Thermospray Methods for Rapid, Sensitive, and Nonchromatographic Speciation of Chromium Oxidation States

Abstract

A sensitive technique for speciation and quantification of Cr(III) and Cr(VI) has been developed using thermospray (TSP) sample introduction with inductively coupled plasma atomic emission spectrometry (ICPAES). For unacidified solutions, the sensitivity for Cr(III) was found to be lower than that for Cr(VI). The sensitivity for Cr(III) was further depressed to a negligible level by adjusting sample and thermospray operating parameters. The low sensitivity for Cr(III) was thought to result from the precipitation of that species to form Cr(OH)(3), which deposited within the vaporizer. For acidic solutions (1% v/v HNO(3)), the sensitivities for both species were essentially identical. On the basis of these results, methods for speciation of Cr(III) and Cr(VI) were developed. With samples buffered to pH 4.4, Cr(VI) could be selectively determined. With acidic sample aliquots (1% v/v HNO(3)), the total chromium concentration could also be determined, and the Cr(III) concentration could be calculated by difference. Parameters affecting Cr(III) sensitivity, such as control temperature, pH, and pump flow rate, were studied in addition to optimal TSP-ICPAES parameters. The limits of detection (LODs) for Cr(VI) and for total Cr were 0.47 and 0.61 μg/L with standard deviations of 1.5% and 2.0%, respectively. Good accuracy and precision of the method were demonstrated for analysis of spiked tap water and lake water samples. Mobile phase ion-pairing chromatography with ICPAES detection provided comparable results for moderately high concentration samples. Accuracy of measurements for Cr(VI) was within 1% of the certified value for NIST standard reference material 2109.