Determination and Correction of Analytical Biases and Study on Chemical Mechanisms in the Analysis of Cr(VI) in Soil Samples Using EPA Protocols

EPA Methods 3060A (alkaline extraction) and 7196A (colorimetric measurement) currently constitute a pair of protocols for the measurement of Cr(VI) in environmental soil samples in EPA RCRA SW-846 Update III. To identify the sources of possible biases, we applied the newly developed Speciated Isotope Dilution Mass Spectrometry (SIDMS) as an alternative detection method to Method 7196A. SIDMS is capable of correcting for species transformation, while Method 7196A is not. In Method 3060A, soluble Cr(III) could be oxidized during extraction, resulting in positive errors of Cr(VI); Cr(VI) could be lost or reduced during neutralization, leading to negative errors of Cr(VI). When sand and soil extracts were analyzed, low recoveries were obtained with Method 7196A. However, SIDMS achieved approximately 100% recoveries. The influence of some soil matrix components on the detection of Cr(VI) using Method 7196A has been evaluated. As expected, reducing agents could cause low recoveries of Cr(VI). Surprisingly, we found that strong oxidizing agents could also result in low recoveries. We identify these method biases and discuss the mechanisms in this paper.