Specific Explosives: TNT, RDX, PETN, HMX, ANFO, TATP, Urea Nitrate

TNT (systematic name 2,4,6-trinitrotoluene, CAS 118-96-7) is an aromatic nitro compound: a toluene ring with three nitro groups substituted at the 2, 4, and 6 positions. Its molecular formula is C7H5N3O6 and its molecular weight 227.13 g/mol. The nitro groups serve as the internal oxidiser: during detonation the nitrogen-oxygen bonds break, releasing energy and producing carbon dioxide, water, nitrogen gas, and carbon soot (it is slightly oxygen-deficient, which accounts for the characteristic black smoke of TNT detonations and the soot contamination of post-blast surfaces).

TNT is manufactured by progressive nitration of toluene using mixtures of nitric and sulfuric acids. The process produces 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) as intermediates, and the mononitrotoluene isomers as very early products. These precursor isomers, particularly 2,4-DNT and 2,6-DNT, are important forensic markers: they are present as impurities in all manufactured TNT and can survive post-blast even when the parent TNT is largely consumed. Their molar ratio provides a manufacturing signature.

TNT's detonation velocity is approximately 6,900 m/s; its density when cast is approximately 1.65 g/cc. It melts at 80.9 degrees Celsius, which made it the first high explosive suitable for melt-pour loading into artillery shells, the defining advance of First World War ammunition manufacture. This casting property is also exploited in Composition B (RDX 60%, TNT 40%) which combines RDX's higher detonation velocity with TNT's melt-castability.

In post-blast debris, TNT is detected by HPLC with UV detection (characteristic absorption at 230 nm and 254 nm), GC-MS (molecular ion m/z 227, base peak at 210), and LC-MS/MS (negative-ion mode, [M-H] at m/z 226). The UK Forensic Explosive Laboratory (FEL, now part of the Defence Science and Technology Laboratory, Dstl), the Swedish National Forensic Centre (NFC), and the US USACIL all use validated HPLC and LC-MS/MS methods for TNT residue quantitation in soil and debris matrices.

RDX (1,3,5-trinitroperhydro-1,3,5-triazine; Research Department Explosive; hexogen; cyclonite; CAS 121-82-4) is a cyclic nitramine: a six-membered ring alternating nitrogen and carbon atoms, with three N-nitro groups attached. Its molecular formula is C3H6N6O6 (MW 222.12 g/mol). RDX is manufactured by the Bachmann process (nitration of hexamethylenetetramine with ammonium nitrate and nitric acid) or the Brockman process; the two processes produce slightly different impurity profiles, which has been exploited in forensic profiling of military RDX batches.

RDX detonation velocity is approximately 8,750 m/s at a density of 1.82 g/cc. Its energy output (heat of explosion approximately 5,400 kJ/kg) makes it the energetic backbone of a wide range of military formulations:

• C-4: 91% RDX in a plasticiser matrix (polyisobutylene and diethylhexyl sebacate). The plasticiser makes C-4 mouldable and mechanically stable. Standard US military explosive.
• Composition B: 60% RDX, 40% TNT. Cast-pourable. Used in grenades, artillery, and bomb bodies.
• Semtex-H: an approximate 50/50 mix of RDX and PETN in a styrene-butadiene binder matrix. Manufactured by Explosia a.s., Czech Republic. Widely diverted to non-state actors through Cold War supply channels.

Forensic detection of RDX relies on GC-ECD (electron capture detection, highly sensitive to nitro groups), HPLC-UV, and LC-MS/MS (negative-ion ESI: [M-NO3] at m/z 257, [M+Cl] at m/z 258). RDX degrades in the environment to the metabolites hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), which are detectable in soil and water even after the parent compound has degraded.

HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane; octogen; CAS 2691-41-0) is the eight-membered ring analogue of RDX: four nitrogen-carbon alternations with four N-nitro groups. Its molecular formula is C4H8N8O8 (MW 296.16 g/mol). HMX achieves the highest detonation velocity of any practical explosive at approximately 9,100 m/s (density 1.91 g/cc). It is produced as a by-product of some RDX manufacturing processes. Its primary applications are in polymer-bonded explosives (PBX) such as PBX-9404 (US Navy) and in the explosive lenses of implosion-type nuclear weapon designs. Its presence in a device is a strong indicator of military supply chain access.

PETN (pentaerythritol tetranitrate; CAS 78-11-5) is a nitrate ester: four nitrate (-O-NO2) groups attached to a central pentaerythritol (neopentyl) carbon framework. Molecular formula C5H8N4O12 (MW 316.14 g/mol). Unlike the cyclic nitramines (RDX, HMX), PETN carries its energetic groups as ester linkages, which makes it somewhat more susceptible to hydrolysis in alkaline environments but very stable under neutral conditions.

PETN detonates at approximately 8,400 m/s (at density 1.77 g/cc) and has a heat of explosion of approximately 5,800 kJ/kg. Its melting point is 141 degrees Celsius, higher than TNT's 81 degrees Celsius, which prevents it from being melt-cast directly; it is typically formulated as pressed charges or suspended in binder matrices. The flexibility of such matrices is exploited in detonating cord, where PETN (typically at 5 to 40 g/m loading) is extruded inside a plastic sheath; the cord can be bent, looped, and cut to length in the field. The laboratory explosives analysis workflow, LC-MS, GC-MS, IC, XRF and SEM-EDX covers confirmatory identification of PETN residues in detail.

Semtex-A (manufactured by Explosia, Czech Republic) is primarily a PETN-based formulation (approximately 76% PETN, 5% RDX by total mass, with styrene-butadiene binder making up the remainder) in a styrene-butadiene binder. Unlike Semtex-H (RDX/PETN roughly equal), Semtex-A is slightly more malleable and was widely supplied to Middle Eastern and African groups through Libya in the 1970s and 1980s. The 1988 Lockerbie bombing (Pan Am 103) used Semtex-A in a device concealed in a Toshiba radio-cassette player; PETN and RDX residues were identified in the aircraft debris by HPLC and GC analysis by the Forensic Explosives Laboratory (then at RARDE, Fort Halstead). The analysis was a landmark in aviation forensic chemistry.

The 2009 Detroit underwear bomber (Umar Farouk Abdulmutallab) carried approximately 80 g of PETN sewn into his underwear. The device failed to detonate fully, producing a flash fire that injured the bomber but did not breach the fuselage. Post-incident analysis by the FBI Laboratory confirmed the PETN identification. Body-borne PETN is now a primary concern in aviation security; advanced imaging technology (AIT) at airports and ion mobility spectrometry (IMS) trace detection systems are specifically calibrated to PETN's vapour pressure and ion signature.

In forensic debris, PETN is detected by HPLC-UV, GC-ECD, and LC-MS/MS (negative-ion ESI: [M-H] at m/z 315; loss of NO2 produces fragments at m/z 269). PETN is notably non-polar and has very low aqueous solubility, which limits its migration from a blast seat in soil; this is both a detection advantage (it concentrates near the seat) and an analytical challenge (requires solvent extraction rather than aqueous rinse).

ANFO is a physical mixture of ammonium nitrate (NH4NO3, 94% by mass) prills and diesel fuel oil (6% by mass). The fuel oil sensitises the oxidiser by providing the carbon and hydrogen needed for an oxygen-balanced reaction:

3 NH4NO3 + fuel oil approximates 3 N2 + 7 H2O + CO2 + heat

The precise stoichiometry is adjusted based on the fuel oil grade and density; commercial ANFO blending at quarry sites typically uses calibrated proportioners. ANFO's detonation velocity in a 200 mm diameter borehole (confined geometry) is approximately 4,500 to 4,800 m/s, substantially below secondary explosives but sufficient for rock fragmentation at the scale of commercial mining.

