The challenge of detecting explosives

  • Despite the fear that terrorists could launch an attack using biological or chemical weapons, the most realistic threat is still posed by explosives.
  • Terrorists have access to a range of explosives that can either be homemade using chemicals such as ammonium nitrate, or bought through the black market, such as military-grade Semtex.
  • Modern explosive detection systems can detect and determine the nature of a chemical compound within seconds, but a universal detection system for all kinds of explosives has yet to be developed.

Despite the possibility that terrorists could use nuclear, radiological, biological or chemical weapons, by far the greatest terrorist threat remains the continued use of improvised explosive devices (IEDs) comprising conventional explosives.

The 7 July 2005 London bombings, the 11 March 2004 Madrid attacks, the almost daily insurgent bombings in Iraq, and 30 years of Provisional Irish Republican Army (PIRA) bombing campaigns are testimony to the ongoing reliability, availability and relative ease of deploying IEDs.

Much recent attention has been on the cheapness, availability and sheer variety of chemicals and components that can be used for relatively crude devices made from homemade explosives (HME), the low-level expertise required to fabricate them, and the difficulties in detecting them in transit.

Equipment and training to detect many more kinds of explosives used by terrorists is urgently needed. On 16 August, EU interior ministers meeting in London agreed to spend EUR350,000 (USD444,000) to implement a programme of research into improvised explosives, with the emphasis on liquid explosives and developing effective detection technologies for airports and other vulnerable locations.

From the PIRA to Al-Qaeda

During the 1970s, PIRA detonated thousands of IEDs in Northern Ireland and mainland Britain. A variety of explosives were used in devices ranging from small letter bombs and crude incendiary devices to car bombs and large truck bombs (vehicle-borne IEDs). The campaign targeted military barracks, shops, bars, restaurants, public buildings, railway stations and other such places. In 1971 alone, over 1,000 PIRA IEDs were deployed and detonated on British territory.

Documents found in Al-Qaeda hideouts in Afghanistan were used to train operatives in the making of explosive mixtures such as triacetone triperoxide (TATP), which was reportedly used by the 7 July London bombers. Al-Qaeda operatives and affiliates have used recipes for making a version of the US military explosive Composite-4 (C-4, or its European equivalent, Semtex), urea nitrate (used in the 1993 World Trade Center attack), and ammonium nitrate (fertiliser). These operatives have also researched the use and handling of nitromethane (a commercially available racing fuel), PETN high explosive, blasting cap detonators and shaped charges to destroy buildings.

Explosives chosen for attacks are often made from readily available materials either acquired locally or through complex smuggling operations. For example, the Madrid bombs were composed of ECO, a type of dynamite manufactured in Spain and normally used in construction and mining.

Ammonium nitrate

PIRA established the use of low-explosive mixtures based on inert ammonium nitrate and diesel or fuel oil (ANFO) and used them for several 'spectaculars', such as the Canary Wharf blast on 12 February 1996. One PIRA technique was to mix ammonium nitrate with nitrobenzene and aluminium powder. Timothy McVeigh, an unaffiliated right-wing extremist, used two tonnes of it - along with along with diesel fuel, nitromethane and the commercially manufactured explosives Tovex and Primadet - to blow up the Alfred P Murrah building in Oklahoma City in 1995.

Ammonium nitrate was also used in the 15 November 2003 Istanbul bombing in Turkey, the 8 November 2003 bombings in Saudi Arabia and the 17 October 2002 Bali attack (which used at least 1,000 kg of the compound), showing that it continues to be a favoured bulk explosive. In March 2004, Islamic separatists in south Thailand were reported to have stolen more than 1,300 kg of the explosive, along with 58 sticks of dynamite and 170 blasting caps, during an armed raid on a quarry. Potassium chlorate (which is used in fireworks) was also seized from a house in East Java in mid-December 2004.

With only around 40 per cent of the explosive power of TNT, bombs made from ammonium nitrate tend to be very large - often comprising several sack loads for bombs of up to 2,000 kg. Consequently, terrorists make the most destructive use of it in truck bombs used to blow up urban targets. The explosive mix must be milled to a particular consistency for its explosive power to be realised, which is a process that can take days and heightens the risk of detection. While this compound is extremely insensitive, it is also extremely hygroscopic (absorbs moisture readily) and must be sealed in airtight containers to remain viable.

