Effects of solvent on the maximum charge state and charge state distribution of protein ions produced by electrospray ionization

The effects of solvent composition on both the maximum charge states and ch arge state distributions of analyte ions formed by electrospray ionization were investigated using a quadrupole mass spectrometer. The charge state di stributions of cytochrome c and myoglobin, formed from 47%/50%/3% water/sol vent/acetic acid solutions, shift to lower charge (higher m/z) when the 50% solvent fraction is changed from water to methanol, to acetonitrile, to is opropanol. This is also the order of increasing gas-phase basicities of the se solvents, although other physical properties of these solvents may also play a role. The effect is relatively small for these solvents, possibly du e to their limited concentration inside the electrospray interface. In cont rast, the addition of even small amounts of diethylamine (<0.4%) results in dramatic shifts to lower charge, presumably due to preferential proton tra nsfer from the higher charge state ions to diethylamine. These results clea rly show that the maximum charge states and charge state distributions of i ons formed by electrospray ionization are influenced by solvents that are m ore volatile than water. Addition of even small amounts of two solvents tha t are less volatile than water, ethylene glycol and 2-methoxyethanol, also results in preferential deprotonation of higher charge state ions of small peptides, but these solvents actually produce an enhancement in the higher charge state ions for both cytochrome c and myoglobin. For instruments that have capabilities that improve with lower m/z, this effect could be taken advantage of to improve the performance of an analysis, (C) 2000 American S ociety for Mass Spectrometry.