Quantitative trace-level speciation of arsenite and arsenate in drinking water by ion chromatography

Abstract

We describe an improved method for the determination of inorganic arsenic in drinking water. The method is based on comprehensive optimization of the anion-exchange ion chromatographic (IC) separation of arsenite and arsenate with post-column generation and detection of the arsenate–molybdate heteropoly acid (AMHPA) complex ion. The arsenite capacity factor was improved from 0.081 to 0.13 by using a mobile phase (2.0 mL min⁻¹) composed of 2.5 mM Na₂CO₃ and 0.91 mM NaHCO₃ (pH 10.5). A post-column photo-oxidation reactor (2.5 m × 0.7 mm) was optimized (0.37 µM potassium persulfate at 0.50 mL min⁻¹) such that arsenite was converted to arsenate with 99.8 ± 4.2% efficiency. Multi-variate optimization of the complexation reaction conditions yielded the following levels: 1.3 mM ammonium molybdate, 7.7 mM ascorbic acid, 0.48 M nitric acid, 0.17 mM potassium antimony tartrate, and 1.0% (v/v) glycerol. A long-path length flow cell (Teflon AF, 100-cm) was used to measure the absorption of the AMHPA complex (818 ± 2 nm). Figures of merit for arsenite/arsenate include: limit of detection (1.6/0.40 µg L⁻¹); standard error in absorbance (5.1 × 10⁻³/3.5 × 10⁻³); and sensitivity (2.9 × 10⁻³/2.2 × 10⁻³ absorbance units per ppb). Successful application of the method to fortified surface and ground waters (100 µL samples) is also described.