Speciation of chromium in environmental matrices.

Published methods for single and sequential extraction, separation and determination of chromium(III) and (VI) in sewage sludge, soil and dried plant material were critically evaluated. The results obtained illustrate the difficulties of speciation analysis in such complex matrices. A method based on single extraction with 0.43 mol L[-1] acetic acid followed by on-line ion chromatography with chemiluminescence detection was found to be simple, sensitive, free of interferences, and more importantly the integrity of the chromium species was maintained. Instrumental parameters affecting the sensitivity of the chemiluminscence signals were optimised using a multivariate optimisation approach. A linear calibration was obtained from 0.01 to 50 mug L[-1]. Limits of detection were found to be 0.002 mug L[-1] for both Cr(III) and Cr(VI). Limits of determination were found to be 0.02 mug L[-1] for both Cr(III) and Cr(VI). The precision, determined using a 10 mug L[-1] solution of Cr(III) and Cr(VI) was 10.6 +/- 0.52mug L[-1] (4.9%) for Cr(III) and 10.1 +/- 0.50 mug L[-1] (4.9%) for Cr(VI) (n=6). Validation of the method was carried out using two water reference materials certified for total chromium content; LGC CRM 6010 49 mug L[-1] +/- 4 mug L[-1] and NIST SRM 1643d 18.53 mug L[-1] +/- 0.20 mug L[-1]. Results obtained showed good agreement with the certified values; 49.21 mug L[-1] +/- 1.81 mug L[-1] and 19.03 mug L[-1] +/- 1.50 mug L[-1] respectively. The developed method was used to study the distribution of Cr(III) and Cr(VI) species in soils. The species extraction procedure was validated using a certified reference material (CRM): BCR CRM 483. The changes in the distribution of both species in soil that had been through similar pre-treatment steps as in the production of reference materials were studied. This investigation showed that contamination or chemical oxidation could alter the chromium species present within a natural sample during production steps such as milling, homogenisation and sieving. As a result the metal speciation in the product is different from that in the starting material. In order to limit the effect of oxidation jet rather than ball milling is recommended.