One hundred fifty–microliter serum samples were spiked with calibration and internal standards and diluted with a 5% methanol solution containing 0.1% acetic acid. Samples were extracted using solid-phase extraction (HLB Oasis, Waters Chromatography) and analyzed using an high performance liquid chromotography system coupled to a 6460 Agilent Triple Quadrupole mass spectrometer (Agilent Technologies) [27 (link)].