Arsenic Speciation in Biological Samples Using XAS and Mixed Oxidation State Calibration Standards of Inorganic Arsenic

Abstract

The speciation of elements without pre-edge features preformed with X-ray absorption near edge structure (XANES) can lead to problems when the energy difference between two species is small. The speciation of arsenic (As) in plant samples was investigated using the mixtures As₂S₃/As₂O₅, As₂S₃/As₂O₃, or As₂O₃/As₂O₅. The data showed that the energy separation (eV) between As₂O₅ and As₂S₃ was 5.8, between As₂O₃ and As₂O₅ was 3.6, and between As₂S₃ and As₂O₃ was 2.1. From the intensity of the white-line feature and the concentration of As species, calibration curves showing a limit of detection of approximately 10% were generated. In addition, an error of ±10% was determined for the linear combination–XANES (LC-XANES) fitting technique. The difference between the LC-XANES fittings and calculations from the calibration curves was <10%. The data also showed that the speciation of As in a sample can be determined using EXAFS (extended X-ray absorption fine structure). Finally, it was also shown that both EXAFS and XANES of the sample should be examined to determine the true speciation of an element. Even though there is a difference of 2 eV between As(III) bound to O and As(III) bound to S, in the EXAFS region the As(III)–S and As(III)–O ligands are clearly visible. However, distinction between the As(III)–O and As(V)–O ligands in the EXAFS spectra was not clearly visible in this study.