Excitation-Emission Matrix Spectroscopy With Soft x-Rays: A Method for Simultaneous Measurement of Mineralogy and Chemical Speciation in Soils.

Spectroscopic studies of soils typically probe only one element to understand a complex and extremely heterogeneous system. Spatially resolved techniques address some of the inherent heterogeneity of soils, but when surface coatings of Ca and Fe exist on clay mineral substrates consisting of the same elements, heterogeneity persists at micro- or even nano-scales. A more sophisticated approach is to employ multiple speciation methods that can measure elemental quantities, chemical speciation of multiple elements, and structural/crystallinity. We have developed a new approach that combines an energy discriminating fluorescence detector and the high-resolution soft x-ray beamline to simultaneously measure XRF, XAS, and XRD in soils. The resulting image is a spectroscopic technique we termed Excitation-Emission Matrix Spectroscopy, or EEMS. The collection of an EEMS via UV-VIS-based spectroscopy has been successfully used to qualitatively characterize dissolved organic matter, but when the soft x-ray region is probed with a synchrotron a wealth of soil chemical information is obtained. Vertical slices of an EEMS are energy-resolved x-ray fluorescence, diagonal lines in the sample arise from diffraction in the sample; the different slopes of the scattering lines are from the different orders of light and the diffraction patterns of minerals are readily indexed in whole soils. Finally, horizontal slices of the EEM contain XANES spectra; different elements are well separated vertically due to large variation in emission lines. A typical EEMS experiment can provide C, N, O, K, Ca, Mn, Fe, Mg, Al, and Si XANES in a reasonable time period. In this talk, we will present a range of results that highlight the utility of this new technique in understanding soil chemical processes.