2.6　GC-MSmethod

The analytical method was a modified version of the method published by Sieber［21］.Samples were analyzed by GC-MS with a QP2010 gas chromatograph-mass spectrometer(Shimadzu，Japan)equipped with a Agilent DB5-MS column(30m×0.25mm×0.1μm film).Helium was used as carrier gas at a flow rate of 1mL/min.Samples(1μL)were injected in split mode(1∶10)；the injector temperature was 280℃.The oven temperature was increased from 60℃to 85℃at 3℃min—1，then from 85℃to 280℃at 10℃min—1，and at 280℃for 8min.The total run time was 35.8min.The ion-source temperature and interface temperature were 230℃and 250℃，respectively.The detector operated in scan mode from m/z 60 to 750.(https://www.daowen.com)