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 t he equilibrium ion abundances in the solution electrosprayed, One such divergent case is that of free-base octaethylporphyrin, Under conditi ons for which this porphyrin is present in solution predominantly as t he doubly charged, diprotonated molecule, the predominant ionic specie s observed in the electrospray mass spectrum is the singly charged, mo noprotonated molecule, In this paper, direct optical spectroscopic mea surements of the ions in solution (absorption spectra) and in the elec trospray plume (fluorescence excitation spectra) are correlated with t he ion distribution observed in the gasphase (as reflected in the elec trospray mass spectra) to determine at what point in the electrospray process and by what mechanism(s) the transformation from dication to m onocation occurs, The data indicate that the major portion of the doub ly protonated porphyrin species originally present in solution are con verted to singly protonated species relatively fate in the electrospra y process, during the latter stages of droplet desolvation in the atmo spheric/vacuum interface of the mass spectrometer, via the loss of a c harged solvent molecule/cluster.