CHROMIUM SPECIATION WITH ISOTOPE-DILUTION MASS-SPECTROMETRY

Cr(III) and Cr(VI) were determined by positive thermal ionisation isotope dilution mass spectrometry (IDMS). From the spiked samples, Cr(VI) was separated from Cr(III) by extraction with the liquid anion exchanger Amberlite LA-2. Re-extraction of Cr(VI) was performed with an ammonia solution. After electrolytic deposition of chromium from both species the isotope ratio Cr-52/Cr-53 was determined. Detection limits of 1.2 ng g(-1) for Cr(III) and of 2.4 ng g(-1) for Cr(VI), respectively, were achieved. A HCO3-/CO2 buffer solution of pH 6.4 containing Cr(III) and Cr(VI) enriched in Cr-53 was used as spike solution. The stability of the chromium species in this solution was tested in model experiments with labelled radioactive Cr-51. The accuracy of the IDMS method could be shown in two interlaboratory comparisons. Several fresh and treated waste water samples with Cr(III) concentrations in the range of 2-55 ng g(-1) and Cr(VI) concentrations of less than 2.4 ng g(-1) were analysed. The results of the fresh water samples could be explained by the known geochemical behaviour of chromium in natural water systems.