X-ray spectroscopy is a family of analytical techniques that use X-rays to reveal the elemental composition, chemical bonding, and atomic structure of virtually any material. The basic principle is straightforward: when X-rays interact with matter, atoms respond in ways that are unique to each element, producing signals that act like fingerprints. Over the past century these fingerprints have been read in increasingly creative ways, from identifying what elements sit on a periodic table to watching chemical reactions unfold in real time inside a working battery. The field has branched into dozens of specialized methods, but they all trace back to the same insight that Henry Moseley demonstrated just over a hundred years ago: every element emits or absorbs X-rays at characteristic energies, and measuring those energies tells you exactly what you are looking at.
How the Idea Started
In the early 1910s, Henry Moseley bombarded elemental targets with cathode rays and measured the frequencies of the X-rays that came back. He found that each element produced a distinct set of X-ray frequencies, and that these frequencies could be used to identify the charge on an element’s atomic nucleus. That discovery led directly to a reorganization of the periodic table: elements were rearranged by atomic number rather than atomic weight, fixing several inconsistencies in earlier tables, including those developed by Mendeleev.1PubMed. Henry Moseley, X-ray spectroscopy and the periodic table Moseley’s work established the core logic that still underlies every X-ray spectroscopy technique in use today: hit a material with energy, measure the X-ray response, and read off what is inside.
The Major Techniques and What They Measure
X-ray spectroscopy is not a single method but a toolkit. Each technique exploits a slightly different physical interaction between X-rays and atoms, and the choice depends on what you want to learn. The most widely used methods fall into a few broad families.
X-Ray Fluorescence
X-ray fluorescence, or XRF, is the workhorse of elemental analysis. When an X-ray beam knocks an inner electron out of an atom, a higher-energy electron drops down to fill the vacancy and emits a secondary X-ray whose energy is specific to that element. XRF can detect elements from sodium (atomic number 11) all the way beyond uranium, and because the fluorescence energy is unique to each element, you can identify what is present and, with proper calibration, how much of it there is.2The Canadian Journal of Chemical Engineering. Experimental methods in chemical engineering: X‐ray fluorescence Crucially, XRF is non-destructive: you can scan a priceless painting or a soil sample without damaging it, which is why the technique appears everywhere from geology labs to airport security.
X-Ray Absorption Spectroscopy
Where XRF tells you which elements are present, X-ray absorption spectroscopy (XAS) tells you what those elements are doing chemically. XAS works by tuning the X-ray energy through an element’s absorption edge and watching how the absorption changes. The region close to the edge, roughly 10 to 150 electron-volts above it, reveals the element’s oxidation state and electronic configuration. The region at higher energies reveals local atomic structure: how many neighboring atoms surround the absorbing atom, how far away they are, and how disordered the arrangement is.3The Canadian Journal of Chemical Engineering. Experimental methods in chemical engineering: X‐ray absorption spectroscopy—XAS, XANES, EXAFS This makes XAS especially powerful for studying catalysts, batteries, and biological molecules where the chemistry of a single metal atom can determine how the whole system behaves.
X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) takes a different approach. Instead of measuring X-rays that come out of a sample, it measures electrons. When monochromatic X-rays hit a surface, they eject core electrons whose kinetic energies depend on how tightly they were bound. By analyzing those kinetic energies, you get a binding-energy spectrum that identifies each element and its chemical bonding environment.4ACS Central Science. X-Ray Photoelectron Spectroscopy Techniques and Applications XPS is inherently surface-sensitive, probing only the top few nanometers of a material. That makes it ideal for studying coatings, corrosion layers, semiconductor interfaces, and anything else where surface chemistry matters.
