Aim To develop and validate a liquid chromatographic/tandem mass spectrometric (LC/MS/MS) method for the simultaneous quantitation of pseudoephedrine and diphenhydramine in human plasma after oral administration of compound preparation. Methods Samples spiked with the analytes and the internal standard, chlorphenamine, were processed using liquid-liquid extraction. The extract was chromatographed on a Diamonsil C 18 column. The mobile phase consisted of methanol-water-formic acid (70∶30∶1), at a flow rate of 0.65 mL·min -1 . A Finnigan TSQ tandem mass spectrometer equipped with atmospheric pressure chemical ionization source was used as detector and was operated in the positive ion mode. Selected reaction monitoring (SRM) using the precursor/product ion combinations of m/z 166→148 and m/z 256→167 were used to quantify pseudoephedrine and diphenhydramine, respectively. Results The linear concentration ranges of the calibration curves for pseudoephedrine and diphenhydramine were both 2.0-400.0 μg·L -1 . The lower limit of quantification of both analytes was 2.0 ng·mL -1 . Each sample was chromatographed within 3.9 min. Conclusion The method was successfully used in simultaneous determination of pseudoephedrine and diphenhydramine in human plasma after oral administration of a compound preparation containing 60 mg pseudoephedrine chloride and 50 mg diphenhydramine chloride. The method was shown to be suitable for clinical investigation of pseudoephedrine and diphenhydramine pharmacokinetics.
