Basics
X-ray absorption spectroscopy is the general term encompassing both the X-ray absorption fine structure (XAFS) and X-ray absorption near-edge structure (XANES) techniques. In both techniques, X-rays are absorbed by atoms in a sample while the energy of the radiation is varied, and each element type produces a characteristic absorption pattern. This plot of absorption against energy is the X-ray absorption spectrum of the sample. The key feature of these XAS techniques is that since every element absorbs a different wavelength of X-ray radiation, each measurement can be “tuned” to exclusively look for an element of interest, simply by adjusting the wavelength of the X-ray source. This is where a synchrotron facility becomes essential - only synchrotron X-ray sources such as DESY can provide precisely tunable X-rays.
Benefits
Only synchrotron X-ray radiation can provide the intense and continuously tunable X-ray beams needed for XAS. This allows the energy of the beam to be changed to target-specific element types. The element specificity provides an excellent tool for identifying the elements in unknown samples. This, in combination with the high penetration of X-ray radiation, makes the technique very useful for detecting buried layers in materials otherwise inaccessible for analysis. The ability to discriminate chemical oxidation states and compounds containing toxic elements such as arsenic, selenium or chromium at the parts per million level makes these XAS techniques ideal for investigating contamination in the environment. In the XAFS technique, the incoming X-rays release electrons from the target atoms, which interact with nearby atoms, allowing us to learn about the environment surrounding the target atom. The XANES technique, on the other hand, provides the identity of the atoms being hit by the X-rays, and is very sensitive to their oxidation state and geometry, thus being useful for determining the exact chemical species present.
Types of samples
One of the major advantages of XAS techniques is that (unlike diffraction techniques) crystalline samples are not necessary, allowing measurement of solids, liquids, gases, mixtures and amorphous materials. The technique is also non-destructive, making it suitable for sensitive samples (such as artwork, historical artifacts) and samples where material is scarce. The PETRAIII X-ray source is even intense enough to detect trace elements in a sample. In general, the very fast analysis means that structural changes during chemical reactions (e.g. catalytic reactions) can also be probed in situ as they occur.
Applications of X-ray absorption spectroscopy at DESY
Key applications of XAS include materials science, chemical and biochemical research, often for the identification of metals in a sample (e.g. metals, nanoparticles, or in the active sites of metalloproteins/metalloenzymes) and their oxidation states. The fast nature of XAS also makes it useful for studying dynamic samples such as metals during alloying and catalysts during reactions. XAFS has found particular utility in the study of nanoparticles and catalysts. The determination of chemical valence states in high temperature superconductors or the characterisation of impurities, detected for instance by their X-ray fluorescence, are other interesting applications of the XAFS technique. XANES, being a more direct technique for gaining information about targeted atoms, lends itself to the quantification of certain elements in a sample and the determination of their true oxidation states.
Examples of specific applications:
- Observation of structural changes during surface processes on heterogeneous catalysts.
- Determining the true oxidation state of atoms in minerals, oxides, and mixed-metal systems.
- Determining the types of metals present in enzyme-active sites and their local structures.
- Determining structural changes of metals during alloying processes.
- Determining the local structures of target atoms in amorphous composites and highly dispersed suspensions of nanoparticles.
- Determining the local environment around doped atoms in a material.
- Understanding the speciation of metal ions in solutions.
- Determining the chemical composition of impurities in common materials.
- Determining the identity of metals in geological samples.
- Investigating buried or covered parts of microelectronic components.
- Analysis of bulk and surface structures of heterogeneous catalysts.
- Tracking structural changes of heterogeneous catalysts during catalytic reactions.
