Ffa standards

FFA standards were purchased from Sigma-Aldrich (St. Louis, MO, United States) and Cayman Chemical (Ann Arbor, MI, United States). 2-Hydrazinyl-4,6-dimethylpyrimidine (DMP) and 2-hydrazinylpyrimidine (DP) were used for the derivatization of FFAs. DMP and DP reagents were obtained from ChemBridge (San Diego, CA, United States) and J&K (Beijing, China), respectively. 2-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) and ethyldiisopropylamine (DIEA) were purchased from Sigma-Aldrich and used as condensation agents for FFA derivatization. Organic solvents of MS grade, acetonitrile and methanol, were purchased from Fisher Scientific (Pittsburgh, PA, United States). Formic acid of MS grade was acquired from Sigma-Aldrich. Ultrapure water was prepared by using a Mili-Q purification system (Bedford, MA, United States).

AA standards, OA standards, FFA standards, internal standards (IS; norvaline, ¹³C₁-leucine, ¹³C₁-phenylalanine, ¹³C₂-succinic acid, 3,4-dimethoxybenzoic acid, and lauric-d₂-acid), ethyl chloroformate (ECF), and triethylamine (TEA) were purchased from Sigma-Aldrich (St. Louis, MO, USA). N-Methyl-N-tert-butyldimethylsilyl trifluoroacetamide (MTBSTFA) was obtained from Pierce (Rockford, IL, USA). Diethyl ether, ethyl acetate, toluene, and dichloromethane were purchased from Kanto Chemical Co. Inc. (Chuoku, Tokyo, Japan), and other reagents, including sulfuric acid, sodium hydroxide, and sodium chloride, were manufactured by Deajung (Gyeongido, Korea). All chemicals were analytical reagent grade.