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 measur ement) currently constitute a pair of protocols for the measurement of Cr(VI) in environmental soil samples in EPA RCRA SW-846 Update ill. T o identify the sources of possible biases, we applied the newly develo ped Speciated Isotope Dilution Mass Spectrometry (SIDMS) as an alterna tive detection method to Method 7196A. SIDMS is capable of correcting for species transformation, while Method 7196A is nor. In Method 3060A , soluble Cr(III) could be oxidized during extraction, resulting in po sitive errors of Cr(VI); Cr(VI) could be lost or reduced during neutra lization, leading to negative errors of Cr(VI). When sand and soil ext racts were analyzed, low recoveries were obtained with Method 7196A. H owever, SIDMS achieved approximately 100% recoveries. The influence of some soil matrix components on the detection of Cr(VI) using Method 7 196A has been evaluated. As expected, reducing agents could cause low recoveries of Cr(VI). Surprisingly, we found that strong oxidizing age nts could also result in low recoveries. We identify these method bias es and discuss the mechanisms in this paper.