A mass spectrometry and optical spectroscopy investigation of gas-phase ion formation in electrospray

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 gas-phase (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.