Df. Mcmillen et al., FRACTURE-INDUCED AND THERMAL-DECOMPOSITION OF NTO USING LASER IONIZATION MASS-SPECTROMETRY, Combustion and flame, 111(3), 1997, pp. 133-160
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.