Determination of hexavalent chromium by using speciated isotope-dilution mass spectrometry after microwave speciated extraction of environmental and other solid materials

Precise and accurate determination of hexavalent chromium in different types of solid environmental sample is regarded as a technical challenge with significant potential error if historically accepted methods are used. Microwave-assisted alkaline extraction (0.5 mol L-1 NaOH + 0.28 mol L-1 Na2CO3) followed by anion-exchange chromatographic separation and inductively coupled plasma mass spectrophotometric detection has been shown to provide accurate and precise results. To obtain a better understanding of potential species conversion during and/or after extraction steps, speciated isotope-dilution mass spectrometry (SIDMS) (EPA Method 6800) metrology has been successfully applied as a diagnostic tool with the modified accompanying extraction version of EPA Method 3060A. In our study, aggregate materials distributed over a large area of a major western US state were found to contain a high concentration of total chromium (195 +/- 13 to 709 +/- 19 mu g g(-1)) and significant amounts of Cr6+ (141 +/- 6 to 341 +/- 29 mu g g(-1)) which are at least three orders of magnitude higher than the US EPA threshold limit (0.5 mu g g-1). Sediment samples from a major western US state, studied independently, were found to contain less (177 +/- 0.34 mu g g(-1)) or no Cr6+ in the presence of significant total chromium.