To compare the SFE with other common extraction methods,
the effect of solvent type and extraction time on the oil yield
obtained by Soxhlet extraction was also evaluated and the results
are shown in Fig. 4. The experimental results were the average of
three measurements, with error bars showing the standard deviation
of these measurements. Ethanol and hexane were chosen in
this study as the extraction solvent. Fig. 4 shows that the oil yield
rapidly increased with increasing extraction time till ca. 2 h, but
levelled off with further increasing from 2 h to 8 h. However, the
solvent type had a more significant effect on the oil yield, with ethanol
showing much higher oil yield than hexane over the same
extraction time. For example, the oil yield extracted by ethanol
at 8 h was 36.33% which was nearly four times that by hexane at
7.90%. In order to explain the difference, the total ion compound
chromatograms of the Eucalyptus leaves extracts obtained by the
two solvents were acquired using a GC–MS. The results are shown
in Fig. 5 and the detailed compounds information is listed in the
Supplementary information. It is obvious that the compositions