01 January 2005
TNT and nitro groups
A team of University of Florida researchers has invented a way to rapidly detect traces of TNT or other hidden explosives simply by shining a light on any potentially contaminated object, from a speck of dust in the air to the surface of a suitcase.
"We have to find explosives quickly, inexpensively, and particularly, reliably," said Rolf Hummel, a UF professor emeritus of materials science and engineering who heads the lab.
The development provides instantaneous results, gives no false positives, works remotely, and is portable—attributes he says will make it indispensable at all levels of law enforcement, from local police to homeland security.
The method uses photoluminescence spectroscopy, a technique that casts light on a material and measures the range and intensity of the wavelengths of light the material produces in response. The wavelength of the emitted light varies depending on the chemical structure of the material.
Using photoluminescence to reveal the presence of TNT is similar to how "black light" uses ultraviolet radiation to make white clothes glow, but in this case, the black light is a laser, Hummel said. "Once you shine a laser at the sample, the material then re-emits (it) at specific wavelengths that are different for each material—it's a kind of a fingerprint."
TNT's fingerprint is a sharp, distinct photo luminescent peak at a specific wavelength within the electromagnetic spectrum, the researchers discovered. The electromagnetic spectrum encompasses the entire range of electromagnetic waves, from long-wavelength radio waves to visible light to short-wavelength gamma rays. The peak occurs just outside the longer-wavelength, or red, portion of the spectrum that includes visible light. TNT shares this characteristic peak with other explosive materials, such as plastic explosives and nitroglycerin, but not with safe materials.
The key to this common attribute, Hummel said, lies in the explosives' chemical makeup—they all contain at least two "nitro groups," molecules made up of one nitrogen atom bound to two oxygen atoms. The peak is a narrow spectral line and would be easy to miss if you don't know where in the spectrum to search, Hummel said.
The UF discovery of TNT's signal resulted from a U.S. Army Research Office challenge to universities to find a way to make inexpensive, quick, and reliable explosive-detection systems. Out of curiosity, one of Hummel's graduate students tested TNT in the lab's photoluminescence spectrometer. With its high resolution, the machine scanned across the entire light spectrum and caught the explosive's elusive signal. "That's why we detected it the first time," Hummel said.
"This is a very complex phenomenon," said Chuck Schau, a scientist at Raytheon Missile System's Radiation Technology Laboratory who also was conducting experiments on explosive detection using photoluminescence but initially did not observe the TNT peak discovered by the UF team.
Raytheon is now interested in following up on this discovery, he said. That development may include a future for this detection technology that goes beyond airport lines and into uncovering dangerous materials on a much larger scale—though that technology may be years away.
Sample collection for explosives is familiar to anyone who has recently passed through an airport: a swab brushed across an object, such as a suitcase, clothing or even a person, or puffs of air blasted across a filter that can trap tiny amounts of airborne explosives.
The advantages of photoluminescence-based explosive detection are technicians can apply its technology from a remote location, and it requires neither time-consuming and expensive machines nor trained dogs.
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