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

A UV-Vis or IR spectrum changes when you change the molecular environment. Move a carbonyl from a free aldehyde into a conjugated enone and the band shifts by 30 cm⁻¹. The valence electrons that drive those transitions are the bonding electrons. Inner-shell electrons sit far below the valence shell, are screened from chemistry by the outer electrons, and care almost nothing about what molecule the atom finds itself in. The K-alpha line of iron sits at 6.40 keV whether the iron is in haemoglobin, in a steel cartridge case, in red ochre on an Ajanta cave wall, or in iron oxide pigment on a forged currency note.

A primary X-ray photon (from an X-ray tube, a radioisotope source, or a synchrotron) hits the sample and ejects an inner-shell electron, leaving a vacancy. An electron from a higher shell drops to fill it, and the energy difference is released as a characteristic X-ray photon. The transitions are named by which shell the electron drops into. K-alpha is the L-to-K drop, K-beta is the M-to-K drop, L-alpha is the M-to-L drop. Each element has its own set of transition energies, and the set is what the spectrometer reads.

Moseley's 1913 discovery formalised the rest. The square root of the characteristic X-ray frequency is linear in atomic number Z, with a small offset for inner-electron screening. Two practical consequences follow. Every line in an XRF spectrum can be unambiguously assigned to an element, and the periodic table is properly ordered by atomic number rather than atomic mass.

Matrix tolerance is a key practical property of XRF. Point a handheld unit at a paint chip, a piece of glass, a sealed pharmaceutical capsule, a banknote security thread, or the surface of a bronze idol, and the elemental peaks come out at the same energies. Matrix changes the absorption and the relative peak heights, so quantification still needs calibration against matrix-matched standards, but the qualitative call of which elements are present is robust. This is why the Bureau of Indian Standards (BIS) hallmarking centres run handheld XRF for gold purity and why Bruker Tracer 5g units are carried by NSG and BSF teams for explosive triage at suspected device sites.

EDXRF and WDXRF are two architectural answers to the same problem. The sample emits a spectrum of characteristic X-rays; you need to separate the photons by energy or wavelength so you can assign them to elements. EDXRF separates by energy in the detector itself. WDXRF separates by wavelength using a crystal monochromator before the detector.

EDXRF uses a solid-state detector, traditionally a lithium-drifted silicon Si(Li) crystal cooled to liquid-nitrogen temperatures, now almost always replaced by a silicon-drift detector (SDD) that runs on Peltier cooling at room temperature. Each X-ray photon deposits energy as a charge pulse proportional to the photon energy, and a multi-channel analyser builds the full spectrum in parallel. Every element from sodium to uranium reports simultaneously. Resolution at the Mn K-alpha line (5.9 keV) is around 125 to 150 eV on a modern SDD, enough to separate adjacent K-alpha peaks across the periodic table for most casework. EDXRF is what sits inside a Bruker Tracer handheld, a Niton XL3t, an Olympus Vanta and the benchtop Bruker S2 PUMA that the CFSL Hyderabad paint and glass section runs.

WDXRF takes a different path. The X-ray beam from the sample is collimated and directed onto a crystal of known lattice spacing. Bragg's law selects exactly one wavelength to reflect at a given crystal angle. The detector reads only that wavelength, then the crystal angle steps to the next one. Resolution drops to about 5 to 20 eV, an order of magnitude better than EDXRF. Trace detection limits drop into the parts-per-million range. The cost is throughput; a full scan that EDXRF finishes in a minute can take WDXRF an hour. CFSL labs that run trace-metal certifications under BIS IS 15776 use WDXRF when the question requires discriminating concentrations at the parts-per-million level.

Handheld XRF is functionally an EDXRF in a pistol-grip case. The Bruker Tracer 5g, Olympus Vanta and Niton XL3t Goldd+ all run an X-ray tube source (rhodium target, up to 50 kV, up to 195 microampere), an SDD detector and an onboard pattern-recognition library. They report alloy compositions in seconds within Atomic Energy Regulatory Board (AERB) dose limits. NSG explosive-ordnance teams use them for triage of suspect packages, customs at Mumbai and Chennai ports use them for cargo verification, and CFSL field teams use them at scene for metal-fragment identification.

A key limitation of XRF is that it reads only the surface, typically a few micrometres for organic matrices and a few hundred micrometres for metals. It is not a depth profile. A gold-plated counterfeit registers as solid gold on XRF unless the plating layer is removed or penetrated. A paint chip with an acrylic top over a primer over a base coat shows mostly the top layer. The fix is sample preparation; the CFSL Hyderabad paint section mounts cross-sections and runs line scans along the layer stack as routine for hit-and-run vehicle paint comparisons.

A crystalline solid has its atoms arranged in a repeating lattice with a characteristic spacing between planes. When a monochromatic X-ray beam strikes the crystal, reflections from successive planes add in phase only at angles where the path-length difference is an integer number of wavelengths, which is the content of Bragg's law nλ = 2d sin(θ). At those angles a sharp diffraction peak appears; at all other angles the reflections cancel.

Powder XRD is the standard form for forensic casework. You grind the unknown to a fine powder, mount it on a zero-background plate, and rotate it through a range of angles while the detector reads intensity at each. The result is a one-dimensional pattern of peaks plotted against 2θ, with each peak at a position set by a lattice spacing. The pattern is unique to the crystalline phase. Calcite, aragonite and vaterite are all calcium carbonate, share the same formula CaCO₃, would be indistinguishable by XRF, and give three completely different XRD patterns.

