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

Abstract

The ultrafast laser-induced photoionization and photodissociation processes of the nitroaromatic containing explosive and explosive related compounds (ERCs) nitrobenzene (NB), 1,3-dinitrobenzene (DNB), m-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) have been investigated at three laser wavelengths and power densities using a time-of-flight mass spectrometer. Examination of the mass spectra of these compounds reveals the enhanced formation of the molecular ion [M ϩ ] when ultraviolet (332 nm) and visible (495 nm) light is used relative to infrared (795 nm) radiation. In addition, at 795 nm and a power density of 3.5 ϫ 10 14 W/cm 2 , the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated. [5][6][7][8][9][10][11][12], and polyatomic aromatic organic molecules [13][14][15][16][17][18][19][20][21][22][23], has been the subject of intense investigation. These studies have focused on the fundamental interaction of the laser pulse with the atomic or molecular system, and have generally found a number of operative ionization and dissociation mechanisms present, which are sensitive to the laser wavelength and power density. In addition, a number of studies have also been dedicated to the investigation of the analytical usefulness of femtosecond laser ionization toward molecular specific detection. A prime example is the application to the ionization and detection of the explosives and explosives related compounds pioneered by . 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 presence of NO ϩ in the mass spectra of these species naturally raised questions regarding its formation. Mechanistically, it was discovered that the dissociative lifetimes of the intermediate states involved in the ionization process were about a few hundred femtoseconds, and that this rapid dissociation led to the extensive fragmentation observed in the mass spectra. Further, the presence of NO ϩ in the mass spectra of these species was attributed to rapid dissociation of the parent molecule (RNO 2 ) producing the NO 2 fragment. The NO 2 molecule undergoes an additional photon absorptio...

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

“…On increasing the energy to 0.40 mJ/pulse (0.65 J/cm 2 ), a well‐resolved mass spectrum is observable with information in the high‐mass range. As in related work, our spectra did not show signals associated to 46 NO 2 . This is because the short lifetime of the intermediate states involved in the generation of NO 2 + (a few hundred femtoseconds) leads to rapid dissociation to generate the most stable NO + ion …”

“…Thermal decomposition chemistry is fundamentally important in the explosives field, and the determination of the kinetics and mechanisms of the thermal decomposition remains a fundamental aspect of their characterization. Laser‐based mass spectrometry has proved to be a useful method for the characterization of these families of compounds . Moreover, the use of mass spectrometry in studies of the products of decomposition and detonation of explosives has been important as it helped to determine the chemical fragmentation pathways which lead to the dissociation of the parent molecules .…”

“…Laser-based mass spectrometry has proved to be a useful method for the characterization of these families of compounds. [2] Moreover, the use of mass spectrometry in studies of the products of decomposition and detonation of explosives has been important as it helped to determine the chemical fragmentation pathways which lead to the dissociation of the parent molecules. [3,4] In addition, explosives detection is a complex field where analytical chemistry has to face the forensic problem of analyzing the residues of explosive compounds once the explosion has taken place.…”

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

“…The results indicated that ultraviolet and visible wavelengths are better at generating the molecular ion than 795 nm irradiation. In addition, the effect of Coulomb explosion in the photoionization of nitroaromatic compounds has been revealed (Mullen, Coggiola, & Oser, 2009). In study of TATP, femtosecond laser was utilized for both detection of the molecular ion and enhancement of sensitivity leading to detection limit of 670 pg to the molecular ion.…”

Photoemission ambient pressure ionization (PAPI) with an ultraviolet light emitting diode and detection of organic compounds

“…Finally, selectivity of ionization can be achieved by choosing the appropriate reagent ions through which chemical ionization occurs, although oxygen is an excellent reagent gas itself.One successful group of ion sources for API is that of the light-based sources. Photoionization is one such technique that utilizes either very high photon energies from vacuum UV lamps [2,3] or lasers [4] to result in Single Photon Ionization (SPI) or, when a laser is focused to give high photon flux, resulting in Multi-Photon Ionization (MPI) [5,6] or Resonant Enhanced Multi-Photon Ionization (REMPI). [7][8][9] Higher photon fluxes with laser sources permit the use of lower per-photon energies, such as with MPI or REMPI, but this process becomes less efficient at atmospheric pressure due to collisional quenching.…”