The oxidation behavior of the antioxidant 9-phenyl-2,3,4,5,6,7,8,9-octahydro-1H-selenoxanthene (SEL) is studied by means of an instrumental on-line set-up consisting of an electrochemical (EC) thin-layer cell coupled directly to the electrospray ionization (ESI) source of a high resolution Orbitrap mass spectrometer (HR-MS). A potential ramp between 0 and 2500 mV vs. Pd/H-2 (10 mV/s) was applied to the EC working electrode in order to screen for emerging oxidation products. Presentation of the obtained data as three-dimensional mass voltammograms visualizes the oxidation/ionization efficiency in dependency of the potential applied. EC oxidation gave rise to the formation of ten oxidation products including the dehydrogenated cationic SEL, various oxygenated as well as dimeric species. Molecular formulae could successfully be assigned for all oxidation products with deviations below 5 ppm between calculated and measured m/z. Further characterization of the oxidation products was achieved by extending the system with online high pressure liquid chromatography (HPLC). All-ion fragmentation of the oxidation products was carried out to confirm the proposed structures. Electrochemical oxidation enabled a first prediction of possible phase-I metabolism reactions of SEL, which are likely to occur in vivo. Therefore, electrochemically assisted electrospray ionization enables method development for the identification of SEL and its potential metabolites by means of HPLC/ESI-MS. (C) 2013 Elsevier Ltd. All rights reserved.