After injection, the temperature started at 120 °C for 1 min and was subsequently increased to 220 °C with a rate of 4 °C/min. Finally, the temperature increased to 290 °C with a rate of 35 °C/min. Prior to the analysis, a dried sample (50 mg) was suspended in aqueous sulphuric acid (72%, 0.5 ml) and stirred at 30 °C for 1 h.