Vapour detection

Researchers in North America and Denmark are developing a prototype device that is capable of detecting explosives based on the physical properties of their vapours.

‘Certain classes of explosives have unique thermal characteristics that help to identify explosive vapours in the presence of other vapours,’ said Thomas Thundat, a researcher at Oak Ridge National Laboratory (ORNL) and at the University of Tennessee who conducted the research with his colleagues at ORNL and the Technical University of Denmark.

The scientists have shown that their technology is capable of the trace detection of explosives. They have also shown that it is capable of distinguishing between explosive and non-explosive chemicals and of differentiating between individual explosives, including TNT, PETN and RDX.

Typical sensors use ion mobility spectrometers, which ionise tiny amounts of chemicals and measure how fast they move through an electric field. While these instruments are reliable, they are also expensive and cumbersome, leading many researchers to try to find a cheaper, more portable device for detecting explosives.

Much of this research focuses on ‘micromechanical’ devices, tiny sensors that have microscopic probes on which airborne chemical vapours deposit. When the right chemicals find the surface of the sensors, they induce tiny mechanical motions and those motions create electronic signals that can be measured.

These devices are relatively inexpensive to make and can sensitively detect explosives. However, they are often unable to discriminate between chemicals that may be dangerous or benign. They may detect a trace amount of TNT, for instance, but they may not be able to distinguish that from a trace amount of gasoline.

Thundat and his colleagues realised they could detect explosives selectively and with extremely high sensitivity by building sensors that probed the thermal signatures of chemical vapours.

They started with standard micromechanical sensors with microscopic cantilever beams supported at one end. They modified the cantilevers so that they could be electronically heated by passing a current through them. They then allowed air to flow over the sensors. If explosive vapours were present in the air, they could be detected when molecules in the vapour clung to the cantilevers.

By rapidly heating the cantilevers, they could discriminate between explosives and non-explosives. All the explosives they tested responded with unique and reproducible thermal response patterns within a split second of heating.

Thundat and his colleagues have so far demonstrated that they could detect very small amounts of absorbed explosives, with a limit of 600 picograms. They are now improving the sensitivity and making a prototype device, which they expect to be ready for field testing later this year.