and ultrasound, were evaluated for the extraction of four arsenic species, arsenite (As(III)), arsenate (As(V)), monomethylarsonate (MMA) and dimethylarsinate (DMA), from a silt loam soil to which species had been added at a concentration of 20 mg kg⁻¹. The best extraction was by a two-stage procedure: shaking for 24 h in the presence of 0.1 mol l⁻¹ phosphoric acid followed by shaking for 24 h in 1.0 mol l⁻¹ sodium hydroxide solution. The arsenic species in the extracts were separated by high performance ion-exchange liquid chromatography, derivatized to hydrides by reaction with tetrahydroborate(III) in a multi-mode sample introduction system (MSIS) and quantified by ICP-OES. Detection limits in solution ranged from 0.4 (As(III) and DMA) to 1 (MMA and As(V)) µg l⁻¹, corresponding to 10 and 25 µg kg⁻¹ in a 0.2 g soil sample and 5 ml of extractant. The most significant change over time was that As(III) was converted to As(V). When each species was added individually, arsenic was 100% recovered over a period of several months. When all four species were added together, the recovery was 89%. As the precipitation of humic acids was slow, the sodium hydroxide extract could be acidified and analyzed without loss of analyte species.