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X-Ray Microscopy and Imaging

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X-Ray Microscopy and Imaging: Micro-XANES and Chemical State Mapping

X-ray absorption near edge structure (XANES) is a spectroscopic technique that provides information about the local coordination environment around absorbing atom. When used with a microprobe, micro-XANES has the unique capability of probing the chemical state at any sample point of interest with submicron resolution. In fluorescence mode, even chemical state of trace elements with part-per million (ppm) concentration can be studied. Micro-XANES has thus become a powerful interrogation tool for a wide array of studies, including microbe-metal-mineral interaction [1], metal trafficking in eukaryotic cells [2], and metal impurities in photovoltaic materials [3].

Many metals have absorption edges in the hard x-ray regime that are suitable for XANES. At 2-ID-D we have made micro-XANES measurements from the Cr K-edge (5.99 keV) to the Pu L3-edge (18.06 keV). Typically x-ray fluorescence maps were first acquired to locate regions of interest on the sample. Then at selected regions, the incident energy was scanned with a pair of Si(111) crystals in channel-cut mode. In this way, the position of the x-ray spot on the sample was stable usually within 0.5 micron during the XANES scan.

 

In one study, pseudomonas fluorescens adhered to a surface were exposed to Cr(VI) solution (1000 ppm). X-ray fluorescence maps revealed that there was increased chromium concentration near but external to the microbes. Further micro-XANES analysis indicated that chromium at these extracellular locations had largely been reduced to Cr(III) [4].

1. S. Glasauer et al., Mixed-Valence Cytoplasmic Iron Granules Are Linked to Anaerobic Respiration, Appl. Environ. Microbiol. 73, 993-996 (2007).
2. L. Yang et al., Imaging of the intracellular topography of copper with a fluorescent sensor and by synchrotron x-ray fluorescence microscopy, Proc. Natl. Acad. Sci. 102, 11179-11184 (2005).
3. T. Buonassisi et al., Engineering metal-impurity nanodefects for low-cost solar cells, Nature Materials 4, 676-679 (2005).
4. K. M. Kemner et al., Elemental and Redox Analysis of Single Bacterial Cells by X-ray Microbeam Analysis, Science 306, 686-687 (2004).

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