Importance of gas-phase proton affinities in determining the electrospray ionization response for analytes and solvents

The effect of gas‐phase proton transfer reactions on the mass spectral response of solvents and analytes with known gas‐phase proton affinities was evaluated. Methanol, ethanol, propanol and water mixtures were employed to probe the effect of gas‐phase proton transfer reactions on the abundance of protonated solvent ions. Ion–molecule reactions were carried out either in an atmospheric pressure electrospray ionization source or in the central quadrupole of a triple‐quadrupole mass spectrometer. The introduction of solvent vapor with higher gas‐phase proton affinity than the solvent being electrosprayed caused protons to transfer to the gas‐phase solvent molecules. In mixed solvents, protonated solvent clusters of the solvent with higher gas‐phase proton affinity dominated the resulting mass spectra. The effect of solvent gas‐phase proton affinity on analyte response was also investigated, and the analyte response was suppressed or eliminated in solvents with gas‐phase proton affinities higher than that of the analyte. Copyright © 2000 John Wiley & Sons, Ltd.