Laser Trace Vaporization of Trace Explosive Materials
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Laser Trace Vaporization of Trace Explosive Materials Michael Papantonakis1, Robert Furstenberg1, Christopher A. Kendziora1, Viet Nguyen1, Jakob Großer2, R. Andrew McGill1 1 Functional Materials and Devices Section, U.S. Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC, U.S.A. 2 Bundesamt für Wehrtechnik und Beschaffung, F.-Sauerbruch-Str.1, 56073 Koblenz, Germany
ABSTRACT The low vapor pressure of many energetic materials presents a challenge for detection by non-contact methods. We address this limitation by illuminating energetic materials including TNT and RDX with infrared lasers tuned to strong molecular absorption bands to efficiently heat trace amounts present on substrates. This substantially increases their vapor signatures for direct detection, obviating the need to swab surfaces for solid particles or to collect headspace vapors for extended time periods. The instantaneously generated vapor produced by Laser Trace Vaporization (LTV) can be detected by any number of techniques which can accommodate vapor sampling or spectroscopic analysis. Currently the testbed for LTV incorporates a tunable quantum cascade laser (QCL) to illuminate the sample and an ion mobility spectrometer (IMS) to validate the signal enhancement. The LTV technique works well with all tested substrates, though the thermal and spectroscopic properties of the substrate can influence the efficiency of the vaporization. Computational results from laser heating along with experimental thermal kinetic measurements were used to optimize LTV laser irradiation parameters. In addition to a range of LTV results for different explosives and substrates, we explore the effects of wavelength-dependent heating on the sample and substrate.
INTRODUCTION In military and homeland security applications, trace explosives detection is a key component to mitigate the threat of improvised explosive devices (IEDs). Persons who are involved in the manufacture, transportation or emplacement of any type of IED become contaminated with trace quantities of explosives and subsequently contaminate any surfaces with which they come into contact, even after 10’s of fingerprint contacts [1] and despite care taken to remove the presence of those explosives, e.g., by washing of hands. The ability to detect trace amounts of explosives on a person or surfaces such as on luggage, boarding passes, or car door handles provides valuable information to security forces, allowing them to take appropriate action depending on the situation, including the option to carry out secondary screening. Most commonly used military, commercial and home-made explosives have a very low vapor pressure and remain present on a surface in solid form for extended periods of time. This is very advantageous from a forensics standpoint, but at the same time presents a sampling challenge for vapor-based detection technologies. As a result, current sampling protocols resort to techniques such as mechanical swabbing or using air jets to dislodge particles which are subsequently collected.
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