Optimization of data processing minimizes impact of self-absorption on phosphorus speciation results by P K-edge XANES

Bulk soil phosphorus speciation by X-ray absorption spectroscopy (XAS) using fluorescence yield-mode measurements is an important tool for phosphorus research because of the low soil P contents. However, when measuring in fluorescence mode, increasing the concentration of the absorbing atom can dampen the XAS spectral features because of self-absorption and affect the linear combination (LC) fitting results. To reduce the self-absorption for samples of high P contents, thick boron nitride diluted samples are produced, yet the effects of self-absorption on P speciation results via LC fitting of P K-edge XANES spectroscopy, and the possible benefits of data processing optimization are unknown. Toward this end, we produced a series of ternary standard mixtures (calcium-iron-aluminum phosphates) and an example soil sample both diluted using boron nitride over a range from 1 to ~900 mmol kg⁻¹ for the soil sample and up to ~6000 mmol kg⁻¹ for the standard mixture. We show that by optimizing background subtraction and normalization values, consistent results with less than 10% error can be obtained for samples with up to 300 mmol kg⁻¹ P. Our results highlight the applicability of optimized P K-edge XANES fitting across a wide range of concentrations encountered in natural environments.