GC–MS was performed with an Agilent 6890 gas chromatography instrument coupled to an Agilent 5973 mass
spectrometer and an Agilent ChemStation software (Agilent Technologies, Palo Alto, CA, USA). A capillary column
(30 m × 0.25 mm i.d.) coated with 0.25 m film 5% phenyl
methyl siloxane was used for separation. The column temperature was set at 100 ◦C and held for 5 min for injection, then
programmed at 5 ◦C min−1 to 145 ◦C and held for 25 min at
the temperature of 145 ◦C, then at 5 ◦C min−1 to 200 ◦C, and
finally, at 20 ◦C min−1 to 280 ◦C. Split injection (2 l) was
conducted with a split ratio of 10:1 and high-purity helium
was used as carrier gas of 1.0 ml min−1 flow-rate. The spectrometers were operated in electron-impact (EI) mode, the
scan range was 40–550 amu, the ionization energy was 70 eV
and the scan rate was 0.34 s per scan. The inlet, ionization
source temperature were 250 and 280 ◦C, respectively.