3 4 dimethylphenol solution

5-hydroxymethylfurfural (HMF) was analysed by GC-MS. The equipment used was an Agilent Technologies 6890 N-MSD-5973 N gas chromatography-mass spectrometer (GC-MS). Chromatography was performed according to Loira et al. (2013) with a DB-WAX column (30 m × 0.25 mm × 0.25 µm) (J&W Scientific, Folsom, CA, USA). The method was calibrated using external standards (Merck, Hohenbrunn, Germany) . A liquid extraction with dichloromethane was done before the chromatographic separation. 2.5 mL of must was mixed with 250 µL of dichloromethane and 25 µL of a 3,4-dimethylphenol solution (10 mg/L; Merck, Hohenbrunn, Germany) as IS; 0.37 g of NaCl was added too and stirred in vortex for 5 min. After centrifugation at 7500 rpm and 4 °C for 15 min, the dichloromethane phase was recovered and 1 µL was injected into the GC-MS. The split ratio used was 20:1. The helium flow rate was 1 mL min -1 . The oven temperature programme was 60 °C for 1 min, followed by a 3 °C min -1 ramp until 150 °C, followed by a 10 °C min -1 ramp to 260 °C, then held for 2 min. The mass spectrometer operated at 70 eV. The analysis was performed in SIM mode.