ELECTROCHEMICAL ORIGIN OF RADICAL CATIONS OBSERVED IN ELECTROSPRAY IONIZATION MASS-SPECTRA

A variety of experimental data is presented that implicates electrochemical oxidation of analytes in the electrospray (ES) needle as the mechanism for formation of molecular radical cations observed in the ES ionization mass spectra of alkyl-substituted metalloporphyrins, polycyclic aromatic hydrocarbons (PAH's), and other compound types. Analyte structural characteristics and solution-phase half-wave oxidation potentials (which correlate with gas-phase ionization energies) can be used to evaluate the likelihood of forming and observing a particular compound as a radical cation. Use of an appropriate solvent is critical in the observation of radical cations generated by the ES process. In addition to dissolving the analyte and providing a stable electrospray, the solvent(s) must "stabillize" or otherwise protect the radical cation from reactions in solution. Appropriate solvent systems (e.g., methylene chloride/0.1 % trifluoroacetic acid) are much the same as used in traditional studies of electrochemically generated radical cations. The ability to produce radical cations in the ES process expands the utility of ES ionization mass spectrometry to include compound classes not normally amenable to the technique (e.g., neutral, nonpolar compounds such as PAH's) and provides for generation of a different type of molecular species than normally produced in positive-ion ES ionization (i.e., M.+ versus (M + H)+, (M + Na)+, etc.).