FAST-ATOM-BOMBARDMENT IONIZATION OF PORPHYRINS - STUDIES OF REDUCTION PROCESSES AND USE IN COUPLING OF THIN-LAYER CHROMATOGRAPHY WITH MASS-SPECTROMETRY

The positive-ion fast atom bombardment mass spectra of porphyrins usually contain substantial signals corresponding to ions from the reduced forms of the protonated molecule, specifically those ions that incorporate extra hydrogen atoms. Occurrence of such reduction reactions is not unexpected since the same processes have been throughly characterized in ammonia chemical ionization. Several authors have also noted the presence of these ions in fast atom bombardment mass spectra. However, this work documents the effect of several experimental parameters on the extent of these reactions. The extent of reduction is measured with precision for a number of different porphyrins from two different solvent matrices commonly used in fast atom bombardment ionization. Time dependences of the signal are estalished for the lifetime of the solvent in the mass spectrometer source. For several porphyrins, the extent of reduction is great, but constant, suggesting a steady-state reduction process at the surface of the bombarded liquid. An analogy to electrochemically induced reduction is drawn, and a correlation to electrochemical reduction potentials is firmly established. Finally, the use of fast atom bombardment mass spectrometry (under conditions in which a steady ion signal can be obtained) for the measurement of porphyrin mass spectra directly from thin-layer chromatography plates, and with an intermediate extraction device, is described. Full mass spectra are obtained for about 50 ng of porphyrin within the chromatogram. © 1990, American Chemical Society. All rights reserved.