Since microwaveheating is sample dependent and the microwave instrument
allowed pressure and temperature control only in a pilot vessel,
to ensure uniform extraction conditions, only samples of the same
type were processed together within each extraction cycle.
Depending on the food type, a sample post-treatment was
sometimes required after MAS. The pasta and bread samples did
not require any sample post-treatment and were injected directly
into the LC–GC system (after reconcentration of the hexane
extract). On the other hand, the biscuits and cakes required a sample
post-treatment and, after MAS, the sample extract was transferred
into a separatory funnel, washed with successive aliquots
of water and small amounts of methanol (avoiding agitation of
the sample during the first wash in order to prevent formation of
stable emulsions) until clear extracts and good phase separation
were obtained. As an alternative, once cooled, the vessels were
opened and added with about 20 mL of water and 3–4 mL of
methanol (without mixing), and left to rest for about 20 min at
20 C. An aliquot of the hexane extract was then washed with a
double volume of water in a screw-cap vial (vortex 1 min). The
hexane extract was directly used for LC–GC injection or an aliquot
(5 mL) was concentrated to 1 mL before injection, using an evaporation
system consisting of a centrifuge (Univapo 100 H, Uniequip
System; Martinsrieder, Munich, Germany) and a vacuum pump.
To remove interfering olefins (eluting in the MOAH fraction),
some biscuits and cake samples were epoxidised prior to LC–GC
determination. After MAS and wash with water, an aliquot of the
sample was added with 100 mg of a clean vegetable oil and underwent
epoxidation according to the procedure described by