Detection of Radical Anions by Electrospray‐Ionization Mass Spectrometry

ABSTRACT Radical anions constitute an important class of reactive intermediates. Here, we investigate the suitability of electrospray‐ionization (ESI) mass spectrometry for their qualitative and quantitative analysis. To this end, we have probed solutions of 14 different benzoquinones and nitrobenzenes in the presence or absence of the reductant CoCp 2 . The latter greatly helped in the formation and detection of radical anions, but even without, analytes of sufficiently high electron affinity could accept an electron upon cathodic reduction during the ESI process and, thus, became observable. Analytes with low electron affinities escaped from detection because the corresponding radical anions did not form to a sufficient extent and/or underwent electron detachment during the ESI process and the consecutive ion transfer. Furthermore, we studied the effect of the analyte concentration, different reducing agents as well as the solvent and demonstrated the utility of the present approach for the mechanistic elucidation of reactions involving radical anions.