Aqueous metal salt solutions were used as models to probe the origin of the species observed in the electrospray mass spectrum. A qualitative or semiquantitative correlation among different species was observed between electrospray responses and calculated equilibrium aqueous solution concentrations. Quantitative correlations were obtained, however, when ions that were identical in charge and similar in type were selected for comparison. In these experiments the ions experienced very similar electrospray-related processes and their effects on the responses were canceled in a comparison of these ions. Consequently, the relative abundances of these ions in the electrospray mass spectrum closely matched the calculated relative abundances in aqueous solution. Our results suggest that the basic principle that determines ionic distribution in the electro spray mass spectrum in aqueous solution chemistry.

The effects of heat on the electrospray mass spectra of eight globular proteins in solution were studied. These ranged from hardly noticeable to a dramatic shift in the mass spectrometric profile and a concomitant increase in ion abundance. This change is believed to be the result of thermal denaturation of the protein species in solution resulting in a transition from a more compact to a less compact conformation. We accounted for this transition by means of a recently proposed model based on aqueous solution acidhse equilibria. For cytochrome c, profiles calculated by means of this model agree well with experimental data. The AH of the denaturation reaction of cytochrome c in aqueous solution containing 0.2% acetic acid was calculated from experimental data to be 103.8 f 9.2 kJ mol-I, in good agreement with previous measurements.Crown copyright (Canada)

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Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada The shape of the profile described by the relative abundances of multiply charged ions of proteins in the electrospray mass spectrum can be described by means of one or more Gaussian functions. An aqueous solution equilibrium model of the distribution of multiply charged ions of equine cytochrome c and myoglobin has been shown to match qualitatively the shape of the distribution of these ions in an electrospray mass spectrum. Monotonic functions such as the quadrupole mass spectrometric transmission efficiency may alter the centroid of the profile, but the shape of the ion abundance pattern appears to be controlled by the aqueous solution chemistry of the proteins. NRCC No. 32938.