Laser- and Optical-Based Techniques for the Detection of Explosives
Environment: Trace Gas Monitoring
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Monts, D. L., Singh, J. P. and Boudreaux, G. M. 2006. Laser- and Optical-Based Techniques for the Detection of Explosives. Encyclopedia of Analytical Chemistry.
- Published Online: 15 SEP 2006
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A wide variety of optical- and laser-based techniques have been and are currently being investigated as means of identifying and characterizing explosive materials. These efforts are hampered by the very low volatility of explosive compounds at room temperature and their tendency to ignite at higher temperatures where their vapor pressures are beginning to reach a regime where the species can be readily detected. Traditional, condensed-phase absorption spectroscopy (both infrared (IR) and ultraviolet/visible (UV/VIS)) requires larger samples than are frequently available and, although useful as confirmatory techniques, do not provide signatures that enable unique identification of species. The choice of nontraditional techniques for analysis of explosive materials is dependent upon the amount of sample available, the sample matrix, and sample-imposed constraints upon the measurement. Raman spectroscopy can identify and quantify condensed-phase explosive materials, but the detection limit depends upon substrate, excitation wavelength, and illumination area: limits of detection (LODs) vary from 0.05 ng for 2,4,6-trinitrotoluene (TNT) on glass to 10 µg for nitroglycerin (NG) on silica gel. The other detection techniques considered require volatilization of either the explosive compound itself or more often of characteristic decomposition products, such as NO, NO2, or N2O. Using IR irradiation, researchers have demonstrated that photoacoustic spectroscopy (PAS) has detection limits ranging from 0.55 ppb (parts per billion) for vapor-phase NG with 9 µm excitation to 220 ppm (parts per million) for vapor-phase 2,4-dinitrotoluene (DNT) with 6-µm excitation. IR laser differential absorption detection of dissociation products NO, NO2, and/or N2O can achieve detection limits of a few picograms. Laser-induced fluorescence (LIF) detection of the photofragments (typically NO) of explosive compounds yields detection limits in the tens to hundreds of ppb (by weight) range. Low LODs can be achieved using ion detection techniques: resonance-enhanced multiphoton ionization (REMPI) spectrometry is capable of LODs for detecting vapor-phase explosives compounds in the hundredths to tens of ppb range; ion mobility spectrometry (IMS) has vapor-phase detection limits for common explosive compounds of typically 200 pg, but for some species can be significantly lower: for example, 1 pg for TNT.