I18 Techniques

X-ray fluorescence (XRF) is used to determine what chemical elements are in a sample and how they are distributed. In XRF mapping the synchrotron beam illuminates a small section of the sample, the sample is moved in a grid pattern so that, pixel by pixel the XRF signal can be collected. This provides a 2D image of multiple elements simultaneously, which can be used to correlate the distribution of elements in the sample. I18 also has a tomography stage, allowing for 3D maps to be collected of small samples which can mounted in a capillary (~1mm diameter).

X-ray Fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the incident X-ray photons (synchrotron beam) and subsequently release X-ray photons when electrons transition from the higher energy levels of the atom to fill the vacant inner shell. Each secondary X-ray photon emitted from the sample has a specific energy which corresponds to the atom from which it has originated. By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample. Typically a special type of detector called an energy-dispersive detector is used to precisely measure the energy of each photon. The plot of the number of photon counts versus their energy, the X-ray spectrum,  shows a number of peaks which are directly associated with specific elements, so by just glancing at the spectrum it is possible to quickly deduce which elements are present in the sample.

Applications

X-ray fluorescence mapping is applied to an extraordinary variety of samples, each offering unique insights into the world around us and beyond. From biological materials such as plants and seeds to assess health and nutritional value, to human and animal tissues supporting vital medical research. XRF has been used to examine dinosaur fossils and space debris to reveal their composition and piece together their history, while ancient artefacts and artworks undergo analysis to better understand their materials and guide conservation efforts. Cutting-edge materials and catalysts are studied to observe chemical changes and performance at the microscopic level, driving innovation in science and technology and geological samples are explored to gain critical understanding of Earth's structure, environment, and how it's changing over time.

X-ray absorption spectroscopy (XAS) is used to identify different chemical species present in a sample. At characteristic wavelengths the X-ray absorption of an element changes dramatically. XAS spectra contains fine structure informtation, revealing the electronic and geometrical environment of the element of interest. Some information is provided directly (like the oxidation state) while other information can be obtained by comparing to a library of standards. A range of XAS techniques are available, including X-ray Absorption Near-Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS), Resonant Inelastic X-ray Scattering (RIXS) and X-ray Emission Spectroscopy (XES).

Applications

XAS is important in gaining structural understanding of a range of materials, including biomaterials, novel materials with special electronic properties such as superconductivity, dilute species in fluids, and complex inhomogeneous materials. It can provide information on bio-remediation processes, study minute minerals returned from space missions and be used to understand chemical reactions such as heterogeneous catalysis and hydrothermal synthesis of industrial materials.

See below XAS on I18 used to determine the mineralogy of comet 81P/Wild 2 particles collected in aerogel by the Stardust mission.

Optical microscope images (left) of the Stardust aerogel keystones and tracks and their corresponding Fe-K XAS spectra (right). To learn more about the results of this study view the paper: Hicks, L.J., MacArthur, J.L., Bridges, J.C., Price, M.C., Wickham‐Eade, J.E., Burchell, M.J., Hansford, G.M., Butterworth, A.L., Gurman, S.J. and Baker, S.H., 2017. Magnetite in Comet Wild 2: Evidence for parent body aqueous alteration. Meteoritics & Planetary Science, 52(10), pp.2075-2096. (link)

X-ray Emission Spectroscopy (XES) is a technique complementary to X-ray absorption spectroscopy (XAS), it provides valuable information with respect to the electronic structure (local charge- and spin-density) as well as the nature of the bound ligands. In XES a core electron is excited by an incident x-ray photon and then this excited state decays by emitting an x-ray photon to fill the core hole. The energy of the emitted photon is the energy difference between the involved electronic levels. The analysis of the energy dependence of the emitted photons is the aim of the X-ray emission spectroscopy. XES is both element- and site-specific, making it a powerful tool for determining detailed electronic properties of materials.

The high-energy-resolution fluorescence detection technique, HERFD, makes it possible to overcome some of the main limitations of conventional XAS. This technique consists of measuring the x-ray absorption spectrum via monitoring the intensity of a fluorescence line corresponding to a specific excited state decay process using a narrow energy resolution. This is generally achieved through the use of a crystal analyser to select a narrow energy band from the sample’s emission line.

X-ray diffraction (XRD) is a technique used to study the structure of crystalline materials. When an X-ray incident beam interacts with a sample, scattering of the X-rays occurs. A diffraction pattern is the variation in intensity observed when the scattered X-rays undergo constructive and destructive interference as a result of their interaction with the sample. The intensities of the diffracted X-rays depend on the kind and arrangement of atoms in the crystal structure. A primary use of the technique is the identification and characterisation of compounds based on their diffraction pattern.

See below XRD on I18 used to analyse the degraded mordant gilding on early fourteenth-century paintings by Pietro Lorenzetti.

Sample (left) Pietro Lorenzetti, Christ between Saints Paul and Peter, about 1320. Detail, during cleaning, showing the mordant gilding on Christ’s robes. © Ferens Art Gallery, Hull Museums. Corresponding micro-XRD pattern (right). To learn more about the results of this study view the paper: Howard, H., Najorka, J., Schofield, P. F., & Geraki, K., 2023. Degradation of Fourteenth-century Mordant Gilding Layers: Synchrotron-based Microfocus XRF, XRD, and XANES Analyses of Two Paintings by Pietro Lorenzetti’, Studies in Conservation, 69(3), pp. 193–208. (link)