"Certain families of explosives have unique thermal properties that help detect explosive vapors in the presence of other vapors," say American researchers
A team of researchers from Tennessee and Denmark has developed a new method for the sensitive detection of explosives based on the physical properties of their vapors. Their technology, now being developed into prototype devices to be used in field tests, is described in the latest issue of the scientific journal Review of Scientific Instruments.
"Certain families of explosives have unique thermal properties that help detect explosive vapors in the presence of other vapors," says Thomas Thundat, a researcher at Oak Ridge National Laboratory (ORNL) and the University of Tennessee.
In the paper describing their research, the scientists show that their method is capable of detecting traces of explosives. They also show that it is able to differentiate between chemicals that are explosives and those that are not and to distinguish between the different explosives themselves such as: TNT, PETN and RDX.
Many researchers have been working on explosives sensors for a long time. Conventional sensors use an ion mobility spectrometer, which ionizes tiny amounts of chemicals and measures the speed of their movement through an electric field. While these devices are fast, sensitive and reliable, they are also expensive and somewhat bulky - limitations that have led many researchers in recent years to try and find portable and cheaper devices for detecting explosives.
Many of these studies focus on "micromechanical" devices - tiny sensors with microscopic detectors that chemical vapors in the air are attached to. When the appropriate chemicals are attached to the surface of these sensors, they cause minute mechanical movement, and these movements create electrical signals that can be measured.
These devices, relatively speaking, are not that expensive, and are capable of sensitively detecting explosives, but sometimes they have the disadvantage of not being able to distinguish between similar chemicals - both dangerous and not. They are able to detect trace amounts of TNT, for example, but they are unable to distinguish between traces of gasoline.
In their search for better micromechanical sensors, the researchers realized that they could detect explosives selectively and at a high level of sensitivity by building sensors capable of measuring the thermal signatures of the chemical vapors.
They started with standard sensors - devices with microscopic beams supported at one end. They modified these beams so that they could be heated electrically by passing a current through them. Next they allowed air to pass through the sensors. If explosive vapors were present in the air they could be detected when their bundles were tied to the beams.
Or then, by heating the beams for a fraction of a second, they were able to differentiate between explosives and non-explosives. All the explosives they measured gave unique and permeable thermal reaction patterns within a fraction of a second of heating. In their article, the researchers showed that with their method it is possible to detect very small amounts of adsorbed explosives - in the range of 600 picograms (one thousandth of a nanogram or one trillionth of a gram). The researchers are currently trying to improve the sensitivity of the device and are building a prototype that can be used in field trials later this year.
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