High-sensitivity detection of TNT

Abstract

We report high-sensitivity detection of 2,4,6-trinitrotoluene (TNT) by using laser photoacoustic spectroscopy where the laser radiation is obtained from a continuous-wave room temperature high-power quantum cascade laser in an external grating cavity geometry. The external grating cavity quantum cascade laser is continuously tunable over Ϸ400 nm around 7.3 m and produces a maximum continuouswave power of Ϸ200 mW. The IR spectroscopic signature of TNT is sufficiently different from that of nitroglycerine so that unambiguous detection of TNT without false positives from traces of nitroglycerine is possible. We also report the results of spectroscopy of acetylene in the 7.3-m region to demonstrate continuous tunability of the IR source.quantum cascade lasers ͉ high-power lasers ͉ continuous-wave operation ͉ room temperature operation ͉ TNT detection D etection of illegally transported explosives has become important since the global rise in terrorism subsequent to the events of September 11, 2001. Although not a choice of suicide bombers, 2,4,6-trinitrotoluene (TNT) is a potent explosive for which techniques for detection on a person's body or in one's baggage is considered important for assuring safety of airports and air travel. As with detection of other similar compounds, such as chemical warfare agents, any detection scheme that claims to detect these targets must exhibit acceptable receiver operational characteristic (ROC) that assures detection at very low levels without an unacceptable level of false alarms (1, 2). The molecular mass of TNT (C 7 H 5 N 3 O 6 ) is almost exactly identical to the molecular mass of nitroglycerine (C 3 H 5 N 3 O 9 ) even though the chemical compositions of the two molecules are very different (TNT, 227.131 Da vs. nitroglycerine, 227.0872 Da). The nearly same molecular masses often lead to problems for unambiguous detection of TNT using techniques that rely on measuring the molecular mass of the species. On the other hand, the differences in the chemical structure between TNT and nitroglycerine lead to noticeably different infrared (IR) absorption signatures (3), making it possible to distinguish between the two. However, the detection of TNT in vapor phase is hampered by its low vapor pressure of Ϸ2 ϫ 10 Ϫ4 torr at 25°C. In this work, we report on studies of detection of TNT by using room-temperature (RT) quantum cascade laser (QCL)-based photoacoustic spectroscopy (QCL-PAS). The high sensitivity afforded by laser-based photoacoustic spectroscopy (L-PAS) (4) shows that the vapor-phase detection of TNT at an ambient temperature of Ϸ25°C is possible.Previously, CO and CO 2 lasers have been used for photoacoustic (PA) spectroscopic detection (3, 5) of vapors of explosives. However, both of these laser sources are step tunable, and neither of the lasers is able to access the strong absorption features of TNT that lie in the 6.0-7.5 m region. Quantum cascade lasers (QCLs), with their continuous tunability, should be the right sources for the detection of TNT and other species that do...

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