Determination of hexavalent chromium at the level of the California Public Health Goal by ion chromatography

Chromium is a primary drinking water contaminant in the USA with hexavalent chromium, Cr(VI), being the most toxic form of the metal. As a required step in developing a revised state drinking water standard for chromium, the California Department of Health Services recently issued a new Public Health Goal (PHG) of 2.5 μg/l for total chromium and 0.2 μg/l for Cr(VI). Hexavalent chromium can be determined (as chromate) by ion chromatography, as described in US Evironmental Protection Agency Method 218.6; however, the method as originally published does not allow sufficient sensitivity for analysis at the California PHG level of 0.2 μg/l. Modification of the conditions described in Method 218.6, including the use of a lower eluent flow-rate, larger reaction coil, and larger injection volume, significantly increases the method sensitivity. The modified method, which uses IonPac NG1 and AS7 guard and analytical columns, an eluent of 250 mM ammonium sulfate–100 mM ammonium hydroxide operated at 1.0 ml/min, a 1000 μl injection volume, and postcolumn reaction with 2 mM diphenylcarbazide–10% methanol–0.5 M sulfuric acid (using a 750 μl reaction coil) followed by UV–Vis detection at 530 nm, permits a method detection limit for chromate of 0.02 μg/l. This results in a quantitation limit of 0.06 μg/l, which is more than sufficient for analysis at the California PHG level. Calibration is linear over the range of 0.1–10 μg/l and quantitative recoveries (>80%) are obtained for chromate spiked at 0.2 μg/l in drinking water. The modified method provides acceptable performance, in terms of chromate peak shape and recovery, in the presence of up to 1000 mg/l chloride or 2000 mg/l sulfate.