Conventional High Explosives: TNT, RDX, PETN, HMX, Composition B, C-4 and Semtex

2,4,6-trinitrotoluene is synthesised by the stepwise nitration of toluene with mixed acids (nitric acid and sulphuric acid). The three nitro groups are added sequentially: mononitrotoluene, then dinitrotoluene, then trinitrotoluene. Industrial production generates the 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) isomers as persistent by-products and manufacturing impurities. These DNT isomers are the primary post-blast markers for TNT identification because they survive the detonation partially unreacted and are present in concentrations detectable by GC-ECD.

The melting point of 80.35°C is the physical property that made TNT indispensable in military manufacturing. Below the melting point it is a stable yellow crystalline solid. Above it, it melts into a liquid that can be poured into shells, bomb casings, and molds, allowed to solidify, and machined to precise dimensions. The melt-casting process (termed melt-pour in US Army Materiel Command and UK Defence Equipment and Support terminology) was used to fill artillery shells at industrial scale from World War I through the Cold War era and continues in some modern munitions.

TNT production peaked during World War II. The British Ministry of Supply operated filling factories at Chorley (Royal Ordnance Factory Chorley, Lancashire), Risley, and elsewhere. The US Army operated the Tennessee Ordnance Works and the Radford Army Ammunition Plant. India established the Ordnance Factory Itarsi in Madhya Pradesh (part of the Ordnance Factories Board, now absorbed into the Munitions India Limited) for TNT production. Germany's IG Farben and subsidiary explosives producers manufactured TNT at the Leverkusen and Bitterfeld plants. By 1945, global production had exceeded several million tonnes.

Post-blast TNT residue chemistry centres on the DNT impurities and on 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene (the amine decomposition products of partial reduction), which together with the parent TNT form a characteristic analytical cluster detectable by GC-ECD or LC-MS/MS. The IR absorption spectrum of TNT has characteristic peaks at 1530 cm-1 (asymmetric NO2 stretching) and 1340 cm-1 (symmetric NO2 stretching), alongside aromatic ring deformation bands at 825 cm-1 and 740 cm-1. Raman spectroscopy shows a dominant peak at 1357 cm-1 (symmetric NO2 stretch) useful for standoff detection with portable Raman instruments.

RDX (cyclotrimethylenetrinitramine, also called hexogen in European nomenclature and T4 in older British military terminology) was first synthesised by the German chemist Georg Friedrich Henning in 1898. Its explosive properties were recognised separately in multiple countries between 1920 and 1940. The name Research Department Explosive derives from its development in the UK's Royal Arsenal at Woolwich during the interwar period. Large-scale Allied production during World War II was coordinated through the Anglo-American Technical Cooperation on Explosives.

The molecular structure of RDX is a six-membered ring alternating carbon and nitrogen atoms (trimethylene), with three N-nitro groups on the ring nitrogens, formula C3H6N6O6. Molecular weight: 222.12. Melting point: 204°C. Detonation velocity: 8,750 m/s at crystal density 1.82 g/cm3. Because RDX melts above 200°C (compared to TNT at 80.35°C), it cannot be melt-cast alone at moderate temperatures. The standard solution is to blend RDX with TNT in Composition B (60% RDX, 40% TNT by mass), which melts at TNT's melting point and pours into shells carrying the combined explosive power of both components.

HMX (cyclotetramethylenetetranitramine, octogen) is the eight-membered ring homologue of RDX. Formula C4H8N8O8. Molecular weight: 296.16. Melting point: 276-280°C (alpha polymorph). Detonation velocity: 9,100 m/s. HMX was first synthesised by the US Army as an unwanted by-product of Bachmann's process for RDX manufacture and was later developed as an explosive in its own right when its superior performance was recognised. HMX co-occurs with RDX in many military formulations and is detected alongside RDX in post-blast residue as a diagnostic for military-grade explosives (as opposed to commercially manufactured RDX of lower purity).

Plastic explosives are formulations of a secondary high explosive (typically RDX or PETN or both) with a polymeric binder and plasticisers that produce a workable, mouldable material at room temperature. The high explosive content is typically 85-95 per cent by mass. The binder holds the explosive together mechanically without sensitising it, and the plasticiser modifies the rheological properties to allow hand-shaping.

C-4 (Composition C-4) is the US military standard plastic explosive, standardised under MIL-C-11092C. Composition by mass: approximately 91% RDX, 5.3% di(2-ethylhexyl)sebacate (plasticiser), 2.1% polyisobutylene (binder), 1.6% mineral oil. Density: 1.601 g/cm3. Detonation velocity: 8,050 m/s. C-4 is white to off-white and is odourless. It is used by US and NATO militaries in demolition charges, breaching charges, shaped charges, and improvised firing. US Army Engineer demolition M112 block: 1.25 lb (567 g). The analytical signature of C-4 is dominated by RDX with the specific ratio of plasticiser components detectable by GC-MS.

