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A new method Is presented for the detection of thioethers after separation by high-performance liquid chromatography (HPLC). The method Is based on the postcolumn oxidation of the thioethers by bromine, which Is generated electrochemlcally In the column effluent. Detection Is performed by amperometric measurement of the excess of bromine downstream, after reaction with eluting thioethers. For the determination of ampicillin In plasma and urine a superior sensitivity and selectivity compared to the usual UV detection Is shown. Plasma concentrations from 0.2 to 10 pg/mL can be determined, with a precision of ±6 %. Attention has been paid to the kinetics of the reaction of bromine with various penicillins and their degradation products. Ranitidine can be determined In plasma after sample cleanup and concentration by liquid-liquid extraction with a lower limit of detection of 2 ng/mL. On the other hand, a much better reproducible method was developed without extraction steps, with which ranitidine levels from 10 to 300 ng/mL can be measured, with a precision of ±10% in the 100-300 ng/mL range. With this procedure an Internal standard does not have to be used.

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