Reduction Reactions in the Ion Source in Electron Ionization Mass Spectrometry

Theoretically, electron ionization mass spectra should contain exclusively peaks of ions due to the monomolecular fragmentation of molecular ions. However, spectra of aromatic compounds with halogen atoms and nitro groups contain ion peaks with the composition not consistent with this postulate. Based on a detailed study of such ions by high-resolution mass spectrometry, we showed that their formation is associated with the reduction of the initial molecules at the metal walls of the ion source before their ionization. As a result, products of the reduction reaction of the corresponding composition are ionized. Nitro compounds are reduced to the corresponding amines, while halogenated compounds lose the halogen atom, turning into unsubstituted substrates. Water molecules on the metal surface of the ion source act as reduction agents. This process does not depend on the geometry of the source and proceeds in any mass spectrometer.