Phase speciation by extended x-ray absorption fine structure spectroscopy

The application of x-ray absorption spectroscopy methods to both materials and life sciences is well appreciated. However, the power of extended x-ray absorption fine structure (EXAFS) spectroscopy as a quantitative structural technique has largely been limited by its application to the microscopically homogeneous systems, in which the local environment around each absorbing atom in the sample is the same. The growing interest in time-resolved EXAFS studies of systems in physics, chemistry, biology, and materials science has reintroduced the requirement for an analytical tool to probe heterogeneous mixtures in situ. While long being recognized as a premiere technique for this role, EXAFS studies of mixtures have been particularly difficult due to the strong model dependence and correlations between parameters in the fit. To circumvent these drawbacks, we introduce two new techniques in EXAFS analysis: the principal component analysis and the residual phase analysis. Using a test case of a heterogeneous mixture of two organometallic Co compounds, we demonstrate that these new EXAFS modeling techniques, together with the existing one, the multiple datasets fit method are the most suitable and adequate methods for phase speciation. In addition, we discuss the application of these data analysis approaches to biological systems. (C) 2002 American Institute of Physics.