26 July 2005: 'Pen' detects peroxide-based explosives

Researchers have developed a pocket-sized device for detecting sub-milligram quantities of peroxide-based explosives such as those reportedly used in the recent bomb attacks in London.

’We’ve prototyped and tested the peroxide explosives detector (PET) in our laboratories, as well as in field experiments, and it works,’ said PET’s patent holder, Ehud Keinan of Israel’s Haifa Technion. ’Now it’s ready for commercialisation and use by all law enforcement agencies and anyone dealing with security.’ 

There is strong interest from some of the world’s top security organisations, Keinan told Chemistry World, although London’s Metropolitan Police counter terrorism unit declined to comment. The new device is the size of a large fountain pen and costs less than ?15 per unit. 

The PET ’pen’ shows a strong colour change when any peroxide-based explosive is detected. Suspect material is collected or swiped with a silicone-rubber test pad and inserted into the pen. Three test chemicals are then sequentially injected into the transparent chamber: a suitable organic solvent; followed by an aqueous solution of strong acid, which decomposes any putative explosive and releases hydrogen peroxide; and finally a mixture of a dye and a peroxidase enzyme. 

If a peroxide-based explosive is present in the original sample, the solution turns a deep blue-green in about three seconds. Sub-milligram quantities of an explosive can be detected by this pronounced colour change. 

’The simplicity of the chemistry...is beautiful,’ said Andrea Sella, of University College London. He warns, though, that it is probably not suited to the high throughput screening needed by airport security.

One reason the second wave of attacks might have failed is the physical instability of peroxide-based explosives, such as triacetone peroxide (TATP), during long-term storage. ’When TATP dries out, it can sublime’, said Sella. ’But if it is stored in an airtight box, like Tupperware for example, TATP recrystallises and, as a result, its mean particle size changes radically. This can lead to intimate contact between the detonator and the explosive being disrupted, and probably explains why all the second wave of London bombs failed to detonate.’

Recent reports suggest that hexamethylene triperoxide diamine (HMDT), rather than TATP, was the explosive responsible for the attacks in London on 7 July. It is not yet known if the same explosive was used in the failed attacks on 21 July.

Conventional scanning techniques rely on the nitrogen content of many explosives. But it would be difficult to detect HMDT by conventional techniques, says PET patent holder Keinan, despite the presence of nitrogen atoms.

’These are tertiary amines, which are very different from the nitrogen atoms in conventional explosives, where they appear in the form of nitro groups,’ said Keinan. ’H MTD is almost as difficult to detect as TATP by conventional methods. Our PET responds to both TATP and HMTD equally well.’ Lionel Milgrom