The optimized procedure for sample preparation is as follows: 100μL of serum was mixed with 100μL of sodium acetate with a pH of 4.0 (0.1molL−1) in a1.5mL Eppendorf tube. Then25μL of IS(3(4-chlorophenyl) propionic acid,0.8μgmL−1 in methanol ),75μL of methanol and 50μL of pyridine were added and the mixture was shaken for 10s. Finally ,20μL of methyl chloroformate was added and the mixture was carefully mixed and sonicated for60s.The desired analytes were extracted with 200μL of hexane for 1 min and 100μL of hexane layer was transferred into a 400μL glass insert placed in a 1.5mL glass vial,and crimped.