Method detection limits were determined and compared for analysis of liquid injections of organic explosives and related compounds by gas chromatography-mass spectroscopy (GC/MS) using electron impact (EI), negative ion chemical ionization (NICI), and positive ion chemical ionization (PICI) detection methods. Detection limits were rigorously determined for a series of dinitrotoluenes, trinitrotoluene, two nitroester explosives, and one nitramine explosive. GC/MS was performed on a Hewlett-Packard 5989 Gas Chromatograph/Mass Spectrometer (quadruple) operating in the three ionization modes. Methane gas was used as reagent gas in both PICI and NICI. The gas chromatograph was fitted with a 12 m HT5 column with a 0.22 mm I.D. and a 0.1um film thickness. An initial temperature of 70 degrees C was held for 3 minutes, followed by a temperature increase to 185 degrees C at a rate of 8 degrees C min.-1 and a subsequent increase to 250 degrees C at a rate of 25 degrees C min.-1. The final temperature was held for 5 minutes. NICI was found to give the lowest detection limits for each of the explosives examined, with the exception of RDX. Detection limits in the MS scanning mode were dependent on the chemical structure of the explosive and varied from the low picogram range to a few nanograms. The use of NICI generally gave a factor of less than 10 decrease in LOD relative to EI and PICI, although larger decreases in LOD were observed for 3,4-DNT. 5 tables and 24 references