2.4.1. GC–MS Conditions

The samples were homogenized separately. Five grams of the samples were taken into a 20 mL glass vial, and then 10 μL of 2-nonanone (0.0113 g L⁻¹) was added after adding 1.5 g NaCl. A 50/30 μm DVB/CAR/PDMS (SigmaAldrich, St. Louis, MO, USA) extraction head with sorptive properties for aromatics was selected. According to the instructions for use, the extraction head was required to be aged at 270 °C for 1 h before the initial use, and then activated at 270 °C for 30 min before each use and set aside [23].

Chromatographic separation was carried out on a HP-INNOWax column coated with 100% polyethylene glycol of 60 m × 0.25 mm i.d., 0.25 µm film thickness (Agilent, Inc). The oven temperature started at 40 °C, ramped to 130 °C at a rate of 5 °C min⁻¹, ramped to 150 °C at a rate of 3 °C min⁻¹, ramped to 230 °C at a rate of 6 °C min⁻¹ and held for 2 min. Finally, the oven temperature ramped to 250 °C at a rate of 10 °C min⁻¹ with a total run time of 45 min.

Electron impact (EI) mass data from m/z 30 to 500 were acquired and with an ionization voltage of 70 eV. The ion source temperature was maintained at 230 °C. In addition, the temperature of the quadrupole was maintained at 150 °C. The carrier gas was 99.999% high-purity helium, and the flow rate was 1.1 mL min⁻¹.