Negative‐ 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 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.