Abstract: | 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 18O2, indicate that the M + 14]?˙ ion is formed by the electron-capture ionization of analytes altered by surfaceassisted reactions involving oxygen. This conversion is also observed under low-pressure conditions following source pretreatment with O2. Experiments with 15N]aniline, 2,3,4,5,6-2H5] aniline and 13C6]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. |