Rapid determination of omeprazole in human plasma by protein precipitation and liquid chromatography–tandem mass spectrometry

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 × 2 mm, 3 μm 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.