Highly Sensitive Analysis of Norethisterone in Human Serum by LC Coupled with Atmospheric Pressure Photoionization Tandem Mass Spectrometry

High-performance liquid chromatography coupled with tandem mass spectrometry has been used for sensitive and specific quantitative analysis of norethisterone (NE) in human serum. NE and the internal standard fentanil were isolated by solid-phase extraction. Chromatographic separation was achieved on a 4.6 mm × 50 mm, 5-μm particle, C₁₈ column. The mobile phase was 70:30 (v/v) methanol–0.5% aqueous formic acid. NE and the internal standard were detected by multiple-reaction monitoring of precursor/product ion combinations at m/z 299.4/231.2 and 377.1/188.1, respectively; an atmospheric-pressure-photoionization source was used in positive-ion mode. Linearity was established in the concentration range 0.2–49.24 ng mL^?1 and the lowest limit of quantification was 0.2 ng mL^?1. Recovery of NE ranged from 92.54 to 102.74% and relative standard deviations were <15%. The method was used for a pharmacokinetic study of NE in healthy postmenopausal Chinese female volunteers.