The GC analyses were carried out on an Agilent 6890 GC–FID apparatus equipped with an SPTM-2560 (‘Supelco’) (100m×0.25mm I.D., 0.20 m film thick- ness) bicyanopropyl polysiloxane fused-silica capillary column. The injector was kept at 250◦C and the detector at 260◦C. Helium at a constant pressure of 42.7 p.s.i. was used as a carrier gas with reten- tion time locking. The column was maintained at 50◦C for 2min and then programmed to 190◦C at 5min and kept for 20min at 190◦C. Identification of compounds was performed by comparing their relative retention indices with those of authentic samples.