Energy-Dispersive and Wavelength-Dispersive Spectrometry
When paired with an electron microscope, X-ray spectroscopy becomes a microanalysis tool. The electron beam excites X-rays from a tiny spot on the sample, and either an energy-dispersive spectrometer (EDS) or a wavelength-dispersive spectrometer (WDS) collects them. EDS is faster and simpler but has lower energy resolution; WDS resolves closely spaced peaks better but requires more time and beam current. The two methods converge in accuracy when certain conditions are met, such as X-ray energies above about 1,000 electron-volts and well-separated peaks.5Journal of Electron Microscopy Technique. Quantitative EDS and WDS X‐ray microanalysis of semiconductor materials: Principles and comparisons Modern EDS systems, following the same measurement protocol originally developed for WDS, can achieve equivalent accuracy and precision at substantially lower electron dose, even when peaks overlap, provided enough counts are collected.6PubMed. Is scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) quantitative?
Resonant Inelastic X-Ray Scattering
Resonant inelastic X-ray scattering, or RIXS, is a more exotic technique that has grown rapidly in recent years. It tunes the incoming X-ray energy to an absorption edge and then measures the energy and momentum of the scattered X-rays. That combination lets researchers probe collective excitations in materials: magnons, phonons, plasmons, and orbital excitations. RIXS has become an important tool for studying quantum materials such as cuprate and nickelate superconductors, where it can characterize magnetic and electronic excitations that are central to understanding how superconductivity works.7Nature Reviews Methods Primers. Resonant inelastic X-ray scattering For the newly discovered nickelate superconductors in particular, RIXS has made substantial contributions to mapping out their electronic and magnetic properties.8Acta Physica Sinica. Resonant inelastic X-ray scattering applications in quantum materials
Synchrotrons, Free-Electron Lasers, and Why the X-Ray Source Matters
Most of the techniques above can be done with a conventional lab X-ray source, typically a sealed tube with a copper or molybdenum anode. But the experiments that push the field forward almost always use a synchrotron or a free-electron laser, because these facilities produce X-ray beams that are orders of magnitude brighter, and, critically, tunable in wavelength. A home lab source generates X-rays at fixed energies set by the anode material; a synchrotron lets you dial in whatever energy you need. That tunability made absorption-edge techniques like XAS and RIXS practical, and it enabled anomalous dispersion methods in crystallography that rely on subtle changes in diffraction intensity near an element’s absorption edge.9PubMed Central. Synchrotron Radiation as a Tool for Macromolecular X-Ray Crystallography: a XXI Century Perspective
X-ray free-electron lasers (XFELs) pushed the boundaries further by delivering pulses short enough to freeze molecular motion in time. These sources, along with table-top high-harmonic-generation setups, give researchers the ability to focus on individual atoms inside complex molecules and watch electronic and nuclear motion at their natural scales of attoseconds and angstroms.10Journal of Physics B: Atomic, Molecular and Optical Physics. Roadmap of ultrafast x-ray atomic and molecular physics In one striking demonstration, XFEL-based X-ray scattering tracked the ground-state structural dynamics of a diplatinum molecule with better than 100-femtosecond time resolution, resolving oscillations along the platinum-platinum bond with a period of 0.28 picoseconds.11PubMed. Ultrafast X-Ray Scattering Measurements of Coherent Structural Dynamics on the Ground-State Potential Energy Surface of a Diplatinum Molecule That kind of experiment would be impossible with any conventional source.
