How It Works: The Mechanical Bomb Sniffer

Mechanical Sniffer

Trevor Johnston

A guide to the machine that could change how we screen for explosive threats

Last fall, David Atkinson and a team at the Pacific Northwest National Laboratory announced that they had “sniffed” certain explosives in ambient air for the first time. Most common explosives-RDX, PETN, blasting gels-exist in very low ambient concentrations, often in the parts-per-quadrillion range. Existing detectors are not that sensitive, meaning security forces need to test suspects directly, as in airports. A version of Atkinson’s machine could simply sniff targets, speeding the process. “It could change the way we do screening for explosive threats,” he says.

Scientists spike a glass slide with residue from a few known explosives. A vacuum pump within the detector sucks air through a one-inch wide opening at a rate of between one and five liters per minute.

The vapor passes through a copper tube toward an ionization source. Nitrate ions, which have a high charge affinity, collide with the highly polar explosive molecules, so they tend to stick together, forming an adduct-or cluster molecule.

To ensure that many of the explosive molecules in the airstream are ionized, Atkinson’s team used a long, copper reaction tube to extend the reaction period to approximately two seconds.

Electric fields on the front of the commercial mass spectrometer guide the charged ions through a 600-micron-wide passage into the mass spectrometer itself.

The mass spectrometer analyzes the sample and determines the molecular weight. Currently, Atkinson’s team can detect at least nine explosives, including PETN, RDX, C-4, Semtex, smokeless powder, and some blasting gels.


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