After mixing with the internal standards tetrahydropalmatine (IS1) and rosmarinic acid (IS2), plasma samples were pretreated by protein precipitation using acetonitrile. The HPLC analysis was performed on an Agilent Eclipse plus C18 (4.6 mm × 100 mm, 1.8 μm) column with mobile phase consisting of 0.1% formic acid aqueous solution and acetonitrile at a flow rate of 0.3 mL min−1.