The effect of small cations on the positive electrospray responses of proteins at low pH

Solutions consisting of protein and small molecule mixtures have been subjected to electrospray ionization to study the influence of small molecule/cation components at high concentrations on the electrospray responses of proteins. Emphasis was placed on solutions consisting of equal parts methanol and water and containing 1 vol % acetic acid. The results, therefore, are relevant to low pH solutions with significant organic content, a commonly used set of conditions in electrospray ionization mass spectrometry that tends to denature proteins. A variety of small cations/molecules were selected to sample a range of chemical characteristics. For example, sodium and cesium cations were studied to represent metal ions, tetrabutylammonium and tetramethylammonium cations were studied to represent quaternary ammonium compounds with different surface activities, and octadecylamine and glycine were studied to represent species that compete for protons but have different surface activities. A methodology for measuring relative ion suppression efficiencies was developed and applied for protein ions derived from bovine cytochrome c. The form of the small cation (i.e., metal ion, quaternary ammonium ion, or protonated molecule) did not appear to be a factor in determining the efficiency with which protein ion signals were suppressed. The extent to which ions are expected to concentrate on the surface, however, was the major factor in determining the ion suppression efficiency. It was found that the ion suppression efficiency of the most surface active species in this study was comparable to that of a protein on another protein after normalization by charge. These results are particularly relevant to the development of mixture analysis strategies based on ionization and tandem mass spectrometry applied to mixtures of whole proteins.