A rapid, sensitive and reliable method was developed to quantitate omeprazole in human plasma using liquid chromatography-tandem mass spectrometry. The assay is based on protein precipitation with acetonitrile and reversed-phase liquid chromatography performed on an octadecylsilica column (55 mm x 2mm, 3 microm particles), the mobile phase consisted of methanol-10 mM ammonium acetate (60:40, v/v). Omeprazole and flunitrazepam, the internal standard, elute at 0.80+/-0.10 min with a total run time 1.35 min. Quantification was through positive ion mode and selected reaction monitoring mode at m/z 346.1-->197.9 for omeprazole and m/z 314.0-->268.0 for flunitrazepam, respectively. The lower limit of quantitation was 1.2 ng/ml using 0.25 ml of plasma and linearity was observed from 1.2 to 1200 ng/ml. Within-day and between-day precision expressed by relative standard deviation was less than 5% and inaccuracy did not exceed 12%. The assay was applied to the analysis of samples from a pharmacokinetic study.

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