2.7. Sample preparation
The prepared sample (30 mg) was weighed in a 5 mL glass centrifuge tube and then mixed with 2 mL of ethanol. Extraction was performed with ultrasonication for 30 min. Sample was centrifuged at 4000 r/min for 10 min, then the supernatant was collected, and 1 mL of ethanol was added into the residue for further extraction. The twice supernatant was united and evaporated to dryness under a gentle stream of nitrogen. Then it was redissolved in acetonitrile for the next analysis.