The forensic significance of ANFO is inseparable from the 19 April 1995 Oklahoma City bombing, in which Timothy McVeigh and Terry Nichols loaded a Ryder rental truck with approximately 2,200 kg of ANFO sensitised with liquid nitromethane (a racing fuel that improves ANFO's oxygen balance and detonation reliability). The device was initiated by a two-stage fuse assembly attached to commercial blasting caps and boosters. The explosion destroyed the Alfred P. Murrah Federal Building, killing 168 people, and produced a crater approximately 8.5 metres wide and 2 metres deep in the parking lot. Post-blast analysis by the FBI Laboratory, the ATF, and the Institute for Research in Security Science (IRSS) identified ammonium nitrate residues, nitromethane traces, and physical evidence linking McVeigh to the procurement chain.

Commercially, ANFO is joined by emulsion explosives (water-in-oil emulsions with AN as the oxidiser phase, structured to hold oxidiser droplets in sub-millimetre suspension for close proximity to fuel) and heavy ANFO (mechanical blends of ANFO and emulsion). These formulations are produced by companies including Orica (Australia), Maxam (Spain), Dyno Nobel (US/Norway), and Solar Industries (India). Solar Industries is the largest Indian manufacturer; its products are licensed under the Explosives Rules 2008 and distributed under PESO oversight to mining operations in Rajasthan, Jharkhand, Chhattisgarh, and the northeast.

TATP (triacetone triperoxide; acetone peroxide; CAS 17088-37-8) is a cyclic organic peroxide formed by the acid-catalysed condensation of acetone and hydrogen peroxide. Its molecular formula is C9H18O6 (MW 222.24 g/mol). TATP is not a nitro compound and carries no nitro groups; its energetic release comes from the decomposition of peroxide (-O-O-) linkages, producing primarily acetone vapour and molecular oxygen with release of entropy rather than the nitrogen oxide gases that characterise nitramine and nitrate ester detonations.

TATP detonation velocity has been measured at approximately 5,300 m/s in the pure crystalline form, lower than military secondary explosives but sufficient for lethal device construction. Its primary hazard is formation: the reaction of acetone with hydrogen peroxide (typically 30% or 50% concentration) in the presence of an acid catalyst (sulfuric, nitric, or hydrochloric acid) produces TATP crystals that precipitate from the reaction mixture. The synthesis can be performed with commercially available precursors, making TATP accessible to device makers without supply-chain exposure. The synthesis is also highly dangerous: temperature excursions cause explosive decomposition or fire; the product is shock-sensitive, highly volatile (vapour pressure approximately 6.9 Pa at 25 degrees Celsius), and sublimes at room temperature, reducing its shelf life but also distributing its detectable vapour signature.

TATP achieved international forensic prominence through a series of major attacks:

• The 7 July 2005 London bombings used TATP as the initiating charge for main charges of hydrogen peroxide-based explosive. Metropolitan Police Service forensic analysis confirmed TATP in recovered device fragments.
• The 2016 Brussels bombings (Zaventem airport and Maelbeek metro station) used devices with TATP as an initiator for TATP-based main charges; Belgian Federal Police and IRCGN (French Gendarmerie forensic institute) joint analysis confirmed the formulation.
• The 2019 Sri Lanka Easter Sunday attacks (coordinated bombings at three hotels and three churches) used TATP-initiated devices. Sri Lanka Police's Special Investigations Unit and international forensic assistance (US FBI, Interpol) established the TATP connection within days.

Detection of TATP presents particular challenges. It contains no nitro groups; conventional ion mobility spectrometry calibrated for nitro-explosives may miss it. Dedicated TATP detection modes (using a different ionisation chemistry) are now standard on modern IMS instruments. In laboratory settings, TATP is detected by GC-MS (characteristic fragmentation with base peak at m/z 43, acetyl cation; molecular ion at m/z 222), LC-MS/MS (positive-ion mode, [M+NH4] at m/z 240), and by Raman spectroscopy (characteristic peak at 876 cm-1). In India, the National Investigation Agency (NIA) and CFSL laboratories use GC-MS and Raman spectroscopy as the primary identification methods for peroxide-based improvised explosives.

