Counter-ion and solvent effects in electrospray ionization of solutions of alkali metal and quaternary ammonium salts

Abstract

Despite the wide use of electrospray ionization (ESI) mass spectrometry, the factors determining the relative signal intensities of different analytes are not yet fully understood. We have performed a systematic study to assess the influence of selected counter-ions (Br−, ClO4−, BF4−, BPh4−) on the ESI response of alkali-metal (Li+, Cs+) and quaternary ammonium ions (NEt4+, NBu4+) over a concentration range of 0.025–2.5mmolL−1. Specifically, we have subjected solutions containing equimolar mixtures of these analytes in H2O, CH3OH, CH3CN, and tetrahydrofuran, respectively, to ESI and determined the relative signal intensities of the ions thus produced. Especially in the aprotic solvents, variation of the counter-ion can significantly change the outcome of the ESI process, which we ascribe to ion pairing. The smaller counter-ions Br−, ClO4−, and BF4− undergo more selective ion pairing in that their tendencies to pair up with the cations strongly decrease in the order Li+>Cs+>NEt4+>NBu4+. In contrast, ion pairing of the larger BPh4− counter-ion with the different cations is much less selective. As a result, the smaller counter-ions Br−, ClO4−, and BF4− suppress the ESI response of Li+ and Cs+ ions significantly more strongly than does BPh4−. Additional evidence for selective ion pairing comes from the observation of ionic aggregates enriched in the alkali-metal ions relative to the quaternary ammonium ions.