Oxygen effects in amperometric liquid chromatographic detection of omeprazole at a mercury electrode

Abstract

The determination of omeprazole, a reducible benzimidazolyl pyridyl methyl sulphoxide, was investigated in a reversed-phase liquid chromatographic system by electrochemical detection at a static mercury drop electrode. The mobile phase was composed of acetonitrile and phosphate buffer (pH 7.6). In the presence of oxygen dissolved in the mobile phase, omeprazole could be indirectly detected at potentials between +0.2 and −0.8 V vs. Ag/AgCl, where the compound is not electroactive. The resulting chromatographic peak is explained by the interference of omeprazole with the electrochemical reduction of oxygen residuals in the mobile phase. The adsorption of omeprazole at the mercury electrode is suggested as a prerequisite for the appearance of the peak, as the effect was not observed for compounds with limited adsorption properties. In a thoroughly deoxygenated system, the oxygen level was decreased to about 0.2% of the air-saturated value. Detection at −1.2 V, where omeprazole is reducible, resulted in a detection limit of 5 pmol (2 ng).