Development and validation of a stereoselective liquid chromatography–tandem mass spectrometry assay for quantification of S- and R-metoprolol in human plasma

A stereoselective liquid chromatography–tandem mass spectrometry assay was developed and validated for quantification of S- and R-metoprolol at concentrations of 0.5–50 μg/L in human plasma. Metoprolol was extracted from plasma by liquid–liquid extraction with ethyl acetate (82% recovery). Chromatographic separation of the enantiomers was achieved on a chiral Chirobiotic T column using an isocratic mobile phase consisting of methanol/acetic acid/ammonia (100/0.15/0.15, v/v/v). An ion trap mass spectrometer with an electrospray interface was used for detection in the positive mode, monitoring the m/z transition 268 → 191 for metoprolol. Standard curves for S- and R-metoprolol fitted quadratic functions (r2 ≥ 0.9995) over the range 0.5–50 μg/L in plasma, with 0.5 μg/L representing the limit of quantification. In this range, relative standard deviations were <6% for intra-day precision and <10% for inter-day precision. The accuracy was within the range of 92–105%.