Ammonium nitrate currently costs about GBP130 (USD245) a tonne. Almost two million tonnes are used by British farmers every year and mining companies use it to create explosives. Although available in many countries, it has long been banned in the Irish Republic and Northern Ireland, where the farmland border areas, particularly South Armagh, were obvious havens for its use. It is tightly restricted in the EU, where a version is also made that does not combine well with diesel oil. It can be treated to make it less usable in bombs, but terrorists may be able to reverse the process or concoct a homemade version.


Semtex is based on PETN-RDX (Penaerythrite tetranitrate - Research Development Explosive), a plastic explosive formerly made by Explosia in the Czech Republic. It is a favourite explosive of terrorist groups, particularly PIRA, as only small amounts are needed for horrendous results. Semtex is highly stable and malleable, is usable over a large temperature range, has a long shelf-life of around 20 years and - most important for terrorist operations - is virtually undetectable.

Although Al-Qaeda used over 270 kg of C-4 to bomb the USS Cole in 12 October 2000, along with Semtex it has been used mainly as a trigger or booster for ammonium nitrate IEDs, since ammonium nitrate will support and promote combustion initiated in another material. There are many examples of this, such as the April 1992 PIRA bombing of the Baltic Exchange, which used a combination of Semtex and around a tonne of ammonium nitrate, or the 24 April 1993 Bishopsgate bombing in London, which had the explosive equivalent of 1,200 kg of TNT.

While new batches of Semtex have been manufactured with a detectable odour and a three-year shelf life, most terrorist or other illegal stocks will be original minimal-odour Semtex. Libya has been a main source in the past, which received a shipment of almost 700 tonnes in the 1980s. A further 1,000 tonnes was exported to Syria, North Korea, Iraq and Iran. However, Semtex is not cheap. Since it is highly restricted to licensed users, on the global black market it sells for as much as USD1,300 per kg.

Since the 1988 Pan Am flight 103 bombing, the US and other Western nations established a practice of tagging C-4 made in designated factories with selected chemical marking agents to facilitate their detection. This does not, however, solve the problem of detecting illicitly acquired Semtex and other plastic explosives.

Triacetone triperoxide

One of the first homemade explosives was PIRA's 'Co-op mix', a blend of sodium chlorate and nitrobenzene, used in the 1930s bombing campaign. More than 60 years later, the 7 July London bombings brought to light the particular risk of common household substances being used to make small explosive devices. In those incidents, which killed 52 people, security sources believe the terrorists used triacetone triperoxide (TATP), a blend of hydrogen peroxide, acetone, and an acid. Such substances along with the items needed for mixing and storing can be purchased from hardware stores, hairdressers, or stolen from school laboratories. Shoe bomber Richard Reid used it in his failed attack, as well as Palestinian suicide bombers in Israel.

Once made into crystals, TATP is highly explosive and can be used as an initiator. Its explosive force comes from a rapid release of gas rather than a burst of energy. It is highly unstable, being extraordinarily sensitive to friction, and must be kept cool and dry before it can be used, which must be within around seven days.

Liquid explosives

A liquid form of TATP (which can also be plasticised), nitrate and nitro containing compounds such as nitroglycerin fell under suspicion, as they could all be manufactured at home combined with fertiliser as a raw material and disguised as a drink fluid. Industrial solvents and fuel nitromethane are also fairly easy to obtain as they are used as fuel for model aeroplanes and racing cars. For a workable explosive, nitromethane would be combined with oxidiser such as ammonium nitrate, but these mixtures would be odour-detectable.

Detection measures

Large vehicle bombs require detection equipment with a stand-off ability capable of identifying explosives in larger volumes at greater distances. This includes investigating unique physical and chemical phenomena that identify the presence of explosives. Current stand-off techniques under development are limited with respect to the distance and type of explosives that can be detected.

For airport security, much effort has been focused on direct detection of explosive materials in the carry-on and checked luggage of air passengers, but techniques are needed to detect and identify residual traces on the body and clothing that indicate a passenger's recent contact with the many chemicals used in explosives.

