Before and after each oil sample blank values were analyzed. After sampling had been carried out, the SPME device was placed immediately into the GC–MS instrument. For separation of volatile compounds a 60 m × 0.25 mm (inside diameter) RTx-5 (Restec) non-polar column, with a film thickness of 0.25 μm was attached to a Hewlett-Packard HP-6890 model gas chromatograph equipped with an HP-5972 mass selective detector. The following column temperature program sequence was used.