Thus for each position on the sample the

Info icon This preview shows pages 704–706. Sign up to view the full content.

energies will yield the distribution of oxidation state within the sample. Thus, for each position on the sample, the fluorescence yield at three energies has to be measured: one before the edge, one after, and one at the redox-sensitive maximum (see also Sutton et al. [545]). Applications As the position of the absorption edge is dependent on the oxidation state XANES spectra can be used for the determination of the valence state. This can be done by using the edge position, i.e., determination of position in com- parison to the variation observed for the end member valence states. However, the observed position of the edge may be affected by resonances occurring in the edge region that are due to multiple scattering or transitions of the photoelectrons to localized energy levels (these features are very sensitive to the local geometry around the absorber and therefore structure dependent). Another possibility is the use of the whole XANES spectra: if the end mem- ber spectra of each oxidation state are clearly known from available model compounds the oxidation state of the unknown sample can be determined by linear combinations of spectra of the model compounds (e.g., Garvie and Buseck [546]). As the structure of the XANES is also a “fingerprint” of the absorbing element in a given phase (e.g., examples in Fig. 7.143a), linear combinations of end member spectra can be used to determine the host phase of a given ele- ment and its relative abundance in a sample (e.g., Welter et al. [547]; Calmano et al. [548]; Vincze et al. [549], Salbu et al. [506]). The pre-edge feature that probes 1s- > 3d transitions (especially sensitive to oxidation state) is only affected by the first co-ordination shell around the absorber (usually oxygen). Thus, it will show much less dependence on changes in the medium-range environment or bulk structure than the structure of the spectra occurring at the main edge. The pre-edge position varies significantly with oxidation state and shows variation in intensity due to the geometry of oxygen-coordination polyhedron (octahedral, tetrahedral, etc. see Fig. 7.143b; Wilke et al. [550]). In the case of Fe, the difference between the pre-edge po- sition (measured as centroid) of ferric and ferrous Fe does not vary with the coordination geometry. Therefore, a linear variation of the position with redox ratio can be expected if the coordination does not change with oxidation state. Exploiting this effect, it was shown to give moderately precise estimates for the oxidation state (approx. 10 15% error) by the use of general cali- bration curves (Fig. 7.143c) (Bajt et al. [551] 1995; Wilke et al. [550]). These calibrations were applied to several geological problems where knowledge of
Image of page 704

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

684 D. Rammlmair et al. Integrated pre-edge intensity 7100 7120 7140 7160 7180 7200 4 Fe 2+ - Staurolite 6 Fe 2+ - Siderite 4 Fe 3+ - FePO 4 6 Fe 3+ - Andradite garnet Energy (eV) Energy (eV) 7100 7105 7110 7115 7120 7125 Normalized absorbance 4 Fe 2+ - Staurolite 6 Fe 2+ - Sideritet 4 Fe 3+ - FePO 4 6 Fe 3+ - Andradite garnet (b) (c) Normalized absorbance (a) Fe(II) Fe(II) Fe(III) Fe(III) Centroid position (ev) 0.35 0.40 Fig. 7.143. ( a ) XANES spectra collected at the Fe K-edge of several model com-
Image of page 705
Image of page 706
This is the end of the preview. Sign up to access the rest of the document.
  • Spring '14
  • MichaelDudley

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern