As the name suggests, X-ray spectroscopy is based on the detection and measurement of particles with wavelengths that fall in the X-ray region of the electromagnetic spectrum (0.01-10 nm). This type of radiation can interact with the electrons of the inner shell of an atom. Therefore, X-ray spectroscopy can provide information on the chemical and elemental properties of a sample.
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The pioneering work in the development of X-ray spectroscopy led by William Lawrence Bragg and William Henry Bragg (Physics Nobel Prize winners in 1915), opened up to a revolution in the chemical and structural characterization of numerous substrates. The technique finds nowadays multiple applications in many areas of science and technology, including structural biology, material science, astronomy, and engineering.
Principles and applications of X-Ray Spectroscopy
Upon bombardment of an atom with a high-energy beam, X-ray photons with energy ranging from tens to thousands of electron volts (eV) are absorbed, exciting electrons from the atom inner shell (1s, 2s, or 2p orbitals) to higher levels. When electrons return to lower energy levels, X-ray photons are emitted in a way that is characteristic of the atoms that make up that particular element.
X-ray spectroscopy measures those energy changes, whether in absorption or emission, enabling the identification of the atoms within various materials. For these reasons, it is an excellent technique for qualitative and quantitative analysis of material composition.
The chemical sciences and engineering are the fields that mainly benefit from X-ray spectroscopy since it allows the analysis of materials that would otherwise not be possible with other techniques. For instance, it is possible to analyze ceramic materials in their solid-state.
Also, it is an elite technique to study heterogeneous catalysts as well as the formation of metal complexes and their electronic properties, which is fundamental for the development of novel materials such as solar cells.
Being a non-destructive technique, X-ray spectroscopy also finds application in archeometry, not only for the determination of the chemical composition of ancient artifacts but also in fraud prevention.
Intriguing applications of X-ray spectroscopy are found in astronomy and astrophysics. X-rays can be used to probe the high-temperature plasma of galaxies and investigate neutron stars and black holes. X-ray absorption and emission measurements can help determine the elemental composition and physical conditions of these entities.
Special telescopes are equipped with detectors that measure X-rays and count the number of photons collected (intensity), their energy (usually 0.12-120 keV) and wavelength, or how fast the photons are detected (counts per hour), providing information about the object that emitted them.
Recently, X-ray spectroscopy has also found application in food and agricultural sciences. Techniques that use soft X-rays (with lower energy), which are less harmful, have been developed for food inspections. It is possible for instance to discriminate the origin of olive oils, or to determine a series of elements (i.e. K, Ca, Fe, and Zn) in milk-based products, particularly useful to optimize the production of infant formula milk. In general, dairy sample preparation strategies and quantification procedures are quite simple and can be easily implemented, making X-ray spectroscopy a promising tool for routine analysis in the dairy industry.
Structure determination via XRD – applications in crystallography
One of the most known applications of X-rays is in the characterization of crystalline materials, based on the principle of X-ray diffraction (XRD). XRD enables the determination of the three-dimensional structures of complex molecules, such as natural fibers, polymer composites, DNA, and proteins.
The Bragg’s law (nλ = 2dsinθ), which correlates the angle and wavelength of the incident radiation to the distance between crystal lattice planes, is at the basis of the technique. The wavelength of X-rays is of the same order as the Angström (Å, 10-10 m) and thus is in the same range of atomic distances. When a monochromatic beam of X-rays impacts on a sample, constructive interference scatters the X-rays at specific angles from each lattice planes of the crystal.
Diffracted X-rays are then collected by a detector, producing XRD peaks whose intensities depend on the atomic position within the lattice planes. XRD can therefore provide information on structures, phases, crystal orientations, and other structural parameters.
The distribution of the electron cloud of the molecule can be calculated following diffraction data analysis, allowing the determination of the coordinates in the three-dimensional space of each atom of the molecule, including bond lengths and bond angles.
This ability makes X-ray diffraction the most accurate technique for the structural determination of molecules, with the limitation, however, that can only be used on crystalline or semi-crystalline samples.
Among various examples of XRD applications, particularly related to biomolecules, there is the notorious study of the DNA diffraction pattern by Rosalind Franklin, which was crucial in the determination of the structure of the double helix.
Despite the many advantages associated with X-ray spectroscopy, the technique comes with limitations too. Among them, small sample quantities pose a problem. Also, longer exposure to the X-ray beam may cause damage to the samples. Depending on the instrument and technique, potential harm for users due to the nature of the radiation itself requiring particular consideration.
Nevertheless, there are no doubts about the power of X-ray spectroscopy concerning the determination of the elemental composition of a sample and the structure elucidation of complex molecules. There have been remarkable advances over the years and current studies are looking at the development of different approaches to extend the applicability of the technique.
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