Field Explosives Detection: IMS, ETD, Canines and Raman

Ion mobility spectrometry separates ions in the gas phase by measuring their drift velocity through a buffer gas under an applied electric field. In the classical drift-tube configuration, a vapour or swab sample is ionised by a radioactive source (typically Nickel-63 or Americium-241) or, in newer instruments, by a photoionisation lamp or corona discharge. The resulting ions are released in pulses into a drift tube filled with nitrogen or dry air at atmospheric pressure. A continuous electric field drives the ions toward a detector (a Faraday cup or electrometer), but ions of different sizes and shapes drift at different velocities because larger, more structurally complex ions collide more frequently with the buffer gas molecules and are retarded more. The time between ion release and detector arrival, the drift time, is converted to a reduced mobility value (K0) that is characteristic of the compound at a given field strength and temperature.

Explosive molecules ionise under the negative-ion mode used in most field IMS instruments. Common explosives produce characteristic negative ions: RDX generates an adduct ion at m/z 284 (the [RDX + NO3]- cluster), TNT gives the [M - H]- ion at m/z 226, PETN yields m/z 315, and TATP is unusual in that it ionises poorly in negative mode and requires specialised atmospheric pressure chemical ionisation or UV photoionisation for reliable detection.

Two major architectural variants exist. Linear drift-tube IMS instruments, including the Smiths Detection Ionscan 600 (deployed at over 300 airports globally), the Smiths IONSCAN 500DT, and the earlier 400B series, use a single drift tube with a Nickel-63 source. The Morpho (now IDEMIA) Itemiser series, used extensively by the US Transportation Security Administration and the UK Border Force, adds a second ionisation region and uses a different drift tube geometry. Differential mobility spectrometry (DMS) instruments such as the Smiths Detection SABRE 5000 and the Bruker RapidFire filter ions using a high-frequency alternating field rather than a linear field, offering improved selectivity at the cost of a more complex ion separation model.

Time-of-flight IMS (ToF-IMS) instruments separate ions in microsecond resolution, allowing continuous sampling rather than pulsed gating, and have been developed for both portal and handheld configurations. The GE EntryScan portal, now marketed as the EntryScan 4 by Rapiscan Systems (OSI Systems) following GE's security division passing to Morpho/Safran and then to Rapiscan, deployed at several US federal buildings, uses ToF-IMS to screen whole-body vapour emissions without a physical swab.

In the US, the TSA's approved ETD device list is governed by the TSA Qualified Products List (QPL), updated periodically and administered under the TSA's Aviation Security R+D program at Atlantic City. In the UK, the Defence Science and Technology Laboratory (DSTL) and the Centre for the Protection of National Infrastructure (CPNI) issue guidance on approved ETD devices for aviation and crowded-places screening. In India, the Bureau of Civil Aviation Security (BCAS) mandates ETD screening at Category I and Category II airports under the BCAS Security Programme, with approved equipment lists maintained by the BCAS Technology Division.

The olfactory superiority of trained canines over instruments for field explosives detection is real but not absolute. Dogs detect vapour-phase compounds at concentrations routinely below 1 part per trillion for high-vapour-pressure explosives such as EGDN (ethylene glycol dinitrate), the component responsible for the characteristic odour of dynamite. For low-vapour-pressure compounds such as PETN and RDX, which sublime very slowly at ambient temperature, trained canines still outperform most IMS systems because they integrate vapour traces from packaging seams, handling residue on surfaces, and odour plumes that instruments require close-contact swabbing to sample.

What a canine detects is not a single compound but an odour signature, a mixture of primary explosive vapours, decomposition products, solvent residues from manufacture, and plasticiser volatiles from blending. Semtex H, a Czech-manufactured plasticised explosive used in several high-profile attacks including the Lockerbie bombing of 1988, contains RDX and PETN together with a plasticiser. Dogs trained to its odour signature detect the composite, not a single molecule. This is both a strength (it makes evasion harder) and a training challenge (the odour signature varies between lots and between fresh and aged samples).

