2.1 Chemicals and reagents
TGC (CAT Number: T440015) and internal standard TGC-d9 (CAT Number: 440017) were purchased from Toronto Research Chemicals INC (Toronto, Canada). HPLC-grade and acetonitrile were obtained from Sigma-Alorich (St.Louis, USA). Ammonium formate was provided by Guangfu Fine Chemical Research Institute (Tianjin, China) and formic acid was provided by Kemiou Chemical Reagent Co. Ltd (Tianjin, China).