Then 25 ìL of IS (3-
(4-chlorophenyl) propionic acid, 0.8 ìg mL.1 in methanol), 75 ìL of
methanol and 50 ìL of pyridinewere added and the mixturewas shaken
for 10 s. Finally, 20 ìL of methyl chloroformate was added and the mixture
was carefully mixed and sonicated for 60 s. The desired analytes
were extracted with 200 ìL of hexane for 1 min and 100 ìL of hexane
layer was transferred into a 400 ìL glass insert placed in a 1.5 mL glass
vial, and crimped.