Speciation of chromium in simulated soil samples using X-ray absorption spectroscopy and multivariate calibration

X-ray absorption near-edge structure (XANES) spectroscopy has been applied to the speciation of 24 simulated soil samples spiked with known concentrations (8–1015 ppm) of hexavalent and trivalent chromium. For XANES spectroscopy, multivariate calibration models, such as partial least-squares (PLS) regression provide greater capabilities than traditional univariate calibration models. Using normalized XANES spectra (5961.8–6114.8 eV), a one-factor PLS model was able to determine the percentage of hexavalent chromium with a root-mean-squared-cross-validation (RMSECV) error of 6.1%. The corresponding univariate calibration model based upon the hexavalent chromium pre-edge feature had a RMSECV of 9.1%. PLS was also used to determine total chromium content in the simulated soil samples. Using unnormalized XANES spectra (5980.3–6099.8 eV), a two-factor PLS model was able to predict the concentration of chromium with an RMSECV error of 12.1 ppm. This work suggests that a single XANES measurement can be used for both the quantitative analysis and speciation of soil samples contaminated with greater than 10 ppm chromium.