X-ray Techniques in Forensic Analysis: XRD, XRF and X-ray Imaging
X-ray techniques: production, XRD with Bragg's law, EDXRF vs WDXRF, forensic applications, Indian labs.
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X-ray diffraction (XRD) and X-ray fluorescence (XRF) are two complementary, non-destructive analytical techniques used routinely in forensic laboratories. XRD identifies the crystalline phase of a sample by applying Bragg's law (nλ = 2d sin θ) to produce a diffractogram matched against the ICDD Powder Diffraction File database. XRF identifies the elemental composition of a sample by measuring characteristic secondary X-rays emitted after primary-beam excitation. Together, they answer two distinct but linked questions about the same exhibit: what elements are present, and what crystalline phases those elements form.
Three X-ray techniques are central to modern crime laboratory practice: X-ray diffraction (XRD) for crystalline phase identification, X-ray fluorescence (XRF) for elemental composition, and X-ray imaging for radiographic examination of sealed exhibits. The underlying physics is concise, the governing equations are exact, and the casework applications span paint, glass, soil, gunshot residue, counterfeit currency, gemstones, and illicit drugs.
XRD and XRF share the same wavelength regime and both are non-destructive, making them the preferred pairing in forensic labs where exhibit preservation matters. The key distinction: XRD identifies what crystalline phase a sample is; XRF identifies what elements it contains.
By the end of this topic you will be able to:
- Derive and apply Bragg's law (nλ = 2d sin θ) to calculate d-spacings from diffractogram peak positions.
- Distinguish XRD from XRF in terms of what each technique identifies and the underlying physical mechanism.
- Compare EDXRF and WDXRF across resolution, speed, portability, and detection limits.
- Select the appropriate X-ray technique for a given forensic exhibit type: paint, glass, soil, GSR, counterfeit currency, illicit tablets, or gemstones.
- Identify the principal Indian institutions housing forensic XRD and XRF facilities and describe the regulatory framework governing X-ray source use.
- Bragg's law
- nλ = 2d sin θ. The condition under which X-rays scattered from successive crystal planes interfere constructively. Foundation of XRD.
- d-spacing
- The perpendicular distance between adjacent lattice planes in a crystal, expressed in angstroms. A diffraction pattern is essentially a list of d-spacings unique to a phase.
- Characteristic X-rays
- Sharp emission lines produced when an inner-shell vacancy in an atom is filled by an outer electron. Their energies are element-specific (Moseley's law) and form the basis of XRF identification.
- K-alpha line
- The characteristic X-ray emitted when an electron drops from the L-shell to fill a K-shell vacancy. Brightest line for most elements; the workhorse line in EDXRF.
- Continuous (Bremsstrahlung) radiation
- Smooth background X-ray spectrum produced when high-energy electrons decelerate in the anode. Element-independent. The 'hump' under the characteristic lines.
- EDXRF
- Energy-dispersive X-ray fluorescence. A solid-state detector resolves the entire emission spectrum at once by photon energy. Cheap, portable, faster.
- WDXRF
- Wavelength-dispersive X-ray fluorescence. An analyser crystal disperses fluorescence by wavelength (Bragg's law again). Slower, costlier, much higher resolution.
- Anode target
- The metal block (commonly Cu, Mo or Rh) that electrons strike inside an X-ray tube to produce the X-ray beam. Choice of anode sets the K-alpha wavelength used for excitation.
X-ray fundamentals: how the beam is produced
X-rays for laboratory work are produced in a sealed X-ray tube. Electrons boiled off a tungsten filament are accelerated through 20 to 60 kV onto a metal anode. Two distinct spectra come out of the anode at the same time.
- Continuous spectrum (Bremsstrahlung). Electrons that decelerate inside the anode lose energy as a broad, smooth X-ray continuum. The short-wavelength cutoff is set by the tube voltage (λ_min = 12,398 / V, in angstroms and volts). This part of the spectrum carries no element information.
