CHEMICAL ELECTRON-TRANSFER REACTIONS IN ELECTROSPRAY MASS-SPECTROMETRY - EFFECTIVE OXIDATION POTENTIALS OF ELECTRON-TRANSFER REAGENTS IN METHYLENE-CHLORIDE

Cyclic voltammetry (CV), UV/visible absorption spectroscopy, and electrospray mass spectrometry (ES-MS) are used in conjunction to study the mono- and/or dications produced in solution from the reaction of three model compounds (beta-carotene, cobalt(II) octaethylporphyrin (Co(II)OEP), nickel(II) octaethylporphyrin (Ni(II)OEP)), in three different solvent/electron-transfer reagent systems (methylene chloride/0.1% trifluoroacetic acid (TFA) (v/v), methylene chloride/0.1% TFA/2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (v/v/200 mu M), methylene chloride/0.1% TFA/0.1% antimony pentafluoride (SbF5) (v/v/v)). The reactions were carried out on-line with ES-MS by means of flow injection. Correlation of the CV data for these analytes with the ionic species determined to be in the solution on the basis of UV/visible absorption spectra and/or on the basis of the ionic species observed in the gas phase by ES-MS, along with our previously published data on these solvent/reagent systems, allowed an effective oxidation potential range, E, to be assigned to these solvent/reagent systems: methylene chloride/0.1% TFA (v/v), 0.6 V less than or equal to E(TFA) < 0.7 V; methylene chloride/0.1% TFA/ DDQ (v/v/200 mu M), 0.8 V less than or equal to E(TFA/DDQ) < 1.0 V; methylene chloride/0.1% TFA/0.1% SbF5 (v/v/v), 1.3 less than or equal to E(TFA/SbF5) < 1.5. Knowledge of the solvent/reagent oxidation potentials and the electrochemical redox potentials of a particular analyte of interest allows a solvent/reagent system to be chosen to selectively ionize an analyte, or a range of analytes, to a desired ionic state for subsequent analysis by ES-MS. The ability to detect, in the gas phase, an ionic species produced in solution by chemical electron transfer will depend, however, on the stability of the ion in the given solvent/reagent system and on whether the ion can survive intact the transfer to the gas phase via the ES process.