Semtex is a range of plastic explosives manufactured by Explosia a.s. at their Pardubice facility in the Czech Republic. Semtex H (the most widely deployed variant) contains approximately 41% PETN and 41% RDX in a styrene-butadiene rubber binder, with the balance being plasticisers and additives. Semtex is characterised by its red-orange colour (from oil-soluble dyes), its distinctive rubber odour, and its co-detection of both RDX and PETN at roughly equal mass fractions. Semtex A contains only PETN as the explosive ingredient. Semtex SE contains HMX. Post-blast Semtex identification typically shows both RDX and PETN peaks on GC-ECD with a characteristic ratio, confirmed by LC-MS/MS.

Between 1975 and 1981, Czechoslovakia supplied approximately 700 to 1,000 tonnes of Semtex to Libya via the state arms exporter Omnipol, after which sales were halted. Libya in turn supplied quantities of the explosive to various militant groups including the Irish Republican Army and Palestinian factions. The proliferation of Semtex was facilitated by the lack of any detection mechanism: the material had no vapour signature detectable by existing field instruments and no chemical marker distinguishing it from other plastic explosives.

The 1988 Lockerbie bombing (Pan Am Flight 103, 21 December, 270 deaths, attributed to a Libyan intelligence operation placing a Semtex H IED in a Samsonite suitcase on a feeder flight from Malta to Frankfurt) and the 1989 UTA Flight 772 bombing over Niger (171 deaths, also attributed to Libyan intelligence) catalysed international action. The Convention on the Marking of Plastic Explosives for the Purpose of Detection (Montreal Protocol on Plastic Explosives), adopted by ICAO in March 1991 and entering into force in 1998, requires that all plastic explosives manufactured or held after entry into force contain one of four approved detection agents (taggants) at specified minimum concentrations.

The approved taggant for the vast majority of commercial and military plastic explosives is DMDNB (2,3-dimethyl-2,3-dinitrobutane). The ICAO-specified minimum concentration is 0.1 per cent by mass. DMDNB has a high vapour pressure (approximately 2.7 ppm at 20°C) relative to RDX and PETN, and IMS instruments are calibrated to detect DMDNB at concentrations consistent with plastic explosives containing the taggant at the specified level. Other approved taggants include ethylene glycol dinitrate (EGDN) at 0.2%, para-mononitrotoluene (p-MNT) at 0.5%, and ortho-mononitrotoluene (o-MNT) at 0.5%.

In post-blast analysis, the presence of DMDNB alongside RDX and/or PETN confirms that the explosive was a post-1998 manufactured tagganted plastic explosive, as opposed to legacy pre-taggant Semtex or an improvised explosive formulation such as TATP or urea nitrate. The absence of DMDNB in a post-blast sample containing RDX and PETN is analytically significant and is reported.

PETN (pentaerythritol tetranitrate) is a nitrate ester explosive synthesised by the nitration of pentaerythritol with fuming nitric acid. Formula C5H8N4O12. Molecular weight: 316.14. Melting point: 141.3°C. Detonation velocity: 8,400 m/s at crystal density 1.77 g/cm3. Oxygen balance: -10.1 per cent, the closest to zero of the four main secondary explosives, indicating relatively efficient detonation chemistry.

PETN's primary commercial application is in detonating cord (det cord, PETN cord, or Primacord), a flexible tube containing PETN at a standard loading of 3.6 g/m (for #2 cord), 7.2 g/m (#4 cord), or higher loadings for specialist applications. Det cord propagates detonation at approximately 6,400 metres per second through the PETN core and is used to simultaneously initiate multiple charges in mining, demolition, and quarrying operations. Global det cord producers include Orica (Australia), Dyno Nobel (US-Norway), and Indian Explosives Limited (a subsidiary of Orica operating in India under joint venture arrangements).

Flexible sheet explosives (Detasheet, manufactured by Ensign-Bickford in the US; Primasheet, a broadly similar product) contain PETN as the active ingredient in a flexible polymer binder. They are used in metal-cutting shaped charges, aircraft cockpit canopy-cutting systems, and explosive welding (cladding) in the metals industry. The forensic significance is that flexible PETN sheet explosives are physically and analytically indistinguishable from improvised PETN-based flat charges used in vehicle-borne IEDs and person-borne suicide vests.