- Characteristic spectrum. Some incoming electrons knock out an inner-shell (K or L) electron of an anode atom. An outer-shell electron drops in to fill the vacancy and emits an X-ray photon at an energy fixed by the anode element. For a copper anode the dominant line is Cu Kα at 1.5418 Å. This is the line used for XRD work.
Two practical points examiners like to test. First, the anode is water-cooled because more than 99 percent of the input electron energy becomes heat, not X-rays. Second, Moseley's law(√ν proportional to Z) is the physics behind XRF: the characteristic line energy depends only on atomic number, which is why XRF can identify elements regardless of chemical state.
XRD: principles and Bragg's equation
X-ray diffraction (XRD)works because the wavelength of X-rays (around 1 Å) is comparable to the spacing between atomic planes in a crystal. When a monochromatic X-ray beam hits a crystalline sample at angle θ, reflections from successive planes interfere constructively only when path difference equals an integer number of wavelengths. That is Bragg's law
nλ = 2d sin θ
where n is the diffraction order (usually 1), λ is the X-ray wavelength, d is the spacing between lattice planes, and θ is the angle of incidence (half the deflection 2θ recorded by the detector). The output is a diffractogram: a plot of intensity against 2θ, with sharp peaks at angles where Bragg's condition is met.
Every crystalline phase has its own unique set of d-spacings, catalogued in the ICDD PDF (Powder Diffraction File)database. Phase identification is a pattern-matching exercise: measure the d-spacings of the unknown, look them up, name the phase. A typical Indian forensic XRD bench (CFSL Pune physics division, for instance) runs a copper-anode source (Cu Kα, 1.5418 Å), a graphite monochromator and a scintillation or position-sensitive detector, scanning 2θ from about 5 to 80 degrees.
What XRD answers in forensic casework:what crystalline phase is this?Examples include distinguishing α-quartz from amorphous silica in a soil sample, confirming the pigment in a paint chip (rutile vs anatase TiO₂, lead chromate vs lead carbonate), identifying explosives (RDX, PETN, ammonium nitrate) by their lattice pattern, and confirming the active ingredient in seized tablets without dissolving them.
XRF: EDXRF vs WDXRF
XRF asks a different question from XRD:what elements are in this sample, and roughly in what proportion?A primary X-ray beam knocks out an inner-shell electron from the sample atoms. The vacancy is filled from an outer shell, and a secondary (fluorescent) X-ray is emitted at an energy characteristic of the element. Sort those secondary photons by energy or wavelength and you have an elemental fingerprint.
The two flavours of XRF differ only in how they sort the fluorescent photons.
| Feature | EDXRF (Energy-dispersive) | WDXRF (Wavelength-dispersive) |
|---|---|---|
| Sorting method | Solid-state Si(Li) or SDD detector resolves photons by energy in real time | Analyser crystal (LiF, PET, TlAP) disperses photons by wavelength using Bragg's law; goniometer scans |
| Resolution | ~ 130 to 150 eV (Mn Kα) | ~ 5 to 20 eV; much better separation of overlapping lines |
| Speed | Whole spectrum captured simultaneously, seconds per sample | Sequential or simultaneous on multiple channels; slower |
| Cost and portability | Lower cost, available as handheld units | Bench-top only, higher cost, requires stable power |
| Detection limit | ppm range for mid-Z elements | Sub-ppm achievable for light and mid-Z elements |
| Typical Indian forensic use | Field screening of currency notes, gemstones, GSR on hand swabs | CFSL bench analysis of paint, glass, soil with light-element confirmation |
EDXRF is faster and portable; WDXRF resolves better. EDXRF is the workhorse for handheld field instruments (the Niton or Bruker XRF guns used at crime scenes); WDXRF lives on a lab bench. Both are non-destructive, which is why courts prefer them for irreplaceable exhibits.
