Mass spectrometry is perfectly suited for trace analysis of explosives in residues because of its specificity and sensibility; the electronic impact (EI) mode is the most widely used, although negative ion chemical ionization (NICI) is apparently a softer ionization technique for explosives. In the current study, the NICI mode was used to test two reagent gases: isobutane and methane. This allowed direct comparison between ion trap detection results and those found in an earlier study that used single quadrupolar mass spectrometry detection. A description of the experimental procedure addresses the apparatus, the reagents, and the method. Linearity, detection limits (LOD), and quantitation limits (LOQ) were determined by the XPT-90 210 French AFNOR standard method. The NICI mode was found to be more sensitive than EI mode and well suited for the analysis of nitro esters (ethyleneglycol dinitrate, pentaerythrol tetranitrate, and nitroglycerine). The first NICI experiments indicated that methane was a better reagent gas than isobutane. Suggestions are offered for future related research. 6 tables, 2 figures, and 7 references