The ATF's National Canine Division in Virginia trains dogs using the Odor Signature Program, which uses a standardised panel of explosive odour signatures across at least ten substance families. The US TSA Canine Training and Certification Programme (CTCP) sets the federal standard for aviation security dogs, with handler-dog teams evaluated every 90 days using blind hides at minimum. The UK Metropolitan Police Explosives Ordnance Disposal and Search (EODS) dog teams train under the National Police Chiefs' Council (NPCC) Dog Legislation Officer guidance, with operational certification by the College of Policing. In India, military and paramilitary canine training including explosives detection is conducted at the Remount and Veterinary Centre and College in Meerut (under the Indian Army's Remount Veterinary Corps), which trains both army and paramilitary dogs including CRPF and BSF units, using substance panels validated against IED threats documented in the Indian subcontinent.

The standard ten-substance panel used across US, UK, and NATO-aligned programmes includes: TNT, RDX, PETN, ANFO, black powder, smokeless powder, TATP, HMTD, potassium chlorate mixture, and ammonium nitrate. Individual programmes add or substitute based on regional threat intelligence. Australian Federal Police canine teams certified under the AFP Canine Training Unit add ANNM (ammonium nitrate fuel oil with nitromethane) to their panel, reflecting its use in Australian bush-country vehicle bomb threats.

Colour spot tests are presumptive chemical tests, not confirmatory ones. They detect functional chemical groups (nitrate esters, nitroaromatics, peroxides, nitramines) rather than specific molecules, which means a positive result tells you a compound with that functional group is present, not which specific explosive it is. Used correctly, they triage a scene or a package before instrument or laboratory resources are committed. Used incorrectly, they are a source of avoidable false positives from fertilisers, pharmaceuticals, cleaning products, and food residues.

The Griess test detects nitrite and nitrate ions in aqueous solution. Reagent A (sulfanilic acid in acetic acid) reacts with nitrite to form a diazonium salt, which then couples with reagent B (N-1-naphthylethylenediamine) to form a pink-to-red azo dye. The test is strongly positive for nitrite, moderately positive for nitrate (requiring reduction to nitrite first, typically with zinc dust), and is used to detect inorganic nitrate residues from ammonium nitrate, potassium nitrate, and sodium nitrate in soil, surface, or wipe samples. Griess is not specific to explosives: fertilisers, urine, and some foods also contain nitrate.

Diphenylamine (DPA) test detects both nitrates and nitrites and is used as a general screening reagent for nitro-group compounds. A positive result is a blue-green coloration. DPA is more broadly reactive than Griess: oxidising agents including iodates, permanganates, and some metal oxides can produce positive results. Forensic examiners using DPA must document the sample context to distinguish spurious from genuine positives.

The J-acid test (also called the Janovsky test in some literature, using sodium hydroxide and methyl ethyl ketone) detects polynitroaromatic compounds and is specifically useful for TNT and dinitrotoluenes, producing a characteristic red-purple colour. J-acid is relatively specific to the polynitroaromatic class and is less prone to false positives from inorganic nitrates than DPA, making it useful in the second tier of a sequential spot-test protocol.

The Scott test, developed for cocaine identification in narcotics screening, is not an explosive test. The Aldrich reagent, a formulation based on 2-nitrophenylhydrazine or structurally similar chromophores in field kit form, was developed specifically for nitramine group detection (RDX, HMX, Tetryl) and produces a characteristic colour change distinct from the nitroaromatic tests. Commercial field kits such as the Mistral Group's EXPRAY, the Sirchie NITE field kit, and the Nabis Field Test Kit combine multiple reagents in a sequential protocol to distinguish nitrate esters, nitroaromatics, and peroxide-based explosives.

