GC-MS analysis
The laser captured samples (with the same size as in LCM) were extracted with 100 μL hexane. After sonicating for 60 min, the sample was centrifuged at 12,000 rpm, 25℃ for 10 min. 90 μL of the supernatant was transferred into a brown vial with a glass insert and stored at 4℃ before analysis.
GC-MS analysis was performed on a Thermo-Fisher Scientific gas chromatography (Waltham, MA, USA) combined with triple quadrupole mass spectrometer system (TP-8030) and fitted with a DB-5MS (5% phenyl methyl polysiloxane) capillary column (30 m × 0.25 mm, 0.25 μm) with helium as a carrier gas at 1 mL/min. The column temperature was programmed initially at 120 ℃, and then increased to 200 ℃ at 5 ℃/min, and then increased to 240 ℃ at 10 ℃ / min, held for 5 min. The mass spectrometer was operated in electron ionization (EI) mode at 70eV, with an interface temperature of 250 ℃, an ion source temperature of 250 ℃ and a continuous scan from 50 to 550 atomic mass unit (amu). Peaks were identified by comparison with the mass spectra data from the National Institute of Standards and Technology (NIST) spectral library.
Standards were accurately weighted and dissolved in hexane at a concentration of about 0.5 mg/mL. The appropriate amount of every standard solution was diluted respectively with hexane to a series of concentration for establishment of the calibration curves (Table S2). The limit of detection (LOD) and limit of quantitation (LOQ) were determined by injecting a series of diluted standard solutions until the signal-to-noise (S/N) ratio was 3 for LOD and 10 for LOQ in all quantitative analysis.