X-Ray Spectroscopy: XRF, XRD and X-Ray Absorption

X-rays can cause inner-shell ionisation when the photon energy is sufficient. Filling a vacancy can release a characteristic X-ray photon or energy through a competing non-radiative process. The resulting emission lines depend strongly on atomic number and the electronic transition.

Kα and Kβ designate families of transitions filling K-shell vacancies, while L-series lines fill L-shell vacancies. A line energy supports elemental assignment, but overlapping lines, detector response, escape peaks and scattered radiation can complicate interpretation. Not every observed feature is an unambiguous element peak.

Moseley's relationship connects characteristic frequencies with atomic number for corresponding line series. It explains systematic line behaviour, not perfect immunity to all chemical-state effects. Fine energy shifts and absorption-edge structure can contain chemical information even though routine XRF mainly measures elements.

Low-energy X-rays are readily attenuated by air, windows and the specimen. Light-element capability depends on source, detector, optical path and instrument design. Vacuum or helium can help where appropriate, but neither automatically makes every instrument detect carbon, oxygen or beryllium.

The sampled depth is not a single universal thickness. Incident and emitted energies, material composition and geometry affect the effective volume. Coatings, plating and underlying layers can contribute differently. A surface XRF measurement cannot simply be assumed to represent an entire object.

Quantification requires an appropriate calibration or supported model with relevant matrix and geometry treatment. A factory alloy programme may be inappropriate for a thin paint layer, irregular fragment or plated object. Qualitative peaks and quantitative bulk composition are different levels of conclusion.

Energy-dispersive XRF measures photon energies using a suitable detector and builds a spectrum of counts versus energy. Silicon-drift detectors are common, although other detector types exist. Multiple detectable elements can contribute during an acquisition without guaranteeing coverage of every element in the periodic table.

Wavelength-dispersive XRF uses diffraction optics to select characteristic wavelengths. Sequential and simultaneous WDXRF designs both exist. The technique should not be described as necessarily measuring one element at a time or invariably requiring an hour for a specimen.

WDXRF often offers greater spectral separation for particular overlaps, while EDXRF supports compact and portable configurations. Detection capability and speed depend on the instrument, element, specimen and measurement programme. Fixed universal resolution, price and detection-limit tables are unreliable purchasing or method-selection guidance.

Handheld instruments can support field screening and alloy or material assessments under suitable conditions. Geometry, positioning, measurement time and specimen thickness influence response. A high gold reading on a plated item is not proof of solid gold; additional appropriate examination may be needed.

Micro-XRF focuses excitation for local measurements and elemental mapping. The spot size and interaction volume constrain spatial interpretation. A line scan across a paint cross-section can contain contributions from adjacent layers unless the method demonstrates adequate spatial selectivity.

X-ray equipment requires appropriate radiation protection, training and regulatory compliance in its jurisdiction. The specimen must be positioned and supported according to the equipment procedure. A generic exposure limit or an assumed national licence should not replace the actual applicable requirements.

A crystalline structure produces constructive interference at conditions described by Bragg's law, nλ = 2d sinθ. Here d is the relevant lattice-plane spacing and θ is the angle to those planes. Powder diffraction patterns are commonly plotted against 2θ, so using the plotted angle directly as θ gives an incorrect spacing.

Peak positions reflect lattice spacings, while intensities depend on structure and measurement conditions. Preferred orientation, crystallite size, strain and specimen preparation can affect the pattern. A few matching peaks do not necessarily identify all phases in a mixture.

Polymorphs share chemical composition but differ in crystal structure. Calcium-carbonate phases provide a useful example: elemental analysis alone cannot distinguish calcite and aragonite, whereas suitable diffraction patterns can. Molecular or elemental agreement is therefore different from phase agreement.

Reference collections such as the ICDD Powder Diffraction File support search and identification. Review candidate patterns against all relevant observed features and the acquisition conditions. Database size changes over time and does not determine whether the particular specimen's phases are represented.

