Negative‐ and positive‐ion chemical ionization mass spectra of aromatic amines: Surface‐assisted reactions involving oxygen

Abstract

The 0,-N, and 0,-Ar negative-ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [ M + 141-'. Other ions are observed a t IM -121 -', [ M + Sj-., [ M + 121-', [ 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,, indicate that the [ M + 141 -. 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 0,. Experiments with I "Nlaniline, [ 2,3,4,5,6-*H,] aniline and [ '3C,]aniline show that the [M + 141-' ion corresponds to [ M + 0 -2H]-', resulting from conversion of the amino group to a nitrosogroup. 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.

The investigation of N‒dinitrophenyl derivatives of amino acids by electron/chemical ionization using a particle beam interface1

“…Only a minor portion of DNP-amino acids and DNP-oligopeptides reaches the ion source undecomposed and gives rise to the typical EI or CI spectra when a particle beam interface is used for connection with a liquid chromatograph. The major part suffers surface-catalized decomposition [13], especially decarboxylation and reduction of the nitro groups (for detailled discussions with respect of the various parameters of the PB interface and the ion source [11,12]) resulting finally in the formation of benzimidazole derivatives carrying in 2-position the residue of the respective amino acid. These processes offer a major advantage: CI spectra obtained with a direct inlet system show essentially [M + H] + (in the positive mode) or M −• (in the negative mode [6]) and in EI spectra loss of the complete side chain is observed.…”

“…Reactions of substrate molecules with substances adsorbed at the metal surfaces of the ion source (O 2 , H 2 O, solvents, CI reactand gases, etc.) have been studied in detail [11,12], especially the reduction of aromatic nitro groups, adsorbed solvents being the reducing agent [12][13][14]. It was shown, i.a., [12] that the reactions do occur in the ion source whose temperature has essentially no effect, and that the 5) and the Nle-derivative (7b) is the much lower intensity of the loss of • C 3 H 7 as compared with that of C 3 H 6 for the latter (loss of an i-as compared with n-propyl radical).…”