Detection and identification of explosives compounds using laser ionization time‐of‐flight techniques

Abstract

Resonance-enhanced multiphoton ionization has been used to detect the presence of vapour-phase explosive-type samples in a linear time-of-flight (TOF) mass spectrometer. In particular, nitrobenzene, 2-nitrotoluene, 2,4-dinitrotoluene, 2,4,64rinitrotoluene, ethylene glycol dinitrate, pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-l,3,5-triazacyclohexane (RDX) and SEMTEX (PETN plus RDX plus plasticizer) have been analysed at a laser wavelength of 226.3 nm corresponding to a strong resonant transition of the neutral NO molecule. TOF mass spectra have been recorded and the intensity of the NO+ ion monitored as a function of temperature for comparison with the temperature dependence of the various vapour pressures.The detection and identification of explosives and explosive residues is an area of analytical science which has continued to be of great importance for society. A wide variety of techniques have been investigated, with chromatographic and mass spectrometric methods being the most widely used. ' The use of laser technologies for the detection and identification of organic molecules has been assessed by a number of workers using a variety of approaches. Generally, a pulsed laser is used to effect multiphoton ionization/fragmentation of the parent molecule' with the resulting parent and/or fragment ions being analysed using a mass spectrometer. The degree of fragmentation can be controlled via the laser fluence, and different species can be identified by their fragmentation patterns. Laser ionization can also be used to detect the presence of vapours at atmospheric pressure3 by virtue of known wavelength-dependent ionization fingerprints. Application of some of these approaches to the realm of explosives detection and identification have allowed progress to be made in both atmospheric pressure4 and vacuum',6 environments.The use of tunable lasers has demonstrated that characteristic ionization yields are possible if the wavelength can be ~c a n n e d ,~ due to resonance enhancements in the initial absorption step. In the case of nitrotype explosive molecules which generally have broad, featureless absorption bands occurring at similar wavelength regions, this might not be expected to lead to an obvious fingerprint with which to identify a particular species. However, since these molecules have a tendency to photodissociate on exposure to ultraviolet radiation, the photodissociation products can be probed to yield important information relating to wavelength-dependent fingerprints.', '-I3A series of studies"'-I2 has previously been carried out by this group on some nitroaromatic molecules (nitrobenzene and 2-nitrotoluene) with the intention of identifying a procedure for the detection of explosive vapours using tunable, pulsed lasers. These studies have demonstrated the importance of the NO+ fragAuthor for correspondence. + Permanent address: Department of Physics, University of Ioannina, Ioannina, GR 451 10, Greece. ment ion which provides a prominent signal and which has a very characteristic wavelengt...

Condensed‐phase laser ionization time‐of‐flight mass spectrometry of highly energetic nitro‐aromatic compounds

“…LIMS has been employed to study energetic materials in the gas phase using nanosecond laser pulses . This method allows the rapid screening of many samples without the need for complicated preparation techniques such as are required in gas chromatography/mass spectrometry (GC/MS).…”

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

“…Their studies focused on an understanding of the mass spectra obtained using ultrafast laser pulses in conjunction with time-of-flight mass spectrometry, M ϩ ion formation, and specific molecular fragments that could be assigned to a particular precursor species. Of particular interest was the demonstration of isomer dependent mass spectra for the three isomers of nitrotoluene.The motivation to apply femtosecond laser ionization to the study of explosives and ERCs originated from previous laser ionization work, which found that nanosecond laser ionization in the ultraviolet led to extensive molecular fragmentation [31][32][33][34][35][36]. The extensive fragmentation was determined to be non-useful for species-specific identification, even though abundant NO ϩ ions were observed in the mass spectra [37].…”

“…The motivation to apply femtosecond laser ionization to the study of explosives and ERCs originated from previous laser ionization work, which found that nanosecond laser ionization in the ultraviolet led to extensive molecular fragmentation [31][32][33][34][35][36]. The extensive fragmentation was determined to be non-useful for species-specific identification, even though abundant NO ϩ ions were observed in the mass spectra [37].…”

Ion spectrometric detection technologies for ultra‐traces of explosives: A review

“…Laser ionization-TOF-techniques have been introduced to ionize explosive species (Marshall et al, 1994); femtosecond laser TOF has been used to analyze TNT and other nitro-aromatic molecules (Ledingham et al, 1995;Kosmidis et al, 1997). Recent studies have demonstrated the ability of ultrafast laser ionization to generate analyte molecular and structure-specific ions from nitro-aromatic compounds.…”