N alkanes c8 c30

The GC-MS analysis of essential oil was carried out on a Thermofocus DSQII (Thermo, Wilmington, DE, USA), equipped with an HP-5MS column (30 m × 0.25 mm, film thickness 0.25 μm). The oven temperature was programmed from 60 °C (1 min isothermal) to 300 °C at a rate of 15 °C/min, and then kept for 8 min with a carrier gas at 1.0 mL/min. Both the temperatures of the MS transfer line and the injector were set at 250 °C. One microliter of essential oil sample was injected (in split mode 20:1). The mass spectra were obtained at 70 eV with a mass scan range of 41–400 amu. Compounds of essential oils were identified by matching their mass spectra with data bank mass spectra in NIST5.0 (National Institute of Standards and Technology), and comparing their retention to a homologous series of n-alkanes C₈–C₃₀ (Sigma-Aldrich, Steinheim, Germany) under the same operating conditions.

All 23 standard compounds used for the quantification analysis were purchased from Sigma-Aldrich (purity 99.0%, all chromatographic grade), including butanoic acid, acetic acid, thujone, methyl eugenol, p-cymene, α-pinene, diallyl disulphide, α-terpineol, β-caryophyllene, β-myrcene, 1,8-cineole, D-limonene, linalool, methional, nonanal, phenylethanal, ethyl caprylate, 4-methyl-5-thiazoleethanol, (E)-2-octenal, (E)-2-heptenal, humulene, acetophenone, and linalyl acetate. The internal standard (2,4,5-trimethylthiazole) and n-alkanes (C8–C30) used to calculate the retention index (RI) were of chromatographic grade and purchased from Sigma-Aldrich (St, Louis, MO, USA). Diethyl ether and n-pentane for dilution and preparation of sample were obtained from Thermo Fisher Scientific Inc. (Waltham, MA, USA). High purity helium (99.999% purity) and nitrogen (99.99% purity) were purchased and produced by Beijing Tianlirenhe Trading Co., Ltd. (Beijing, China).