Determination of tocopherols by reverse-phase liquid chromatography and electrochemical detection at a surface-oxide modified platinum microelectrode, without added electrolyte

The reverse-phase separation and electrochemical detection of alpha-, gg-, and delta-tocopherol at a potential of +0.90 V vs. a gold pseudo-reference electrode is possible down to 10(-7)M concentrations, with surface-modified platinum microdisc electrodes in a methanol/water (95:5) solvent mixture. The use of microclectrodes with radii of 10-70 mum, rather than electrodes of conventional size, minimizes problems associated with iR drop and obviates the need for deliberately added electrolyte. These features simplify the analytical procedure. The background response of an untreated platinum microelectrode in the methanol/water (95:5) system at positive potentials is characterized by processes arising from adsorption/oxidation of methanol and formation of surface oxides. Amperometric detection is of little use under these conditions. However, preoxidation of the electrode surface in 2M nitric acid inhibits the methanol adsorption/oxidation reaction but not the tocopherol response and therefore allows highly sensitive amperometric detection.