Simultaneous, sensitive and selective on-line chemiluminescence determination of Cr(III) and Cr(VI) by capillary electrophoresis

Abstract

A novel in-capillary reduction and capillary electrophoretic (CE)–chemiluminescence (CL) method was developed for the sensitive and selective determination of chromium(III) and chromium(VI). The proposed method was based on the in-capillary reduction of Cr(VI) with acidic H 2O 2 to form Cr(III) using the zone-passing technique and chemiluminescence detection of Cr(III). The sample [Cr 3+ and CrO 4 2−], hydrochloric acid, and H 2O 2 (reductant) solution segments were injected for specified periods of time in this order from the anodic end of a capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr 3+ migrates to the cathode while CrO 4 2− ion, moving oppositely to the anode, reacts with acidic H 2O 2, resulted in formation of Cr 3+. Based on the migration time difference of both Cr 3+ ions, they were separated by zone electrophoresis. Running buffer was composed of 0.02 mol l −1 HAc–NaAc (pH 4.7) with 1×10 −3 mol l −1 EDTA. Parameters affecting CE–CL separation and detection, such as reductant concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, stability of luminol–hydrogen peroxide mixed solution were optimized. The limits of detection for chromium(III) and chromium(VI) (3 σ) were 6×10 −13 mol l −1 (mass concentration 12 zmol) and 8×10 −12 mol l −1 (160 zmol), respectively. This method offered potential advantages of simplicity, sensitivity, selectivity and applicability to the determination of Cr(III) and Cr(VI) in environmental water.