Capillary column was HP-5MS (30 m × 0.25 mm × 0.25 μm). The original oven temperature was set at 60 °C for keeping 1 min and risen at 10 °C/min to 180 °C for keeping 1 min, and then risen at 20 °C/min to 280 °C for keeping 15 min. The injector temperature was maintained at 270 °C. The samples (1 μL, dilute to 1% with hexane) were injected with a split ratio of 1:10. The carrier gas was helium at flow rate of 1.0 mL/min. The retention indices (RI) were calculated using a homologous series of n-alkanes (C5–C36) under the same operating conditions.