Watching Batteries Work from the Inside
One of the fastest-growing application areas for X-ray spectroscopy is energy storage. Researchers want to understand exactly what happens inside a battery electrode as it charges and discharges, and soft X-ray absorption spectroscopy turns out to be well suited for the job because it is sensitive to specific elements, chemical states, and surfaces. In situ and operando experiments, where the battery is running while the measurement is being made, have revealed that lithium-ion and electron dynamics behave quite differently in different cathode materials. Work on common cathode chemistries showed that one material exhibited uniform charge dynamics while another showed a pronounced relaxation effect attributed to its phase-transformation mechanism and the way charges move through the electrode’s internal structure.12Nature Communications. Distinct charge dynamics in battery electrodes revealed by in situ and operando soft X-ray spectroscopy These experiments work on both crystalline and amorphous samples, and the technique has expanded beyond lithium-ion systems to next-generation chemistries, where it helps researchers interpret the interfacial phenomena that limit battery performance and longevity.13ENERGY & ENVIRONMENTAL MATERIALS. In Situ/Operando (Soft) X‐ray Spectroscopy Study of Beyond Lithium‐ion Batteries
Proteins, Enzymes, and the Biology of Metals
Many of the enzymes that run essential biological processes depend on metal atoms at their active sites. Understanding exactly how those metal atoms are arranged, down to hundredths of an angstrom, is key to understanding how the enzyme works and how drugs might block it. X-ray crystallography gives you the overall protein structure, but the metal site is often too disordered for crystallography alone to nail down. A combined approach collects a crystallographic data set and polarized X-ray absorption spectra from the same crystal, yielding a structural model with exceptional resolution at the metal center. In one demonstration on myoglobin, this hybrid method pinned down iron-to-ligand distances with an accuracy around two to seven hundredths of an angstrom and bond angles within about seven degrees.14PubMed Central. X-ray structure analysis of a metalloprotein with enhanced active-site resolution using in situ x-ray absorption near edge structure spectroscopy
This kind of precision has direct pharmaceutical relevance. Studies of matrix metalloproteinase-2 (MMP-2), an enzyme implicated in cancer progression, used XAS to show that a highly selective inhibitor coordinates directly to the catalytic zinc atom in a specific geometry, and that the inhibited enzyme snaps back to a conformation resembling the latent (inactive) form. That structural insight helps explain why the inhibitor is so selective and could guide the design of future drugs.15Journal of Biological Chemistry. X-ray Absorption Studies of Human Matrix Metalloproteinase-2 (MMP-2) Bound to A Highly Selective Mechanism-based Inhibitor X-ray fluorescence mapping has also been applied to neuroscience, where it can track the distribution and amount of metals like iron, copper, and zinc in brain tissue, making it a valuable tool for evaluating therapies aimed at neurodegenerative diseases that involve metal accumulation.16PubMed Central. Mapping brain metals to evaluate therapies for neurodegenerative disease
Authenticating Paintings and Uncovering Hidden Art
Art conservation is one of the more surprising fields where X-ray spectroscopy has become indispensable. Scanning macro X-ray fluorescence (MA-XRF) sweeps an X-ray beam across a painting’s surface and maps the distribution of elements like lead, mercury, copper, and zinc, which correspond to specific historical pigments. Because X-rays penetrate paint layers, the technique can visualize materials hidden below the visible surface, revealing compositions, corrections, and even entire paintings that the artist covered over.17Microchemical Journal. In situ macro X-ray fluorescence (MA-XRF) scanning as a non-invasive tool to probe for subsurface modifications in paintings by P.P. Rubens
This capability has proven especially useful for authentication. In one case, MA-XRF analysis of a still life attributed to the 17th-century Spanish painter Francisco de Zurbarán revealed a hidden painting beneath the surface and helped resolve the question of whether the work was genuine. In another case, a painting attributed to the workshop of Rubens turned out to contain a hidden stamp from a canvas manufacturer that placed the painting several hundred years later than originally supposed.18Microchemical Journal. Macro X-ray fluorescence scanning (MA-XRF) as tool in the authentication of paintings No sample needed to be removed, no paint scraped off. The scanning approach provides a full elemental image of the painting, which is far more informative than the isolated point measurements that older XRF instruments could deliver.
Reading the Universe in X-Rays
The same spectroscopic logic that works in a chemistry lab works in space. Hot astrophysical plasmas, the gas in galaxy clusters, the material spiraling into black holes, the remnants of supernova explosions, all emit X-rays whose spectral features encode the temperature, density, chemical composition, and velocity of the emitting gas. High-resolution X-ray spectroscopy from space-based observatories can probe these properties along with the physical processes shaping them, including heating, cooling, turbulence, shocks, and particle acceleration.19Astronomische Nachrichten. The Athena space X‐ray observatory and the astrophysics of hot plasma
The challenge in space is building detectors sensitive enough to extract faint spectral lines from distant objects. A new generation of X-ray microcalorimeters has reached the point where they can split X-ray energies finely enough to study these astrophysical plasmas in detail, probing conditions that cannot be replicated in any ground-based experiment.20Reviews of Modern Plasma Physics. High-resolution X-ray spectroscopy of astrophysical plasmas with X-ray microcalorimeters Future missions are expected to combine high-resolution spectroscopy with high-spatial-resolution imaging, allowing astronomers to map not just the average properties of a plasma cloud but how those properties change from one region to another.
