Ionization of nitromethane in the flame ionization detector

Abstract

The mass spectra observed on sampling the unburnt gases of a flame ionization detector to which nitromethane has been added are due mostly to proton transfer reactions of NH4+ and H3O+. These are formed in the reaction zone from the primary ion HCO+, and pass through the unburnt gases to the sampling orifice. The ions which are detected may be attributed to pyrolysis (CH4NO+ and NO+) or hydrogenation products (CH2NH2+ and CH3NH2+) of nitromethane. The small amount of NO+ observed in the unburnt gases is attributed either to a primary ionization mechanism which produces NO+ from excited nitrogen atoms or molecules in the reaction zone, or to a reaction of H3O+ with HNO2, a pyrolysis product of nitromethane. Observations of CH2NH2+ in the flame and in double resonance experiments performed in an ion cyclotron resonance mass spectrometer give limits on the proton affinity of methanimine (CH2NH) of 847.6–862.3 kJ mol−1.