Synchrotron X-ray Fluorescence Microtomography in Geo-, Cosmo-, and Bio- chemistry

Synchrotron-based X-ray fluorescence computed microtomography (xrfCMT) is a unique method for imaging major and trace element distributions within natural materials nondestructively and with high spatial resolution. The technique is particularly useful in imaging and quantifying elemental abundance in small objects that may be too precious or too difficult to section, or in the analysis of materials in which sectioning may potentially alter elemental distributions. This presentation will highlight how this technique is being applied at beamlines X26A and X27A at the National Synchrotron Light Source (Brookhaven National Laboratory) and at 13-ID at the Advanced Photon Source (Argonne National Laboratory). These instruments utilize 1-10 μm diameter focused, monochromatic X-ray beams to non- destructively measure x-ray fluorescence from a sample as it is translated and rotated within the beam. The resultant fluorescence intensities are then reconstructed as either two-dimensional cross sectional or three- dimensional elemental distribution using a fast fourier transform based computational reconstruction algorithm. Reconstruction of multi-elemental distributions at concentrations down to approximately 1 μg g-1 (element dependent) can be obtained. By collecting and storing full energy dispersive spectra from a multi-channel analyzer for every pixel (rather than regions of interest), it is possible to evaluate a reconstructed spectrum within the object for more robust elemental analysis. For high density matrices in particular, corrections are necessary to account for x-ray absorption by the object of both incoming X-rays and outgoing fluorescent X-rays. These effects limit the size of objects and elements that can be imaged; however reasonable corrections can be made if an estimate of linear absorption coefficient through the material is made. It is also possible to couple fluorescence tomography with microbeam x-ray absorption and diffraction analysis. When coupled to absorption spectroscopy, the xrfCMT analysis is conducted at multiple incident x-ray energies that preferentially target a given oxidation state of an element. This allows for three-dimensional visualization of an element's speciation. Coupled with simultaneous x-ray scattering studies utilizing a CCD area detector, individual mineral reflections can be reconstructed in three dimensions simultaneously with the x-ray fluorescence data. Examples of materials analyzed by this technique at X26A, X27A and 13-ID include interplanetary dust particles, fluid inclusions, plant materials, and heavy metals sorbed to mineral grains.