Determination of Hexavalent Chromium in Drinking Water Using Ion Chromatography

INTRODUCTION Hexavalent chromium, Cr(VI), is the most toxic form of the metal chromium, a primary drinking water contaminant in the U.S. Dissolved hexavalent chromium can be determined as chromate (CrO 4 ) by ion chromatography in drinking water, groundwater, and industrial wastewater effluents as described in U.S. EPA Method 218.6 and Dionex Technical Note 26. Dionex Technical Note 26 uses a 250-μL injection onto a high-capacity IonPac AS7 anion exchange column to separate Cr(III) from Cr(VI) in four minutes. The product of the postcolumn reaction between Cr(VI) and diphenylcarbazide is detected by absorbance at 530 nm, yielding a method detection limit of 0.4 μg/L in reagent water. The California Department of Health Services (DHS) recently issued a new Public Health Goal (PHG) of 2.5 μg/L for total chromium and 0.2 μg/L for Cr(VI). In January 2001, California DHS added Cr(VI) to the list of unregulated chemicals that must be monitored. As a result of this regulation, public water systems are now monitoring for Cr(VI) in drinking water.3 EPA Method 218.6 does not allow sufficient sensitivity for analysis at the California PHG level of 0.2 μg/L. This application update describes modifications to Method 218.6 that significantly increase sensitivity over the existing method. The modifications include lower eluent and postcolumn reagent (PCR) flow rates, a larger reaction coil, and a larger injection volume. The resulting MDL for Cr(VI) as CrO 4 2of 0.02 μg/L is more than sufficient for determinations at the California PHG level.