NEGATIVE-ION AND POSITIVE-ION CHEMICAL-IONIZATION MASS-SPECTRA OF AROMATIC-AMINES - SURFACE-ASSISTED REACTIONS INVOLVING OXYGEN

The O2-N2 and O2-Ar negative-ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [M + 14]-.. Other ions are observed at [M - 12]-., [M + 5]-., [M + 12]-., [M + 28]-. and [M + 30]-.. Ion formation was studied using a quadrupole instrument equipped with a conventional chemical ionization source and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. These studies, which included the examination of ion chromatograms, measurement of positive-ion chemical ionization mass spectra, variation of ion source temperature and pressure and experiments with O-18(2), indicate that the [M + 14]-. ion is formed by the electron-capture ionization of analytes altered by surface-assisted reactions involving oxygen. This conversion is also observed under low-pressure conditions following source pretreatment with O2. Experiments with [N-15]aniline, 12,3,4,5,6-H-2(5)]aniline and [C-13(6)]aniline show that the [M + 14]-. ion corresponds to [M + O - 2H]-., resulting from conversion of the amino group to a nitroso group. Additional ions in the spectra of aromatic amines also result from surface-assisted oxidation reactions, including oxidation of the amino group to a nitro group, oxidation and cleavage of the aromatic ring and, at higher analyte concentrations, intermolecular oxidation reactions.