Rapid LC-APCI-MS-MS Method for Simultaneous Determination of Phenacetin and Its Metabolite Paracetamol in Rabbit Plasma

A highly sensitive liquid chromatographic-atmospheric pressure chemical ionization-tandem mass spectrometric method is developed to quantitate phenacetin and its metabolite paracetamol in rabbit plasma. The analytes and internal standard oxazepam are extracted from plasma by liquid–liquid extraction using ethyl acetate, and separated on a Zorbax SB-C₁₈ column (2.1 mm × 150 mm, 5 μm) using acetonitrile–0.1% formic acid in water (40:60 v/v) at a flow of 0.4 mL min^?1. Detection is carried out by multiple reaction monitoring on a ion-trap LC-MS-MS system with an atmospheric pressure chemical ionization interface. The assay is linear over the range 4–1,600 ng mL^?1 for phenacetin and 3–2,000 ng mL^?1 for paracetamol, with a lower limit of quantitation of 4 ng mL^?1 for phenacetin and 3 ng mL^?1 for paracetamol. Intra- and inter-day precision are less than 7.1% and the accuracy are in the range 97.3–103.5%. The validated method is successfully used to analyze the drug in samples of rabbit plasma for pharmacokinetic study.