Quantitative phase analysis can use suitable calibration or model-based refinement such as Rietveld analysis when adequate structural models and data are available. The process is not an automatic decomposition of every unknown mixture. Unidentified phases, texture and absorption effects can limit the result.

There is no universal five-percent minor-phase detection limit. Sensitivity depends on phase, matrix, overlap, acquisition and analysis. Amorphous material lacks the sharp Bragg pattern of a well-crystallised phase but still produces scattering information. Supported methods can investigate amorphous content; calling its pattern information-free is incorrect.

Use preparation appropriate to the material and the preservation requirements. Grinding can change texture or some materials' state, and reactive evidence needs suitable handling assessment. A visual appearance or FTIR spectrum alone does not provide a universal prerequisite test for crystallinity.

X-ray absorption spectroscopy measures changes in absorption as incident energy crosses an element's absorption edge. The near-edge region, XANES, can be sensitive to oxidation state and coordination. The extended region, EXAFS, contains oscillatory information related to neighbouring atoms and their arrangement.

EXAFS modelling can estimate neighbour distances and coordination parameters within the model and data quality. It does not provide an unrestricted image of every atom. Correlations, disorder and mixtures can limit the uniqueness or precision of fitted structural interpretations.

An absorption edge provides element-focused information, but other constituents and instrumental effects can still influence the measurement. Appropriate standards help assess oxidation-state or structural assignments. A chosen edge does not guarantee complete absence of interference.

Synchrotron facilities provide widely used tunable, intense sources for XAS, and laboratory-based approaches also exist. Feasibility depends on edge energy, concentration, specimen, acquisition mode and required data quality. Access and capability should be confirmed with the actual facility.

Transmission and fluorescence detection have different practical strengths and limitations. Specimen thickness, self-absorption and heterogeneity can distort results. These issues matter when assessing a contaminated soil, pigment mixture or environmental specimen.

XAS can support questions about arsenic or other elements' chemical environments that bulk XRF cannot answer. It remains one component of an interpretation. Oxidation-state information alone does not establish the material's origin, toxicity in a person or legal significance.

Glass and paint examinations can combine elemental, structural and molecular information. XRF may compare elemental patterns, FTIR can address selected molecular components, and microscopy can establish layer or particle context. Agreement in one dimension is not automatically unique-source attribution.

For inorganic gunshot residue, bulk XRF of collected material differs from SEM–EDS examination of individual particles. Collection and preservation must suit the intended tests. Dissolving material for a bulk measurement can destroy the particle morphology needed for a later examination of the same material.

Lead, antimony and barium are associated with some conventional primer residues, but ammunition formulations and alternative sources vary. Particle classification considers composition, morphology and relevant criteria. Neither a bulk elemental signal nor a particle finding by itself proves who fired a gun.

Pharmaceutical examinations can assess crystalline form, excipients and other material properties alongside chemical identity and assay. A polymorph difference may be analytically important but does not independently prove counterfeiting or inadequate clinical performance. Manufacturing and regulatory context are separate questions.

Heritage examinations can identify elemental distributions and support pigment assignments with complementary evidence. Lead detection alone does not uniquely identify lead white, and lead white does not establish that a work predates 1920. Pigments can persist in later use, restorations can add layers and old materials can be reused.

A material known to have been introduced after a claimed period can raise a dating question when its identity, location and relationship to the original work are established. That is different from assigning an exact painting date from one elemental peak.

US, UK, Indian and other laboratories use these techniques according to their capabilities and applicable frameworks. Report the specimen, sampling location, measurement, reference basis and limits. Country-specific institutional assignments should be sourced to the actual service rather than presented as universal forensic practice.

Related reading: Infrared Spectroscopy: FTIR and ATR-FTIR.

Related reading: Scanning Electron Microscopy (SEM) with EDS.

Related reading: Radiochemistry: Radioactivity, Carbon Dating, Neutron Activation.