Determination of chromium(III) and chromium(VI) in mineral water by bidirectional electrostacking and electrothermal atomic absorption spectrometry

A feasibility investigation was carried out on the use of bidirectional electrostacking for simultaneous separation and pre-concentration of Cr(III) and Cr(VI) and their electrothermal atomic absorption spectrometry (ETAAS) determination. The bidirectional electrostacking manifold was improved, and the effects of sample tube cross-section and electrostacking time on the pre-concentration factor were investigated. Results indicate that the sample tube cross-section and electrostacking time are the main influence factors on the electrostacking pre-concentration, as well as the electric field strength and ionic mobility. The method developed was suitable for the simultaneous separation and pre-concentration of micro- or trace cations and anions in low conductivity sample solutions. The dynamic calibration range for both, Cr(III) and Cr(VI), was from 0.5 to 10 mug l(-1) using 20 mul sample volume and ETAAS determination. Standard addition method was used for the pre-concentration of Cr(III) and Cr(VI) in electrostacking procedure. The amounts of 100 ng l(-1) Cr(III) and Cr(VI) were added into one of twin samples, which were pre-concentrated in parallel, for the pre-concentration factor calculation. The limit of detection for Cr(III) and Cr(VI) was 6 and 5 ng l(-1), respectively, for three times standard deviation (S.D.) of blank values (n = 7) and 10 min electrostacking, providing pre-concentration factors of the order of 10 times. The method was applied to analyze a certified reference material of simulated fresh water, and results were coincident with its certified value. (C) 2001 Elsevier Science B.V. All rights reserved.