Detection of Trace Explosives in the Gas Phase - Development of an Infrared Laser Absorption Spectroscopy System for the Security Control at Airports

Kurzfassung

Improvised explosive devices (IEDs) pose a major threat to the general public, especially in crowded places. Early detection can help defuse dangerous situations before damage is caused. Often, however, detection is only possible with a certain amount of effort. Samples have to be taken – often by swabbing with a wipe – before an analysis is possible. This in turn leads to long waiting times or subjective random sampling by security personnel. In addition, measurements are not possible from a safe distance in this way. The question therefore arises as to how early remote detection can be realised from a safe distance without having to take samples and whether such a method can be combined with automated measurement. In view of this question, this paper investigates whether it is possible to detect traces of explosives in the gas phase by means of MIR absorption spectroscopy. The background to this consideration is the property of TATP, which is used in a large number of IEDs, to volatilise relatively quickly and sublime into the gas phase. In the course of this work, a MIR absorption spectroscope setup was designed that is able to detect even low concentrations of substances in the air with the help of multipass cells. For this purpose, ambient air is sucked in and passed through the multipass cells. The light of the laser beam is subject to substance-specific absorptions which are detected and recorded by a sensor. Feasibility tests were carried out with a calibration gas, two liquids that have a similar vapour pressure to TATP, and with the explosive TATP itself. By means of the designed set-up and a LabVIEW software, it could be shown that continuous long-term measurements are possible. For the TATP measurements, quantities in the range of hundred milligrams were used. The results show that TATP concentrations of less than 40 mmol/m3 are already noticeable by strong absorptions in the IR spectrum and therefore a detection from the gas phase is possible.