When it comes to finding hidden explosives, the self-propelled detection system known as a sniffer dog has no equal. But sniffer dogs have to be trained, and that is a delicate process. In particular, the trace levels of explosive vapour involved are so low (because dogs’ noses are so sensitive) that accidental contamination of supposedly residue-free “control” samples is a serious possibility. That confuses the animal and slows down its training. Things would therefore go more smoothly if a trainer could find out instantly whether a sample had indeed been compromised by traces of explosive, so that he could tell whether a dog’s reaction to a supposed blank was justified.
This is no theoretical risk. When Ta-Hsuan Ong of the Massachusetts Institute of Technology monitored one such training session he found that six out of 68 controls were contaminated—and that one out of 28 supposedly “live” samples had no explosive residue. Dr Ong thinks, however, that he has a solution to the problem. The monitoring device he and his colleagues built for their experiment, the details of which they have just reported in Analytical Chemistry, lets handlers check instantly whether an apparent mistake by a dog is a real one. That will both speed the process of training and, if deployed in the field, permit a suspicious object which a dog has nosed out to be double-checked by technology, and to have its precise explosive characteristics logged instantly.
Dr Ong’s invention is a type of mass spectrometer—a device that, by measuring the flight time of ions (electrically charged molecules) in an electric field, is able to work out their mass and therefore their probable composition. A conventional mass spectrometer depends on samples being collected, concentrated and deliberately fed into it, but that would not be a practical way to provide the instant feedback needed for dog training. Dr Ong required something that could pick up chemical compounds directly from the air around a suspicious object.
Sniffers that can detect explosive vapours do, of course, exist. They are used in places like airports. But they are not as sensitive as dogs’ noses. Instead, Dr Ong turned to what is known as electrospray ionisation technology (EIT). Commercial EIT works by applying a high voltage to a reservoir of solvent containing compounds of interest. This both sprays the liquid into the air, creating an aerosol, and ionises the molecules within. An adjacent spectrometer then sucks in the aerosol and conducts an analysis. Dr Ong and his team adapted that approach by using pure solvent and aiming the charged aerosol droplets directly at an air stream being pulled into a spectrometer. This ionises the compounds within the air stream, including any derived from explosives, before they enter the device.
Source: The Economist