One milliliter supernatant aliquot in acetonitrile was transferred into 1.5mLmicro-centrifuge tubes containing 50 mg PSA sorbent and 100 mg anhydrous MgSO4. The vial was tightly sealed and vortexed for 1 min, and then centrifuged for 5 min at 5000 rpm to separate solution from solid. The solution was transferred into the 1.5mL sample vial and then placed in the
autosampler and analyzed by LC–MS