In the negative ion mode, the existing HCD ion source produced NO2 -(m/z 46), NO3 -(m/z 62), and CO3 -(m/z 60), while O3 is one of the neutral products in the air-assisted plasma and should appear at m/z 48, [O3] -, but did not appear in the back ground mass spectrum (Fig. ) . Because the electron affinities of the NO2 (2.3 eV) and NO3 (3.9 eV) species are substantially higher than those of O3 (2.1 eV), the NO2 -and NO3 -ions must prevent the formation of O3 - . The O3 -(m/z 48) ion has been infrequently detected in airexcited plasma . At higher ion source pressures, both TNT and TNB molecules produced [M+NO3-HNO2] -adduct ions, which became major ions as the ion source pressure 228) as major ions .