Direct evidence for preformed ions of porphyrins in the solvent matrix for fast atom bombardment mass spectrometry

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Abstract
The observation that protonated molecules are present in solvents utilized for fast atom bombardment (FAB) mass spectrometric studies has been demonstrated using visible absorption spectrometry. Addition of porphyrins to thioglycerol, a solvent used for FAB analyses, results in partial protonation of the molecule. This reaction can be monitored by observing the shift in visible absorption maxima associated with the molecular transition from free base to protonated structure. A good correlation is observed between the degree of protonation indicated by the appropriate absorption bands and the abundance of the [M + H]+ ion in the FAB spectrum of the corresponding solution. Addition of certain non‐polar porphyrin molecules to thioglycerol does not result in the protonation of the molecule in solution; in these cases, analyses of the corresponding solutions by FAB do not yield [M + H]+ ions. Subsequent addition of trifluoroacetic acid to the solvent has proved sufficient to protonate the analyte molecule, as indicated by the visible absorption spectrum; FAB analyses of these non‐polar porphyins in acidified solvent result in the observation of [M + H]+ ions. These experiments demonstrate that analyses of these analyte molecules requires that they be present as ions in solution prior to analysis by FAB. This study provides experimental evidence for the presence of ions in solutions employed for FAB analysis, suggesting that these ions are essential for the generation of the protonated molecules observed during FAB mass spectrometric analyses.