3 nitrophenylhydrazine 3 nph

Formic acid was of analytical grade and obtained from Sigma-Aldrich (St. Louis, MO, USA). Methanol (Optima LC-MS grade), acetonitrile (Optima LC-MS grade), and isopropanol (Optima LC-MS grade) were purchased from Thermo Fisher Scientific (FairLawn, NJ, USA). Sable isotope-labeled internal standards and the derivatization regents, 3-nitrophenylhydrazine (3-NPH) and N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide (EDC)·HCl were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ultra-pure water was produced by a Milli-Q system equipped with a LC-MS Pak filter (Millipore, Billerica, MA, USA).
All of the standards were purchased from TRC Chemicals (Toronto, ON, Canada), Sigma-Aldrich (St. Louis, MO, USA) and Steraloids Inc. (Newport, RI, USA). They were accurately weighed and dissolved in appropriate solutions to obtain individual stock solutions at the concentration of 5.0 mg mL^− 1. Appropriate amount of each stock solution was mixed to get stock calibration solutions.

MS grade methanol, acetonitrile, isopropanol, and formic acid were purchased from Sigma-Aldrich (St. Louis, MO). Ultrapure water was prepared by the Milli-Q system (Millipore, Billerica, MA). All the reference standards and stable isotope-labeled internal standards were purchased from Sigma-Aldrich (St. Louis, MO), Steraloids Inc. (Newport, RI), TRC Chemicals (Toronto, ON, Canada), and Nu-Chek Prep (Elysian, MN). The derivatization regents including 3-nitrophenylhydrazine (3-NPH) and N-(30(dimethylamino)propyl)-N'-ethylcarbodiimide (EDC) were purchased from Sigma-Aldrich (St. Louis, MO).

The ultrapure water, formic acid, methanol, and acetonitrile were purchased from Fujifilm Wako Pure Chemicals (Osaka, Japan). The 3-Nitrophenylhydrazine (3-NPH) and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) hydrochloride were purchased from Sigma-Aldrich (St. Louis, MO, USA). The standards for the target compounds and internal standards used for quantitative analysis are listed in Supplemental Table S1. The ¹³C₃-3-Hydroxypropionic acid and ¹³C₄-methylmalonic acid were used as internal standards for 3-hydroxypropionic acid and methylmalonic acid, respectively. For the other compounds, 2-ethylbutyric acid was used as the internal standard.