On-line investigation of the generation of nonaqueous intermediate radical cations by electrochemistry/mass spectrometry

Anodic oxidation of triphenylamine (TPA) in acetonitrile was investigated by electrochemistry (EC) combined online with mass spectrometry (MS) through a particle beam (PB) interface, EC/PB/MS. Electrooxidation of TPA generates a TPA*+ radical cation (m/z 245) which dimerizes to tetraphenylbenzidine (TPB, MW 488). TPB is readily oxidized to TPB*+ (m/z 488) and TPB2+ (m/z 244) at the oxidation potential of TPA. In EC/PB/MS, direct monitoring of the oxidation of TPB to TPB*+ radical cation as a function of the electrode potential was achieved via selective ion monitoring of the ion peak at m/z 488. By using the relative intensity ratio of ions at m/z 244 (TPB2+) to 245 (TPA*+), the formation of TPB2+ as a function of the electrode potential was also monitored. EC/PB/MS showed a maximum rate of formation of TPB*+ at +1.2 Vvs Pd, while TPB2+ is generated at a maximum rate at +1.6 V vs Pd. The effect of spectral interference from the electron impact ionization of TPA, on EC/PB/MS results, is also discussed. Finally, a significant signal enhancement is observed in the presence of tetrabutylammonium perchlorate (TBAP) and is reported for the first time. Compatibility of coupling of EC with MS via PB interface for EC/MS studies in nonaqueous solvents is demonstrated. The observation of significant signal enhancement in the presence of TBAP may facilitate other applications of LC/MS.