To determine the limitations of electrospray mass spectrometry for the study of condensed-phase chemistry, it is important to understand the origin of cases for which the electrospray mass spectra, which are a measure of the relative abundances of gas-phase ions, do not reflect the equilibrium ion abundances in the solution electrosprayed. One such divergent case is that of free-base octaethylporphyrin. Under conditions for which this porphyrin is present in solution predominantly as the doubly charged, diprotonated molecule, the predominant ionic species observed in the electrospray mass spectrum is the singly charged, monoprotonated molecule. In this paper, direct optical spectroscopic measurements of the ions in solution (absorption spectra) and in the electrospray plume (fluorescence excitation spectra) are correlated with the ion distribution observed in the gasphase (as reflected in the electrospray mass spectra) to determine at what point in the electrospray process and by what mechanism(s) the transformation from dication to monocation occurs. The data indicate that the major portion of the doubly protonated porphyrin species originally present in solution are converted to singly protonated species relatively late in the electrospray process, during the latter stages of droplet desolvation in the atmospheric/vacuum interface of the mass spectrometer, via the loss of a charged solvent molecule/cluster.An electrospray ion source is an electrostatic spray device that assists the transfer of analyte species in a liquid solution to the gas-phase where they can be detected by a mass Spectrometer.' This analysis technique, termed electrospray mass spectrometry (ES-MS), has rapidly become the mass spectrometry method of choice for the analysis of a wide range of polar, nonvolatile and thermally labile compounds, ranging from elemental species'. to very high molecular weight biop~lymers.~.~ Best analytical results with ES-MS are usually obtained for analytes that are ionic in solution and, in general, it is believed that the ions observed in the gas phase are, at a minimum, a qualitative reflection of the equilibrium ion concentrations in the original liquid sample solution.' A number of experimental studies support this general view."I5 However, an increasing number of reports have shown that the distribution of gas-phase ions observed can be quite different from the distribution known to be in the solution on the basis of calculated solution equilibria.IG2' As a result of these conflicting observations, an important area of fundamental ES-MS research today is an exploration of the various factors that determine the correspondence (or lack thereof) among the ionic species in the original sample solution and the ions ultimately observed in the gas phase. The findings of this research will have ramifications in many ES-MS applications, including, among others, general qualitative and quantitative analys~s ?~ the study of noncovalent solution interactions of biomolecular specie^,'^ and the use of ES-MS for elemental...

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