Determination of Chemical Speciation by XAFS

Chemical speciation of toxic elements in the environment is an active area of research. In a physical (i.e. chemically inert) mixture of compounds, the X-ray absorption spectra in transmission mode or fluorescence mode at low concentration and high dispersion are linear combinations of the spectra of the constituents. As long as the linearity conditions are met, and the relevant constituents are known, linear fitting procedures can then be used directly to determine the composition. However, physical effects such as large particle sizes and aggregation can cause the breakdown of the linearity condition, resulting in systematic errors and incorrect conclusions. Here we present a simple and robust procedure for determination of molecular speciation by XAFS using a Linear Programming (LP) algorithm with constraints (positively and normalization), and errors introduced by particle size effects and other problems on the speciation results are discussed.