Correction of species transformations in the analysis of Cr(VI) in solid environmental samples using speciated isotope dilution mass spectrometry

Speciation of Cr(VI) in solid environmental samples is challenging because of the transformations between Cr(VI) and Cr(III). EPA method 3060A completely extracts Cr(VI) in a hot alkaline solution and preserves the solublized Cr(VI). This procedure, however, can oxidize Cr(III) in some chemical forms. On the other hand, the reverse transformation may occur during neutralization and acidification following the extraction step. We developed a method that is capable of monitoring and correcting for such bidirectional species transformations to determine Cr(VI) in solid samples. In this method, we spike a sample with a Cr-53(VI) spike (enriched in Cr-53) and a Cr-iso(II) spike (enriched in Cr-50). The large quantity of Cr-iso(III) in an easily oxidizable form competes with sample Cr(II) in the oxidization, reducing the method-induced oxidation of sample Cr(III). This method also corrects for the reduction of Cr(VI), The theory is presented and is evaluated experimentally. The analysis of chromite ore processing residue, fly ash, and standard reference material SRM 1645 showed that the oxidation of sample Cr(III) could cause positive biases as high as 163% if no correction is performed.