Sample preparation
Liquid phase extractionThe procedure for liquid phase extraction of sample componentshas been reported previously [8–14]. Briefly, 200 L of plasma wasadded to 800 L acetonitrile, the mixture vortexed for 5 min, fol-lowed by centrifugation at 2500 RCF for 5 min. The supernatant waspipetted to another tube containing 2 mL dichloromethane. After5 min vortexing and 5 min of centrifugation at 2500 RCF, the upperaqueous layer was aspirated for HPLC analysis.