Speciation of chromium by in-capillary reaction and capillary electrophoresis with chemiluminescence detection

Abstract

A sensitive method for the simultaneous determination of chromium(III) (Cr 3+) and chromium(VI) (CrO 4 2−) using in-capillary reaction, capillary electrophoresis (CE) separation and chemiluminescence (CL) detection was developed. The chemiluminescence reaction was based on luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by Cr 3+ ion followed by capillary electrophoresis separation. Based on in-capillary reduction, chromium(VI) can be reduced by acidic sodium hydrogensulfite to form chromium(III) while the sample is running through the capillary. Before the electrophoresis procedure, the sample (Cr 3+ and CrO 4 2−), buffer and acidic sodium hydrogensulfite solution segments were injected in that order into the capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr 3+ ions migrate to the cathode, while CrO 4 2− ions, moving in the opposite direction toward the anode, react with acidic sodium hydrogensulfite which results in the formation of Cr 3+ ions. Because of the migration time difference of both Cr 3+ ions, Cr(III) and Cr(VI) could be separated. The running buffer was composed of 0.02 mol l −1 acetate buffer (pH 4.7) with 1·10 −3 mol l −1 EDTA. Parameters affecting CE–CL separation and detection, such as reductant (sodium hydrogensulfite) concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, were optimized. The limits of detection (LODs) of Cr(III) and Cr(VI) were 6·10 −13 and 8·10 −12 mol l −1 ( S/N=3), respectively. The mass LODs for Cr(III) and Cr(VI) were 1.2·10 −20 mol (12 zmol) and 3.8·10 −19 mol (380 zmol), respectively.