4.3. SBSE-GC-MS Analysis

The SBSE-GC-MS analysis was performed on an Agilent 7890 GC-5975 MSD system (Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a multi-purpose sampler (MPS) (Gerstel, Inc.,). The sample holder was put into the thermal desorption unit (TDU) with a splitless mode. The initial temperature of TDU was 35 °C, then increased to 220 °C at a rate of 120 °C/min, and further maintained for 3 min. Then, the volatile compounds reached a programmed temperature vaporizing (PTV) injector. The pressure was 26 psi, and purge flow to split vent was 50 mL/min at 4 min. The compounds were trapped in the cooled injection system (CIS-4) at −80 °C with liquid nitrogen. Then, the CIS was heated at 10 °C/s to reach 220 °C and held for 3 min. Separation was achieved on a ZB-WAX column (60 m × 0.25 mm ID, 0.5 μm film thickness, Phenomenex, Torrance, CA, USA). The initial temperature was 40 °C, held for 4 min, then increased to 230 °C at a rate of 4 °C/min and maintained for 10 min. The total run time was 61.5 min, with the helium flow rate at 1.5 mL/min. Mass spectrometric detection was performed in selective ion monitoring (SIM) mode. The MS transfer line and ion source temperature was 280 °C and 230 °C, respectively. Selective mass ions were used to quantify the volatile phenols. Four groups were set. Selected ion of group one started from 0 min. Selected ions of group two started from 39 min (guaiacol, cis-whiskey lactone, 4-methylguaiacol, and trans-whiskey lactone). Selected ions of group three started from 44.5 min (o-cresol, phenol, 4-ethylguaiacol, p-cresol, and m-cresol). Selected ions of group four started from 48 min (4-ethylphenol and 4-vinylguaiacol).