Detection of the fluorescence can be performed in

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Detection of the fluorescence can be performed in several ways. The sim- plest way is by using a gas-filled ionization chamber [540]. Such a detector enables the detection of a large solid angle although the dynamic range is limited. The sensitivity for the energy used can be adjusted by the use of the appropriate gas filling. Use of this detector is only possible if only the element of interest is excited due to the low-energy resolution of this type of detector. In a similar way silicon pin diodes can be used to provide a higher sensitivity and much larger dynamic range [541] although the solid angle of detection is smaller. Use of solid-state detectors such as Si(Li), high-purity Ge, or Si- drift detectors provide additional advantages due to the high-energy resolution that can be achieved with these devices. Processing of the detector signal by a multichannel analyzer provides the possibility of separating the fluorescence signal from the elastically scattered background or from fluorescence radiation produced by other elements contained in the sample. Sophisticated treatment of each XRF spectrum that is saved at each energy position of a XANES scan enables the collection of XANES even at very low concentration lev- els [542]. In such a data treatment the fluorescence yield is separated from the background by fitting of the XRF spectrum. This procedure provides a much better separation of background and signal and reduces the noise contribution from elastic and Compton scattering. Micro-XAFS Use of a microbeam enables the investigation of samples that are inhomoge- neous even on a microscopic scale. Analyses can be performed at specific sites, grains, etc. of a given sample or even mappings of the chemical state can be performed. Combined with concentration-distribution maps (from micro-XRF or EMPA) this will provide information on the distribution of the chemical state or structural environment of the mapped element in the sample. The synchrotron beam is usually strongly polarized. Therefore, spectra acquired by micro-XAFS on single crystals of minerals as occurring in rock thin-sections may be affected by the orientation of the crystal toward the beam, especially in strongly anisotropic phases [543, 544]. This additional uncertainty has to be taken into account for a robust interpretation of micro-XAFS data. If elements show gradual lateral change in oxidation state within a sample the distribution of the oxidation state can be mapped by superimposition of XRF maps excited by X-ray energies at prominent maxima of the XANES of
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Methodological Developments and Applications 683 each oxidation state. The position of these maxima varies with oxidation state by a few electron volts and therefore setting the energy at either position will lead to preferential excitation of either oxidation state (e.g., compare spectra in Fig. 7.143a). Normalization of the intensity at these energy positions to the height of the edge jump and subtraction of the two maps collected at the two energies will yield the distribution of oxidation state within the sample. Thus,
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  • Spring '14
  • MichaelDudley

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