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 (8-1015ppm) 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 (5961.86114.8eV), 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 (5980.3-6099.8eV), a twofactor PLS model was able to predict the concentration of chromium with an RMSECV error of 12.1ppm. 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 10ppm chromium.