One approach is to direct passengers through a detection portal that will collect, analyse and identify explosive residues on the body or clothing. The passenger's own body heat may volatilise traces of explosive material for detection as a vapour, or the portal may emit puffs of air that can dislodge small particles as an aerosol. A handheld vacuum wand could also be used to collect a sample for chemical analysis or a passenger's boarding pass could be screened for residues transferred from their hands.

Sniffer dogs are still the best detectors for finding explosives at airports or during searches, and detection research is focusing on technologies that support and enhance canine detection of explosives. But dogs have to be trained, can generally only work for brief periods, have significant upkeep costs, are unable to communicate the identity of the detected explosives residue, may cause problems with passenger contact and require a human handler.

The effectiveness of chemical trace analysis depends on sample collection and analysis and comparison of results with known standards. When trace analysis is used for passenger screening, non-intrusive or minimally intrusive sample collection followed by speedy sample analysis and identification at low cost is the aim for successful countermeasures.

Improving detection technologies

Sensor systems used to identify chemicals used in explosives and CBRN materials need a number of subsystems, including sample collection and processing, presenting chemicals to the sensor, and sensor arrays that recognise and distinguish different chemical profiles or 'signatures'.

Chemical and electronic trace detection techniques include mass spectrometry, gas chromatography, chemical luminescence, or ion mobility spectrometry (IMS), to measure the chemical properties of vapour or particulate matter collected from passengers or their carry-on luggage.

Instruments for use in airports and other security-screening locations can be programmed with a set of signatures from known hazardous and illegal compounds. When a sample matches any of these signatures, the person or object can be re-screened to ensure that, for example, cardiac patients who take nitroglycerin and construction workers who handle explosives are not mistakenly identified as terrorist suspects.

IMS is a promising new technology that can separate and detect electrically charged particles (ions) species at atmosphere pressure that have been sorted according to how fast they travel through an electrical field in a tube. As small ions travel very fast they reach the detector first, with successively larger ions following behind.

The screening process involves wiping an absorbent swab over a passenger's clothing or luggage, then inserting the swab into a small heated chamber, where traces of organic compounds picked up by the swab evaporate and mix with a carrier gas that is swept into the main part of the instrument. Inside the instrument, a radioactive isotope emits high-energy electrons, which collide with the sample molecules and the carrier gas to form ions.

Because IMS only sorts molecules by size, and not by chemical properties or other identifying features, it may not be relied upon to positively identify unknown compounds. However, the technique can make a measurement of many typical explosives chemicals in only a few seconds, as compared with several minutes to over an hour for more conventional techniques such as chromatography and mass spectrometry.

Until a universal solution can be achieved, IMS is most often used in currently deployed equipment. New systems coming on stream will integrate the IMS trace technology with advanced digital imaging, data fusion and communication systems into the automatic fare collection (AFC) equipment commonly used in today's public transit systems.

Research into enhanced mass spectrometry techniques, which identify chemicals in a substance by their mass and charge, has produced the means to analyse samples directly from the environment rather than requiring lengthy pre-treatment in the laboratory. A prototype device developed by chemists at Purdue University in the US detects nanogramme-sized samples, but with recent improvements the device has proven successful at detecting at levels of trillionths of a gramme in lab tests, about 1,000 times less material than previously required.

To enhance this technique, the California Institute of Technology has developed desorption electrospray ionisation (DESI), which involves directing a spray of reactive chemicals onto a surface to dislodge suspicious chemicals and then sucking the mixture into a spectrometer for analysis.

This is enabling the detection of the main substances used in explosives very quickly, and at small enough quantities to detect trace amounts of explosives or residue. It may also overcome the problem of the presence of other compounds that may interfere with analysis.

Although mass spectrometers can deliver highly accurate and reliable analyses of substances, the lab equipment is unwieldy and unsuitable for airport use, and is not always reliable for detecting chemicals incorporated into explosives. The DESI spectroscopy equipment will be lightweight, weighing around 10 kg.

To intercept seaborne shipments containing explosives and CBRN materials, a new ultrahigh-speed gas chromatography chemical profiling system has been developed for port screening. It directly measures odour concentration and intensity with an integrated solid-state gas chromatography sensor, which can identify the chemical species in the vapours inside cargo containers and determines within 10 seconds their concentrations with picogramme sensitivity.

Andy Oppenheimer is Editor of Jane's Nuclear, Biological and Chemical Defence

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