Raman spectroscopy measures inelastic scattering of laser photons by molecular bonds. When monochromatic laser light strikes a sample, the vast majority of photons scatter elastically (Rayleigh scattering, same wavelength), but a small fraction scatter at shifted wavelengths determined by the vibrational frequencies of the chemical bonds in the molecule. The frequency shifts, plotted as wavenumber shifts from the excitation wavelength, produce a Raman spectrum that is characteristic of the molecular structure and effectively acts as a molecular fingerprint.

Handheld Raman instruments have made field explosive identification possible in ways that were impractical before 2005. The Thermo Fisher Scientific TruNarc, used by US DEA, DHS, and multiple European police services, is primarily narcotics-targeted but has an explosives library. The Rigaku Progeny ResQ, adopted by the US Army and several NATO allies for CBRN field response, is specifically marketed for energetic materials identification. The B+W Tek NanoRam, used by multiple European customs services, offers sub-30-second identification for common explosives, pharmaceutical narcotics, and precursor chemicals, all through the original packaging of most commercial containers.

The through-barrier measurement capability is a critical operational feature. Because plastic bags, low-density polyethylene containers, and translucent pharmaceutical packaging are largely transparent at typical Raman excitation wavelengths (785 nm and 1064 nm are the most common in field instruments), an examiner can obtain a spectrum of the contents without opening the package, which matters both for officer safety (avoiding direct contact with an unknown) and for chain-of-custody integrity (the package is not disturbed).

However, Raman has two significant failure modes in field conditions. First, fluorescence interference: many organic compounds, including some polymers, dyes, oils, and biological materials, fluoresce strongly when illuminated by the laser, generating a broad background signal that overwhelms the Raman shifts. TATP, in particular, tends to fluoresce in some mixtures, and highly contaminated post-blast residues almost always fluoresce. The chemistry and vapour properties of TATP that make it problematic for standard IMS are covered in the topic on specific explosives chemistry: TNT, RDX, PETN, HMX, ANFO, TATP and urea nitrate. Instruments equipped with 1064 nm excitation (including some Rigaku and Agilent models) mitigate this by operating at a wavelength where fewer organics fluoresce, at the cost of reduced sensitivity. Second, dark or black samples absorb the laser rather than scattering it, generating heat rather than a Raman signal. Black powder, charcoal-based compositions, and carbon-loaded mixtures are poorly amenable to handheld Raman.

Fourier-transform infrared spectroscopy measures absorption of infrared radiation by molecular bonds. When IR light is directed at a sample, bonds absorb at frequencies matching their natural vibrational frequencies, producing an absorption spectrum. The mid-infrared region (400 to 4000 cm-1) contains absorption bands for most organic functional groups: N-O stretches for nitro groups, O-H stretches for alcohols and peroxides, C-H stretches, carbonyl stretches. An ATR (attenuated total reflectance) accessory allows direct contact measurement of solids and liquids by pressing them against a crystal (typically diamond or germanium) through which the IR beam passes, generating an evanescent wave that samples the surface layer of the material.

Handheld FTIR instruments with ATR probes include the Agilent Technologies 4300 Handheld FTIR, the PerkinElmer Spectrum Two Portable FTIR, and the Bruker Alpha II with an ATR module. These instruments are widely deployed in EU customs operations under the EDEN project (Explosives Detection Equipment Needs) framework and in US DHS chemical identification programmes. The Bruker ALPHA and the Agilent 4300 have been used in post-blast scene examinations in the UK (by DSTL Mobile Laboratory teams) and in Germany (by the BKA Technical Investigation Bureau, Wiesbaden).

FTIR excels where Raman fails: dark and black samples do not absorb the IR beam the way they absorb laser photons, so black powder, charcoal formulations, and carbon-loaded compositions that defeat Raman are measurable by FTIR ATR. Conversely, water and aqueous samples strongly absorb IR light across the fingerprint region, masking analyte peaks, whereas water is essentially Raman-transparent (a major reason Raman dominates in aqueous pharmaceutical and liquid analysis).