The Richard Reid case (December 22, 2001, American Airlines Flight 63, Paris Charles de Gaulle to Miami International) involved Reid attempting to detonate PETN concealed in the hollowed-out soles of his shoes. A passenger observed Reid lighting a match near his shoe, and crew and passengers restrained him. US and UK laboratory analysis confirmed PETN at approximately 185 grams per shoe. Had the PETN detonated at altitude using the TATP initiator Reid also carried, the charge was assessed by FBI laboratory analysis to be sufficient to breach the aircraft fuselage. Reid was convicted of eight counts including attempted use of a weapon of mass destruction.

The Umar Farouk Abdulmutallab case (December 25, 2009, Northwest Airlines Flight 253, Amsterdam Schiphol to Detroit Metropolitan) involved approximately 80 grams of PETN concealed in the crotch area of modified underwear, formulated as a gel mixture with TATP as the initiator. PETN was identified by laboratory analysis at the FBI Laboratory and by the UK's DSTL. The device failed to fully detonate, causing burns to Abdulmutallab and to nearby seats. Abdulmutallab was convicted in 2011 and sentenced to life imprisonment.

Post-blast PETN residue is analysed by LC-MS/MS in negative-ion electrospray ionisation (ESI-) mode. PETN decomposes to erythritol tetranitrate, pentaerythritol trinitrate, pentaerythritol dinitrate, and pentaerythritol as detectable products. The parent PETN [M - H]- ion at m/z 315 and the fragment ions at m/z 241 (loss of NO2 and O), 212, and 101 constitute the diagnostic LC-MS/MS signature. GC-ECD is also used for PETN; electron capture detection achieves sub-picogram sensitivity for the nitrate ester functional group.

Infrared spectroscopy provides compound-class identification and molecular fingerprinting. All four secondary explosives share the NO2 stretching absorptions (asymmetric and symmetric), but at different wavenumbers that allow discrimination.

TNT: characteristic absorptions at 1530 cm-1 (asymmetric NO2 stretch), 1340 cm-1 (symmetric NO2 stretch), 825 cm-1 and 740 cm-1 (aromatic C-H out-of-plane bending). The aromatic ring absorptions distinguish TNT from aliphatic nitro compounds.

RDX: characteristic absorptions at 1573 cm-1 and 1268 cm-1 (NN and NC stretching coupled with NO2 asymmetric stretch), 1027 cm-1 (ring breathing), 795 cm-1. The 1268 cm-1 band is highly specific for the RDX cyclic nitramine structure and is diagnostic against TNT.

PETN: characteristic absorptions at 1640 cm-1 (NO2 asymmetric stretch, shifted from typical nitroaromatic position due to the ester oxygen), 1280 cm-1 (C-O-N stretch), 840 cm-1. The ester C-O-N stretch at 1280 cm-1 is diagnostic.

HMX: very similar to RDX with absorptions shifted slightly due to the larger ring. The alpha-polymorph has absorptions at 1544, 1253, and 916 cm-1.

Raman spectroscopy is particularly useful for standoff and non-destructive identification. TNT has a dominant Raman peak at 1357 cm-1, with secondary peaks at 1537 cm-1 and 826 cm-1. RDX Raman: dominant peaks at 877, 940, 1268, 1315 cm-1. PETN Raman: dominant peaks at 872, 1290, 1656 cm-1. These Raman signatures are used in handheld instruments (Smiths Detection HazMatID, Agilent TRS100, DeltaNu Insight) for non-contact identification of suspected explosive residues in the field.

GC-ECD (gas chromatography with electron capture detection) is the workhorse laboratory confirmation method for nitroaromatic and nitramine explosives. Electron capture detection is exquisitely sensitive to halogenated and nitro compounds due to their high electron affinity; detection limits for TNT, RDX, and PETN are in the range of 1 to 10 picograms injected on-column. A DB-17 or Rtx-TNT2 column with a temperature program ramping from 50°C to 280°C separates TNT (14-16 min), 2,4-DNT (12 min), RDX (17-19 min), HMX (20-22 min), and PETN (16-18 min) under typical analytical conditions. Retention time matching against certified reference standards is required for identification.

LC-MS/MS in negative-ion ESI mode is the confirmatory method for all four explosives and is mandatory for PETN (which has lower thermal stability and can decompose on the GC injector port) and for samples suspected of containing both conventional and peroxide explosives. The Waters Xevo TQ-S and Sciex 6500+ triple-quadrupole instruments are widely deployed in forensic explosives laboratories for this application. MRM (multiple reaction monitoring) transitions are instrument-tuned to each analyte:

TNT: [M-H]- at m/z 226, transitions to 196 (loss of NO2), 180, 150. RDX: [M+NO3]- at m/z 285 or [M-H]- at m/z 221, transitions to 120, 46. PETN: [M-H]- at m/z 315, transitions to 241, 212, 101. HMX: [M+NO3]- at m/z 358 or [M-H]- at m/z 295, transitions to 120, 46.

