The negative mass spectra of TNT and TNB measured by HCDI-MS system are shown in whereas TNB produced only the fragment ion, ([TNB-NO] -, m/z 183) . The formation of the ([TNT-OH] -, m/z 210) ion is due to the presence of a methyl (-CH3) group in TNT molecules. The methyl hydrogen should be more acidic than the ring hydrogen, and the oxygen atom of the neighboring -NO2 group (ortho to the -CH3 group) causes the methyl proton to be abstracted, resulting in the formation of the ([TNT-OH] -, m/z 210) ion from the TNT molecules.