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 ele ctrostacking for simultaneous separation and pre-concentration of Cr(III) a nd Cr(VI) and their electrothermal atomic absorption spectrometry (ETAAS) d etermination. The bidirectional electrostacking manifold was improved, and the effects of sample tube cross-section and electrostacking time on the pr e-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 str ength and ionic mobility. The method developed was suitable for the simulta neous separation and pre-concentration of micro- or trace cations and anion s in low conductivity sample solutions. The dynamic calibration range for b oth, Cr(III) and Cr(VI), was from 0.5 to 10 mug l(-1) using 20 mul sample v olume and ETAAS determination. Standard addition method was used for the pr e-concentration of Cr(III) and Cr(VI) in electrostacking procedure. The amo unts 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 val ues (n = 7) and 10 min electrostacking, providing pre-concentration factors of the order of 10 times. The method was applied to analyze a certified re ference material of simulated fresh water, and results were coincident with its certified value. (C) 2001 Elsevier Science B.V. All rights reserved.