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 , metal trafficking in eukaryotic cells , and metal impurities in photovoltaic materials .
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) .
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