The Unambiguous Detection of TNT

Sulzer et al. have provided an explanation for the unusual dependence of the detection sensitivity of TNT and TNB on the reduced electric field [7]. The proposal is that as E/N is reduced, water clustering to TNTH+ and TNBH+ becomes favourable, which leads to a reduction in the signal intensities of the non-hydrated ions. The adduct ions TNTH+(H2O) or TNBH+(H2O) do not survive in the drift tube owing to fast bimolecular reactions with water which leads to the formation of H3O+ (H2O) as the final product ion. 

Thus the terminal product ion, H30+(H20), does not contain the explosive molecule. Furthermore, this ion is already present in considerable abundance in the drift tube and therefore its production cannot be monitored. It is this unusual reaction (8.2) that leads to the observed reduction in the sensitivity of detection for TNT and TNB as E/N is reduced. In confirmation of this explanation, DFT calculations indicate that Reaction 8.2 is exergonic for TNB (AG298 = -14 kJ mol ). Reaction 8.2 is slightly endoergonic for TNT (AG298 = -b 5 kJ moE ) [7] but this small endoergonicity is readily overcome because the reaction will not only be kinetically driven by the high water number density present in the drift 

If the above explanation is correct then the production of MH + (H2O), where M = TNT or TNB, and its reaction with water will increase as the humidity is increased, which in turn will result in more rapid production of H3O+(H2O) as a terminal product ion. Furthermore, it can be expected that the E/N value at which the MH + intensity reaches a maximum will shift to higher values as the humidity in the drift tube is increased. Experiments by Sulzer and colleagues have confirmed this proposal [7]. 

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