The Radiation Damage Problem
Every X-ray spectroscopy measurement involves bombarding a sample with ionizing radiation, and that radiation can alter or destroy the very thing you are trying to measure. The problem is particularly acute for biological samples. X-ray absorption spectroscopy has been used to characterize the types of damage that ionizing radiation causes in DNA itself: strand breaks with specific chemical termini dominate, with the bond between the 5′-carbon of the sugar and the oxygen in the phosphate group being the most likely to snap.21Biophysical Journal. Investigating DNA Radiation Damage Using X-Ray Absorption Spectroscopy If the technique damages the sample as it measures it, you can end up studying the damage rather than the chemistry you care about.
For dilute biological systems like metalloenzymes in solution, this has been a serious roadblock. At a conventional synchrotron, the long exposure times needed to get a good signal from a dilute sample give radiation damage plenty of time to accumulate. A recent approach uses the ultrashort pulses of an XFEL to outrun the damage entirely, collecting the signal in a shot-by-shot mode before the sample has time to change. Combined with efficient sample-delivery strategies and new spectrometer designs, this method has successfully recorded damage-free absorption spectra from the manganese cluster in photosystem II and iron-sulfur centers in photosystem I, both of which are notoriously radiation-sensitive.22PubMed Central. X-ray Absorption Spectroscopy of Dilute Metalloenzymes at X-ray Free-Electron Lasers in a Shot-by-Shot Mode This approach may open operando studies of enzyme catalysis that were previously impossible.
Machine Learning in Spectral Analysis
Interpreting an X-ray absorption spectrum is not always straightforward. The features in a spectrum depend on the three-dimensional arrangement of atoms around the absorbing element, and extracting structural parameters requires comparing experimental data against theoretical simulations. Traditionally, this fitting process was done by hand or through iterative algorithms that could be slow and require expert judgment about starting parameters. Machine-learning tools have started to change that workflow. Software like PyFitIt uses a two-step approach: first, a set of experimental spectra is decomposed statistically into principal components; then, the spectral profiles recovered from that decomposition are fitted against a library of theoretically calculated spectra using algorithms like gradient boosting of random trees, radial basis functions, or neural networks.23Computer Physics Communications. PyFitIt: The software for quantitative analysis of XANES spectra using machine-learning algorithms The practical benefit is speed and reproducibility: the machine-learning model, once trained, can analyze spectra in a fraction of the time a human would need, which matters when synchrotron beamlines generate thousands of spectra per day.
Hard X-Rays as a Stand-In for Soft X-Rays
Most absorption-edge measurements of light elements like carbon, oxygen, and lithium require soft X-rays, which are easily absorbed by air and by the walls of sample containers. That means the sample usually needs to sit in a vacuum chamber, which rules out studying liquids, working batteries, or geological samples under realistic pressure. X-ray Raman scattering (XRS) offers a workaround. It uses hard X-rays, around 10,000 electron-volts, which penetrate air and sample containers easily, but collects the same electronic-structure information that soft X-ray absorption would give. This makes XRS the preferred method for studying soft-edge elements in vacuum-free and operando conditions: energy-storage materials during charge and discharge, inner-Earth minerals under high pressure, chemical and biological reactions in the liquid phase, and carbon bonding in archaeological and paleontological specimens.24X-Ray Spectrometry. A general overview and comparative interpretation on element‐specific X‐ray spectroscopy techniques: XPS, XAS, and XRS XRS is less commonly discussed than XAS or XPS, but for anyone working on light-element chemistry in real-world conditions, it fills a gap that no other technique covers as cleanly.