Urea nitrate (carbamide nitrate; CAS 124-47-0) is formed by the reaction of urea (readily available as a fertiliser) with nitric acid. The product is a white crystalline salt. It is not a nitro compound in the strict sense (the energetic group is the nitrate anion associated with the protonated urea cation) but decomposes exothermically on initiation. Detonation velocity in the confined form is approximately 3,400 to 4,700 m/s depending on density and confinement; it is substantially less sensitive than secondary military explosives and is classified as an improvised oxidiser-based explosive.

Urea nitrate achieved wide forensic recognition through the 26 February 1993 World Trade Center bombing, in which a vehicle bomb in the underground parking garage contained approximately 606 kg (1,336 lb) of urea nitrate (with fuel oil added as a sensitiser) as the main charge, with hydrogen gas cylinders included as a fragmentation hazard. ATF and FBI forensic analysis identified urea nitrate residues on recovered vehicle parts and debris. The investigation led to the convictions of the bombers under 18 USC 844 (federal explosives statute). In India, the urea fertiliser supply chain is regulated by the Department of Fertilisers under the Fertiliser Control Order, and its bulk procurement for non-agricultural purposes has been flagged in security directives following several IED recoveries.

HMTD (hexamethylene triperoxide diamine; CAS 283-66-9) is a second improvised organic peroxide, structurally related to TATP but incorporating nitrogen in the ring system. Molecular formula C6H12N2O6 (MW 208.17 g/mol). HMTD is synthesised from hexamethylenetetramine (urotropine, sold as a fuel tablet under the trade name Esbit) and hydrogen peroxide with an acid catalyst. It is more sensitive than TATP to friction and impact, making it particularly hazardous to manufacture and handle. HMTD has been identified in devices in the UK, Europe, and the United States, often as an initiating charge where TATP was less accessible. The UK Forensic Explosive Laboratory has published validated LC-MS/MS methods for HMTD detection in post-blast debris. The precursor control frameworks and synthesis route analysis for TATP, HMTD, and urea nitrate are developed further in the topic on homemade explosives: TATP, HMTD, urea nitrate and the precursor control response.

1. Scene safety and explosive risk assessment
   Before any evidence collection, the scene is assessed by an explosives ordnance disposal (EOD) team. Residual intact primary explosives or improvised peroxides (TATP, HMTD) present detonation risk. Collection begins only after EOD clearance.
2. Swab and debris collection
   Dry and wet swabbing of surfaces near the seat of explosion. Swabs are stored in sealed glass vials. Bulk debris (soil, concrete, metal fragments) is collected in unlined metal cans or glass jars. No plastic for TATP-suspected scenes.
3. Field screening
   Ion mobility spectrometry (IMS) for nitro-explosive markers; dedicated TATP IMS mode for peroxide explosives; colorimetric field kits (Expray, MISTRAL) for broad-spectrum triage. Field results are not court-admissible but guide laboratory priority.
4. Solvent extraction
   TNT, RDX, PETN, HMX: acetonitrile or acetone extraction. TATP: refrigerated acetone extraction, immediate analysis to reduce sublimation loss. Urea nitrate: aqueous extraction plus acidified acetonitrile. HMTD: methanol extraction.
5. Laboratory confirmation
   HPLC-UV or GC-MS for nitro-compound identification. LC-MS/MS (negative-ion ESI) for confirmatory identification of TNT, RDX, PETN, HMX. GC-MS positive-ion for TATP. Ion chromatography for nitrate-based improvised explosives.
6. Comparative profiling and source attribution
   Impurity profiles (DNT isomers for TNT batches; RDX/HMX ratio for military formulations; prill morphology for AN) are compared with reference libraries at Dstl (UK), FBI Lab (US), and ENFSI member laboratories (EU) to attribute commercial batch or manufacturing source.