For explosive precursors, FTIR's ability to detect the O-H stretch of hydrogen peroxide (at 3300-3400 cm-1) and the nitro N-O asymmetric stretch (around 1540-1560 cm-1) makes it useful for checking bulk hydrogen peroxide concentration (a precursor control target in the EU Regulation 2019/1148 regime) and for confirming the presence of nitrate esters without requiring a colour test or swab. The precursor chemicals targeted by these controls, including hydrogen peroxide, ammonium nitrate, acetone, and urea, are detailed in the topic on homemade explosives: TATP, HMTD, urea nitrate and the precursor control response.

1. Scene risk assessment
   Determine whether the unknown is a powder, liquid, solid block, or contained package. Assess fluorescence risk (dark/organic matrix) and select Raman vs FTIR accordingly.
2. Colour test triage (if no instrument available)
   Apply sequential colour test protocol: DPA or Griess for nitrate/nitrite; J-acid for nitroaromatics; peroxide reagent for TATP/HMTD class. Document each result with timestamp and photograph.
3. Handheld Raman scan (through barrier if possible)
   If instrument available, scan through original packaging first. If fluorescence overwhelms the signal, switch excitation wavelength if available (785 nm to 1064 nm). Note: do not apply pressure that could initiate a sensitive peroxide.
4. Handheld FTIR ATR contact measurement (if Raman inconclusive)
   Apply ATR crystal to exposed surface sample under controlled pressure. Acquire spectrum and compare against library. Check for nitro N-O stretch and peroxide O-H signatures.
5. Canine confirmation sweep (if available)
   Request dog team sweep of the immediate area and handling surfaces to detect vapour-phase components not sampled by instrument methods. Document handler, dog certification date, and pass-fail outcome.
6. Secure sample and refer for laboratory confirmation
   Package positive presumptive sample per exhibit handling SOP. Submit to lab with full field test record. No field result is stand-alone evidence.

The operational picture across jurisdictions is that field detection exists as a screening and triage layer, not as a substitute for laboratory analysis. Understanding which device component generated the residue being screened, and where to look for it at a post-blast scene, is addressed in the topics on IED anatomy and triage and post-blast scene methodology. The US Code of Federal Regulations (49 CFR Part 1544 and Part 1546 for aviation) does not specify which field methods meet confirmatory standards; it specifies that explosive detection must use TSA-approved equipment and training programmes. The UK's Forensic Science Regulator's Codes of Practice and Conduct (2023, Appendix: Explosives Scene Examination) explicitly states that presumptive field tests require laboratory follow-up before an expert opinion can be given in court. Interpol's Explosives Forensics guidelines (published through the Explosives Forensics Sub-Directorate of the Forensic Support Unit) carry the same principle as a recommended minimum standard for member-state forensic services.

False positives from IMS devices remain a persistent operational problem. Nitroglycerin-based medications (GTN patches worn by cardiac patients), dinitrotoluene residues from commercial fireworks, and TACAN and DEET-based insect repellents have all generated confirmed false positives on deployed IMS systems. The operational response is layered screening: a second swab with a second device of a different type, followed by a behavioural assessment interview, followed by a canine screen if the IMS result is not resolved. A confirmed positive on two independent methods, one IMS and one confirmatory canine alert, has been the operational threshold for further action in several documented cases, including at Heathrow and at Frankfurt Airport in the 2010-2015 period.

In India, the standard operating procedure for airports categorised by BCAS requires ETD screening of all hold baggage and 100% of passengers at Category I airports (those with international operations). Post-blast scene procedures follow CFSL guidelines and the National Investigation Agency (NIA) forensic protocol, which mandates laboratory confirmation of all field presumptives within 72 hours of sample collection. In the US, post-blast scene examination is governed by BATFE guidelines and NIST SP 800-171 chain-of-custody requirements for physical evidence.