X-ray Absorption Fine Structure (XAFS)
X-ray Absorption Fine Structure (XAFS) spectroscopy is basically the measurement of the variation or “fine-structure” of the X-ray absorption coefficient with energy in the vicinity of one of the characteristic absorption edges of the element of interest. XAFS spectroscopy is a synchrotron radiation technique that takes advantage of the high-intensity X-ray beam-lines that are only available at specialized synchrotron source facilities. X-ray fluxes at these beam-lines are upwards of one million times more intense than those available from conventional laboratory X-ray tubes.
There are two, virtually independent, parts to the XAFS spectrum:
- X-ray absorption near edge structure (XANES) which deals with the fine structure near the absorption edge of the element itself,
- Extended X-ray absorption fine structure (EXAFS) which deals with the periodic oscillatory structure above the absorption edge.
Whereas the XANES spectrum can be used simply as a “fingerprint” that reflects the occurrence of the element in question in the material under investigation, the EXAFS spectrum can be mathematically converted into a radial structure function (RSF) that can be regarded as a one-dimensional representation of the local structure around the element in question. Either or both of these spectral regions can be used to obtain information about an element in a complex material.
For
these reasons, XAFS spectroscopy has rapidly become a powerful
probe of the immediate bonding and local structure around individual
elements in materials. The material can be any form – gas,
liquid, or solid, including crystalline materials, amorphous materials
(e.g. glass), and complex multi-component materials (e.g. soils,
rocks, environmental materials, etc.). Most elements in the periodic
table can be examined by the XAFS technique, many at concentrations
down to the parts-per-million level.
Increasingly, XAFS spectroscopy is the technique of choice for resolving questions of an element’s speciation in complex materials, as the technique is non-destructive and minimal sample preparation is required except for pulverization and homogenization of the sample. It is also a direct technique in that the XAFS spectrum obtained represents the weighted sum of the individual spectra of different species of the element present in the material under examination. Depending on the quality of the data, XAFS spectroscopy can be used to:
- Identify oxidation state or states of the element: E.g. it readily discriminates between Cr3+ and Cr6+, or As3+ and As5+.
- Determine the immediate local structure around the element in question: Bond distances, coordination numbers, identity of nearest neighbor atom.
- Resolve and quantify up to 3 or 4 species of an element in a complex mixture: By least-squares fitting of either XANES or EXAFS spectra or both.
- Speciation of trace and minor elements (S, Cr, Zn, As, Se, Pb, etc.) in contaminated soils, environmental wastes, ambient particulate matter, etc.
- Determination of forms of heavy metals in fuels (coal, residual fuel oil, biomass) and their combustion by-products (fly-ash, stack samples).
- Identification of the form and role of key elements in catalysts and other complex manufactured materials (composites, alloys, adsorbents, etc.).
The technique is optimum in terms of sensitivity, information obtained, and scheduling for elements between Titanium (Ti) and Molybdenum (Mo), and, at lesser sensitivity, from Sulfur (S) to Uranium (U). For elements below Sulfur, the technique typically requires the analysis be carried out in vacuo, and should be reviewed thoroughly prior to undertaking.
