2.5　Sample treatment for GC-MS

Serum was thawed and vortex-mixed.Each 150-μL aliquot of the serum sample was deproteinated by addition of 450μL acetonitrile followed by vortex mixing and centrifugation(12，000 rpm，15min，4℃).The supernatant(150μL)was then transferred to a GC vial and volatilized to dryness with nitrogen by use of an SE812 pressured gas-blowing concentrator(Shuaien Technology，Beijing，China).Methoxyamine hydrochloride(20mg/mL，25μL)in dry pyridine was added to the dried sample，followed by vortex mixing for 5min and incubation at 40℃for 90min.Finally，25μL BSTFA with 1% TMCS was added，followed by vortex mixing for 5min and incubation for 60min at 70℃.(https://www.daowen.com)