Forensic applications across exhibit types
Most casework uses XRD and XRF together. XRF identifies the elements; XRD identifies the crystalline phase those elements form. The exhibit categories below illustrate how the techniques are paired in practice.
| Exhibit | Question to answer | Best technique | Why |
|---|---|---|---|
| Paint chip (hit-and-run) | Is the chip on the victim's clothing from the suspect vehicle? | XRF for pigment elements (Ti, Pb, Cr, Fe); XRD for the crystalline pigment phase | Layer-by-layer pigment chemistry plus phase is highly individualising |
| Glass fragment | Container glass vs window vs vehicle glass? | XRF for refractive-related oxides (Si, Na, Ca, Mg, Al); SEM-EDX for trace | Float vs container glass differ in trace element profile |
| Soil / sand | Did this soil come from the crime scene area? | XRD for mineral phases (quartz, calcite, feldspar); XRF for major oxides | Mineral assemblage is a strong geographic discriminator |
| Gunshot residue (GSR) | Did the suspect fire a weapon? | SEM-EDX is the gold standard; portable EDXRF is a useful field screen | Pb-Sb-Ba on hand swabs is the classical triad |
| Counterfeit currency | Genuine ink/security thread or fake? | Portable EDXRF on the note in situ | RBI genuine notes have a known elemental fingerprint of the magnetic ink and security thread |
| Illicit drugs / seized tablets | What is the active compound and the bulking agent? | XRD on intact tablet for the API crystalline form | Polymorph identification (e.g. paracetamol Form I vs II) without dissolving the tablet |
| Gemstone authentication | Natural diamond vs synthetic moissanite, real ruby vs glass? | XRF for trace elements; XRD for crystal structure | Used at GSI / Indian gem labs for certification disputes |
| Suspected radiograph / explosive parcel | Is there a device inside this package? | X-ray imaging (cabinet or portal radiography) | Non-destructive visualisation; supports bomb-disposal decision |
X-ray imaging is the third leg of this bullet. examiners does not test it as deeply as XRD or XRF, but you should know that forensic radiography is used to image swallowed evidence, sealed parcels, bullets lodged in tissue (during autopsy), and to look inside suspect IEDs. For the GSR row above, the courtroom-grade confirmation runs on SEM-EDXRFwhich resolves individual Pb-Sb-Ba particles morphologically rather than just elementally. NDT-style X-ray imaging at airports and at the Bomb Detection and Disposal Squad (BDDS) of state police forces falls in the same family.
Indian institutional context
Three institutions handle the bulk of forensic X-ray casework in India.
- CFSL Pune (Physics Division). Houses both XRD and XRF benches for paint, glass, soil and counterfeit-currency casework. The Physics Division is the canonical Indian forensic home for crystalline and elemental examination.
- BARC (Bhabha Atomic Research Centre), Mumbai. Runs synchrotron-grade X-ray facilities (and accesses the Indus-2 synchrotron at RRCAT Indore for collaborative work). Trace-element work referred from forensic labs sometimes ends up here when sensitivity matters.
- NPL (National Physical Laboratory), New Delhi. The national metrology institute. Calibrates XRF and XRD standards used across Indian forensic labs and runs certified reference materials.
Also worth knowing for institutional MCQs:NFSU Gandhinagar has XRD and XRF in its Forensic Physics laboratory; the GSI (Geological Survey of India)runs XRD as its routine mineral-identification workhorse and sometimes assists forensic labs with soil cases; the AERB (Atomic Energy Regulatory Board)regulates the use of X-ray tubes and radiation sources in any forensic lab under the Atomic Energy (Radiation Protection) Rules, 2004.
What is the difference between XRD and XRF?
State Bragg's law and explain the variables for a forensic XRD.
Why is EDXRF preferred over WDXRF for field work in Indian forensic labs?
What forensic exhibits are best examined by X-ray techniques?
Which Indian institutions house major XRD and XRF facilities for forensic work?
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