The method of analysis followed that described by Fankhauser-
Noti and Grob (2006). Briefly, 10 ml of internal standard solution was
added to 150 mg oil weighed into an autosampler vial and filled up
to 1.5 ml with hexane. The solution was agitated on a vortex mixer
for 20 s and 3 ml injected into GC-mS. A Trace gas chromatograph
(Thermo Finnigan, Milan, Italy), equipped with a programmed
temperature vaporizing (PTV) injector, with an AS 2000 autosampler
and coupled to a mass spectrometer MD 800 (Fisons, Milan,
Italy) was used. The gas chromatography was also supplied with
a pre-column backflush system (optional on this instrument). A
three-way switching valve in the gas supply line directed the gas
either to the injector or towards the T-piece between the precolumn
and the separation column, as described by Fankhauser-
Noti and Grob (2006).