Scattering atomic absorption spectroscopy

Concentrating on metalloenzymes, we have developed a strategy based on stopped flow X-ray absorption spectroscopy (XAS) to elucidate in detail the molecular mechanisms at work during substrate turnover (Fig. 4). Importantly, XAS provides local stmctural and electronic information about the nearest coordination environment surrounding the catalytic metal ion within the active site of a metalloprotein in solution. When the X-rays hit a sample, the electromagnetic radiation interacts with the electrons bound in the metal atom. The radiation can be scattered by these electrons, or it can be absorbed, thereby exciting... 

In atomic fluorescence spectroscopy, an external source is used just as in atomic absorption, as shown in Figure 24-6. Instead of measuring the attenuated source radiant power, however, the radiant power of fluorescence, Pp, is measured, usually at right angles to the source beam. In such experiments, we must avoid or discriminate against scattered source radiation. Atomic fluorescence is often measured at the same wavelength as the source radiation, in which case it is called resonance fluorescence. 

Another valuable tool is extended x-ray absorption fine structure (EXAFS) spectroscopy. This absorption technique makes use of the modulation in absorption coefficient above an element absorption edge, due to backscatterlng of the ejected photoelectron by neighboring atoms. By suitable data treatment, it is possible to determine the elemental type, number, and distance of the atoms coordinated to the absorbing cation. Thus, EXAFS probes the atomic-scale structure in the material, while SAXS and ASAXS probe the microdomain-scale structure. By using both scattering and absorption techniques, a full picture of ionomer morphology may be developed. 

---