The initial oven temperature was 50 ◦C, held for 1 min, ramped at 5 ◦C min−1 to 120 ◦C, held for 40 min, and then ramped at 3 ◦C min−1 to 250 ◦C. Mass detector was conducted with electronic impact (EI) mode at 70 eV, source temperature of 250 ◦C, scanning rate of 1 scan s−1 and mass acquisition range between 35 and 550 amu.