The Lockerbie investigation (1988-2001) involved the analysis of approximately 4 million pieces of debris recovered from the Scottish countryside. The Scottish Borders Forensic Science Service (now part of the Scottish Police Authority's forensic chemistry capability) and the US Federal Bureau of Investigation (FBI) Laboratory's Explosives Unit at Quantico, Virginia, worked in parallel. The critical evidence chain: a fragment of the Toshiba RT-SF16 cassette recorder circuit board (item PT/35(b)) identified from the IED timer circuit; Semtex H identified from residues on a fragment of the Samsonite luggage; and a fragment of shirt collar traced through the retailer's records to Malta. The Semtex H identification rested on the co-detection of RDX and PETN at characteristic ratios by GC and the absence of DMDNB (the device was assembled with pre-taggant Semtex from Libyan stockpiles).

The serial bombings of 12 March 1993 in Bombay (now Mumbai), India, killed 257 people and injured more than 1,400. Twelve coordinated bomb blasts targeted the Bombay Stock Exchange, the Air India Building, hotels, and market areas. Indian forensic investigators at the Central Forensic Science Laboratory and the Directorate of Forensic Science, Maharashtra, identified RDX as the main charge in multiple blast seats. The case established, in the Indian legal framework under the Terrorist and Disruptive Activities (Prevention) Act (TADA) and subsequently under POTA and then the Unlawful Activities (Prevention) Act (UAPA), the forensic chemistry evidentiary standard for high-explosive identification that continues to govern Indian prosecutions.

The 7 July 2005 London Transport bombings involved four simultaneous IEDs: three on London Underground trains at Aldgate, King's Cross/Russell Square, and Edgware Road, and one on a Number 30 bus at Tavistock Square. The main charges used TATP as the explosive (not a conventional secondary HE), but the post-blast investigation confirmed that the device design knowledge required expertise in detonation chemistry. The DSTL Porton Down Forensic Explosives Laboratory, working with the Metropolitan Police Counter-Terrorism Command, produced expert evidence at subsequent inquests and prosecution proceedings.

The forensic chain from a fragment to a charge in conventional HE cases requires: (1) chemical identification of the explosive class and specific compound by GC-ECD or LC-MS/MS; (2) quantification of taggant (DMDNB) presence or absence; (3) impurity profiling (DNT isomers for TNT, trace HMX in military-grade RDX) to indicate manufacturing source; (4) physical comparison of any unfired explosive remaining against seized reference samples; and (5) database comparison against the ATF's United States Bomb Data Center (USBDC) and the UK Home Office explosives database.

1. Scene recovery of residue
   Swab blast seat and adjacent surfaces with cotton or nylon swabs moistened with acetonitrile. Collect solid debris fragments in separate evidence bags. Maintain chain of custody with unique exhibit numbers per swab location.
2. Solvent extraction
   Elute swabs with acetonitrile. Filter extract through 0.22 micron PTFE membrane filter. Prepare aliquots for GC-ECD (concentrated, 100-200 microlitre injection) and LC-MS/MS (diluted to prevent ion suppression, 5-10 microlitre injection).
3. GC-ECD screening
   Inject on DB-17 or Rtx-TNT2 column. 63Ni electron capture detector. Identify nitroaromatic (TNT, DNT) and nitramine (RDX, HMX, PETN) peaks by retention time match against certified reference standards. Calculate response factors and report concentration in ng/cm2 or ng/swab.
4. LC-MS/MS confirmation
   Inject on C18 reversed-phase column with 0.1% ammonium acetate/methanol gradient. Negative-ion ESI. Acquire MRM transitions for each analyte. Confirm identity by retention time, two MRM transitions, and ion ratio within 20% of reference standard ratio.
5. Taggant analysis (DMDNB)
   Analyse aliquot on GC-MS or GC-ECD under DMDNB-optimised conditions. Report presence or absence of DMDNB. If present, confirm plastic explosive is post-ICAO-convention tagganted material. If absent, flag for significance in report.
6. Reporting and expert testimony
   Report explosive identification with analytical method, instrument, column, MRM transitions, calibration curve range, LOD, and LOQ. State the explosive class, specific compound, and any formulation inference (e.g. Semtex H from co-detected RDX+PETN at characteristic ratio). Prepare for cross-examination on alternative sources of the analyte.