M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

Abstract

AbstractThe N2 negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]−˙. This addition is also observed in the N2 positive chemical ionization (PCI) mass spectra. An ion at [M – 15]− is found in the NICI spectra of aminoaromatics such as aniline, 1‐ and 2‐aminonaphthalene and 1‐ and 2‐aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]−˙, [M – 15]−˙ and [M + 11]+˙ ions result from the ionization of analytes altered by surface‐assisted reactions. Experiments with 15N2, [15N] aniline, [2,3,4,5,6‐2H5] aniline and [13C6] aniline show that the [M + 11]−˙ ion corresponds to [M + N – 3H]−˙. The added nitrogen originates from the N2 buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N2 PCI mass spectrum of aniline suggest that the neutral product of the surface‐assisted reaction is 1,4‐dicyanobuta‐1,3‐diene. Experiments with diamino‐substituted aromatics show analogous reactions resulting in the formation of [M – 4H]−˙ ions for aromatics with ortho‐amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.