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We developed a sensitive assay for ritodrine (RTD), a beta(2)-adrenergic agonist, in human serum. This method was based upon the selective and sensitive technique by a tandem mass spectrometry (MS/MS) using a hydrophilic interaction chromatography (HILIC) technique. This method involved a mixed-mode cation-exchange solid-phase extraction of RTD and isoxsuprine, the internal standard (IS), from serum with Waters Oasis (R) MCX cartridges. The detection was made using a Micromass Quattromicro (TM) API LC-MS/MS system with electrospray ionization source in positive ion mode. The separation of the analytes was achieved within 4 min on a silica column with a mobile phase of ammonium acetate (10 mM, pH 4.5) and acetonitrile (10:90, v/v). Multiple reaction monitoring was utilized by monitoring 288.2 -> 121.1 for RTD, 302.2 -> 107.0 for IS. The calibration curve for RTD was linear over a range of 0.5-1000 ng/mL. When 50 mu L serum was used for extraction, the lower limit of quantification was 0.39 ng/ml (97.5 fg on-column). The percent coefficient of validation for accuracy and precision (inter- and intra-day) was less than 9.8% and the recovery was ranged from 83.5 to 94.7% for RTD. This method enabled us to successfully determine RTD in maternal and fetal sera. (c) 2007 Elsevier B.V. All rights reserved.