Cleanup was performed using a silica gel method (USEPA,1996). Deactivated silica gel (1.5 g) was transferred into a 1-cm ID column and a 1-cm layer of anhydrous Na2SO4 was used to cap the silica gel. The column was prewashed with 20 mL of hexane before it was used. The concentrated extract was transferred into the column, and then it was eluted with 8 mL of hexane (fraction I), 6 mL of hexane (fraction II) and 1.5 mL of dichloromethane (fraction III) at a rate of 5 mL/min.