X-ray Absorption Near Edge Structure (XANES) is an analytical method that uses X-rays to probe the chemical and physical state of a specific element within a substance. This technique determines an element’s chemical form and what other atoms it is bonded to. Its element-specificity allows scientists to isolate and study one type of atom even in a complex material.
The Underlying Physics of XANES
The core mechanism of XANES involves directing a beam of X-rays with a precise and adjustable energy at a sample. The X-ray energy is scanned until it matches the amount needed to be absorbed by an atom of the target element. This absorption event provides enough energy to eject a tightly-bound core electron from one of the atom’s innermost shells.
The exact energy required to do this is the absorption edge, a unique characteristic of each chemical element. By plotting X-ray absorption against the X-ray energy across this edge, a distinct spectrum is generated. The entire process is part of a broader technique called X-ray Absorption Fine Structure (XAFS), but XANES focuses specifically on the features at and very near the main absorption edge.
Decoding a XANES Spectrum
Interpreting a XANES spectrum allows scientists to extract two primary types of information about the target element. The first is its oxidation state, which describes how many electrons the atom has chemically shared or transferred, and is revealed by the precise energy position of the absorption edge. An atom in a more oxidized state, like Iron(III), will have its absorption edge appear at a slightly higher energy than the same element in a less oxidized state, such as Iron(II).
The second piece of information comes from the specific shape and smaller features within the spectrum, known as the pre-edge. These features act as a unique fingerprint for the element’s local coordination environment. This environment includes the number of neighboring atoms, their distance, and their geometric arrangement. For example, these spectral fingerprints can be used to distinguish between different mineral forms of an element.
The Synchrotron Requirement
Performing a XANES experiment requires a powerful X-ray source found only at a facility called a synchrotron. A synchrotron is a large-scale research installation, often circular and hundreds of meters in circumference, that accelerates electrons to nearly the speed of light. As these electrons are steered by powerful magnets, they emit intense beams of light, including X-rays.
This specialized source is necessary for two main reasons. First is the intensity of the X-rays produced; synchrotron beams can be billions of times brighter than those from a hospital X-ray machine. This intensity is needed to detect elements present in very low concentrations.
The second reason is tunability, as synchrotrons can produce X-rays of a very precise energy that can be smoothly scanned across an element’s absorption edge. This capability is not possible with conventional laboratory X-ray sources. The reliance on this major scientific infrastructure means that XANES is a highly specialized technique.
Applications Across Scientific Fields
The specificity of XANES makes it a tool across numerous scientific disciplines. In environmental science, it is used to determine the chemical form of toxic contaminants in soil and water. For instance, identifying whether arsenic is in a more mobile and toxic form helps scientists predict its risk to ecosystems and human health.
Materials science researchers use XANES to develop technologies, such as batteries. By observing changes in the oxidation state of elements in a battery cathode as it charges and discharges, scientists learn how to design more durable energy storage devices. This can be done in real-time, or operando, to watch the chemical reactions as they happen.
In catalysis, XANES helps identify the specific atomic sites responsible for chemical reactions. Researchers can study a catalyst, like the one in a vehicle’s catalytic converter, to understand how it converts harmful exhaust gases into less harmful substances. This knowledge guides the creation of more effective catalytic materials.
The technique also finds applications in cultural heritage and art conservation. Scientists can analyze the pigments in a historical painting without damaging the artwork. By identifying the pigment composition, they can help verify a piece’s authenticity or develop strategies for restoration.