Adduct Ion Formation by Aromatic Amines in Thermospray Mass Spectrometry

Abstract

The formation of solvent adduct ions in thermospray and ionspray mass spectrometry was studied for twelve aromatic amines: aniline,N-methylaniline,N,N-dimethylaniline, 3-aminophenol, 3-methylaminophenol, 3-dimethylaminophenol, 2-aminopyridine, 2-methylaminopyridine and 2-dimethylaminopyridine, 2-amino-5,6-dimethyl-4-hydroxypyrimidine, 2-methylamino-5,6-dimethyl-4- hydroxypyrimidine and 2-dimethyl-amino-5,6-dimethyl-4-hydroxypyrimidine. For all compounds, adduct ions, [M+H+An]+, with A being methanol or acetonitrile, were observed in the thermospray mass spectra; water adduct ions were observed for a few compounds. No adduct ions with ammonia were formed when ammonium acetate was added to the liquid chromatographic carrier stream. These observations cannot be explained on the basis of gas-phase ion–molecule reactions of the neutral analyte and protonated solvent or solvent additive molecules. Comparative experiments, changing the pH of the carrier stream in both thermospray and ionspray ionization, showed that the solvent adduct ions present in the thermospray mass spectra are not likely to be formed by ion evaporation processes. Incomplete evaporation of droplets or cluster ions is proposed to be responsible for the observations. With this hypothesis, both the absence of ammonium adduct ions and the dependence of the adduct ion abundances on the N-methylation can be related to the adduct–analyte bond strengths.