FRACTURE-INDUCED AND THERMAL-DECOMPOSITION OF NTO USING LASER IONIZATION MASS-SPECTROMETRY

A surface analysis by laser ionization (SALI) apparatus has been used to obtain, for the first time, real-time photoionization mass spectra of the shear-induced molecular-fragment emission from an explosive. NT O (5-nitro-1,2,4-triazol-3-one) was chosen for these experiments becau se of its potential utility as a reasonably energetic, but very insens itive, explosive. Using vacuum ultraviolet single-photon ionization, t he shear-induced NTO spectra were obtained with a spring-driven sheari ng device installed in a SALI chamber directly beneath the mass spectr ometer sampling region. For comparison, we also obtained spectra under either slow-heating or rapid pulsed-laser heating conditions. The she ar-induced spectra are dominated by a peak at m/z 99, which is not see n in the thermal- or laser desorption spectra. This peak is assigned t o the closed-shell traiza-diketone produced by a nitro-nitrite rearran gement, followed by NO loss and then by rapid bimolecular H-atom remov al. The stability of the cyclic diketone intermediate thus generated c ould help to explain the shock insensitivity of NTO. Laser-desorption spectra were also obtained both on fresh NTO samples and on samples th at have been recovered from marginally sub-critical drop-weight impact tests. Comparison of spectra obtained with and without laser desorpti on, and as a function of temperature, demonstrate that the sequences o f fragment ions observed under laser desorption conditions are the res ult of thermal decomposition, not of ion-fragmentation. The sequence o f thermally generated fragments is dominated by M-16, M-30, M-45, M-46 , and M-59. This series suggests several decomposition pathways, domin ated by the same nitro-nitrate rearrangement and NO loss as the shear- induced decomposition. However, under the lower-density, but higher te mperature, thermal or laser-desorption conditions, subsequent bimolecu lar H-atom removal to produce the closed-shell diketone is evidently s lower than unimolecular ring-opening adjacent to the carbonyl group. W e show how this sequence satisfactorily explains (1) the ''initial'' f ormation of CO2 that has been previously reported, (2) the results of nitrogen double-labeling experiments, and (3) the fact that neither NO 2 nor HONO have been seen as substantial initial products of NTO decom position. (C) 1997 by The Combustion Institute.