After extraction, the stir bar was removed with forceps,
rinsed briefly in distilled water, and dried with a lint-free tissue.For solvent
back extraction the stir bar was placed into a 350 μL glass insert containing
200 μL of the solvent mixture dichloromethane/hexane, 50:50. This insert
was immersed into a 2 mL vial closed by a PTFE/silicone septum and
stirred at 1000 rpm for 40 min again.
For the determination of free medium-chain fatty acids (C6-C12) 2 μL
of this extract was injected into the GC column and the same chromatographic
parameters were applied as in the SPE method.
For the determination of long-chain fatty acids (C14-C18) this
SBSE extract was evaporated and methylated with 0.1 mL of BF3-
methanol 14% during 20 min at 95 C. Reaction was stopped by the
addition of 0.1 mL of water, and after shaking in an ultrasonic bath,
long-chain fatty acidmethyl esters were extracted with 0.1mL of hexane in
an ultrasonic bath for 2 min. GC separation was the same as described
above.