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

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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.