Phase identification searches the pattern against the International Centre for Diffraction Data's Powder Diffraction File (currently PDF-4), which holds reference patterns for more than one million phases. A clean unknown typically matches one or two phases at high confidence. A mixture decomposes through Rietveld refinement into the percentage of each phase. Detection limit for a minor phase is roughly 5 percent by weight; below that the peaks merge into baseline noise.

The forensic uses of powder XRD concentrate in a few high-value applications. Drug polymorphism is a primary application: many pharmaceutical actives crystallise in more than one form, and the polymorph affects bioavailability and patent rights. A counterfeit paracetamol tablet with the wrong polymorph gets flagged on XRD even when elemental and chromatographic analysis matches the genuine product. Pigment identification is another high-value application: titanium white as anatase or rutile dates a paint film (rutile became dominant after about 1950), and lead white identifies a pre-1920 work. Explosive identification is a third: ammonium nitrate, RDX, HMX, PETN and TATP each give a distinct XRD pattern, which is often the most reliable identification when the sample is contaminated with substrate material from the scene.

The Bruker D8 Advance is the workhorse benchtop XRD across CFSL labs and IIT central facilities. The PANalytical Empyrean sits at CFSL Chandigarh and at the IIT Bombay SAIF. The Rigaku SmartLab is at IISc Bangalore. RRCAT Indore runs synchrotron-based diffraction beamlines at Indus-2 for applications that need ultra-high-resolution or in-situ time-resolved patterns.

X-ray absorption spectroscopy scans the X-ray energy across an absorption edge of a chosen element and reads absorbance as a function of energy. The edge sits at the energy where the photon becomes capable of ejecting an inner-shell electron. Two distinct spectral regions carry analytical information. XANES (within about 50 eV of the edge) is sensitive to oxidation state and coordination geometry. EXAFS (50 to 1000 eV above the edge) carries bond distances, coordination numbers and the chemical identity of neighbouring atoms.

The technique is element-specific because the edge energy is element-specific. Tuning to the iron K-edge at 7.112 keV isolates the iron environment without interference from any other element. Tuning to the arsenic K-edge at 11.867 keV distinguishes arsenite (As(III)) from arsenate (As(V)), a speciation relevant to toxicity and provenance.

The primary limitation is the source requirement. Routine XAFS needs a tunable, high-flux X-ray beam, which in practice means a synchrotron. India runs one, Indus-2 at the Raja Ramanna Centre for Advanced Technology (RRCAT) in Indore, at 2.5 GeV electron energy. The BL-9 beamline is configured for EXAFS across most transition metals. Indian forensic-physics groups working on heritage pigment provenance, soil arsenic speciation or lead contamination apply for beam time at RRCAT or collaborate internationally with BESSY-II in Berlin or Diamond Light Source in the UK.

Routine forensic casework rarely requires XAFS. Arsenic speciation in West Bengal groundwater contamination has been studied by IIT Kharagpur with RRCAT, distinguishing the more toxic As(III) from the less toxic As(V) at concentrations where chemical kits struggle. Heritage pigment XAFS at the National Museum Institute Delhi distinguishes genuine seventeenth-century pigment formulations from twentieth-century restoration repaints on disputed manuscripts and miniatures.

Gunshot residue is the canonical forensic XRF application. The classical inorganic GSR signature is the simultaneous presence of lead (from the bullet core), antimony (from the priming compound antimony sulphide) and barium (from the priming oxidiser barium nitrate) in residue swabbed off the suspect's hand. The CFSL Pune workflow swabs the back of the hand and the thumb-forefinger web with a dilute nitric acid pad, runs the pad on EDXRF for a Pb-Sb-Ba check, and escalates to SEM-EDS for confirmatory spheroidal particle morphology only when the elemental signature is present. Modern lead-free primers (sintox, NTA-based) produce a different signature dominated by titanium, zinc, strontium and gadolinium; CFSL Pune's protocol reports both panels.

Paint comparison is the second high-value XRF application. A vehicle paint chip from a hit-and-run scene is mounted in cross-section and line-scanned by EDXRF or micro-XRF along the layer stack. Modern automotive paints contain pigment elements (titanium white, iron oxide, chromium green, lead chromate yellow on older fleets) and additive elements (tin, zinc, calcium from corrosion-inhibitor primers) whose pattern is reasonably distinctive. CFSL Hyderabad's paint section uses the Bruker S2 PUMA EDXRF for elemental fingerprinting alongside FTIR for binder identification.

Glass comparison combines XRF for elemental composition with the Glass Refractive Index Measurement (GRIM) bench for refractive index. Soda-lime, low-iron float, borosilicate and tempered automotive glasses differ systematically in bulk composition; a fragment from a suspect's clothing can be compared against a scene fragment by elemental ratios at parts-per-million precision. CFSL Hyderabad runs LA-ICP-MS for the trace fingerprint and EDXRF for major-element composition.

Counterfeit currency XRF reads the elemental signature of the security thread and the inks of a suspect note against the known signature of a genuine Reserve Bank of India denomination. Counterfeit pharmaceutical work uses XRF for the elemental fingerprint of the API and excipients and XRD for the polymorph of the active. A counterfeit batch shows elemental drift, the wrong polymorph, or both. The combined workflow discriminates substandard product even when the chromatographic assay reports both as nominally pass, because polymorphism affects bioavailability without changing the assay value.

Further applications include soil provenance, art authentication, and heritage characterisation. Soil provenance analysis uses rare-earth patterns by ICP-MS with XRF as a major-element screen. Pigment XRF serves as a dating tool in art authentication: zinc white indicates post-1834, lead white indicates pre-1920, and high-concentration titanium white indicates post-1920. The National Museum Institute Delhi used portable XRF on the Ajanta cave paintings to characterise iron oxide reds, carbon black and lapis-lazuli blue in situ.