Liquid chromatography-tandem mass spectrometry analysis was performed on an AB Sciex 5500 QTrap mass spectrometer (Sciex, Foster City, CA) coupled with an Acquity UPLC system (Waters, Milford MA). The LC separation was achieved using a Zorbax XDB C18 column (4.6 × 50 mm, 5 µm) (Agilent, Santa Clara, CA) at room temperature. The mobile phase solvent A was water containing 0.1% formic acid and mobile phase solvent B was methanol containing 0.1% formic acid. The mobile phase was delivered at a flow rate of 0.3 ml/min. The initial mobile phase composition was 60% solvent A and 40% solvent B. From 0.5 to 4.0 min, solvent B was increased to 100% and conditions held until 5.0 min. At 5.1 min, the mobile phase composition was then returned to 40% solvent B until 6.0 min. The total runtime was 6 min.