Tandem mass spectrometry is shown to improve the effective mass resolution in electrospray mass spectrometry. The technique involves selecting a population of ions within a narrow range of mass-to-charge values and allowing the ions to undergo proton transfer reactions. The shifts in mass-to-charge ratios associated with product ions formed by proton transfer allow for mass and charge assignment. The success of the technique relies on the relative enrichment of ions of a particular charge state that occurs in the mass-to-charge selection step. This approach can be used to extend the polymer mass range amenable to measurement, analyze mixtures that might otherwise be too complex for reliable mass measurements, and improve mass measurement precision when a mixture of cations is present within a given charge state. The technique is illustrated with a quadrupole ion trap using multiply-charged ions of cytochrome c, transfer ribonucleic acid from E. coli, strain W, and a synthetic deoxyribonucleic acid 30-mer.