A TEST for explosives that is 50 times more sensitive than current techniques
has been developed by researchers in the US. Security staff at airports and
embassies could use a spray-on version of the test as a quick and handy way of
finding traces of explosives such as TNT and Semtex.
鈥淭his is great,鈥� says Robert Joseph Simonson, who works on detecting
landmines and unexploded bombs at Sandia National Laboratories in Albuquerque,
New Mexico. He says that a big advantage of this approach is that it picks out
all major compounds containing nitrate, a chemical group found in many common
explosives.
Laboratories normally use chromatography to detect traces of explosives. A
solution of the suspect sample is forced through a stainless steel tube filled
with a silica-based particles. Because different substances flow through the
tube at different speeds, the explosive becomes separated from the other
components of the sample. The concentration of the explosive can then be
determined by bathing this part of the sample in ultraviolet light and measuring
how much light is absorbed.
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鈥淏ut UV absorbance detection is not very sensitive as a detection technique,鈥�
says Victoria McGuffin of Michigan State University in East Lansing. Organic
compounds in soil, for example, can also absorb ultraviolet light, which can
mask traces of explosive.
So McGuffin and her colleague John Goodpaster turned to a technique called
fluorescence quenching. After separating the different fractions with
chromatography, they add a fluorescent chemical called pyrene to the sample and
illuminate it with a laser. Nitrates interact with pyrene, which stops it
fluorescing. 鈥淔luorescence quenching responds only to the nitrated explosives.
So most of the other things in the soil that might absorb UV light aren鈥檛
detected by this method,鈥� says McGuffin. This increases the contrast between
explosives and innocent substances.
The degree of quenching is a measure of the concentration of the explosive.
The researchers say in a forthcoming issue of Analytical Chemistry that
this technique can detect nitrate concentrations of between 10 and 100 parts per
billion. The detection limit for the standard method is measured in parts per
million, says McGuffin.
The new technique can also be used directly to detect traces of explosives on
luggage being used to smuggle explosives through airports or suspicious packages
left at railway stations. Security guards could simply spray a solution of
pyrene on the object and shine a commercial ultraviolet lamp at it. Pyrene
normally fluoresces with a blue-green light. 鈥淚f you see black spots, that would
indicate a quencher, which in this case would be an explosive,鈥� says
McGuffin.
But there鈥檚 a snag with the technique that may limit its usefulness. Pyrene
is carcinogenic. So McGuffin has also tested a common dye called fluorescein,
which can detect even fainter traces of explosives. But fluorescein can
sometimes trigger false positives, she says.