Propionic acid

Unless otherwise specified, all reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA). Water (H₂O, # W6), methanol (MeOH, # A454) and acetonitrile (ACN, # A996) Optima^TM grade HPLC solvents were purchased from Fisher Scientific, Inc (Hampton, NH, USA). Formic acid (# 33015, ACS reagent grade). ¹²C- and ¹³C-SCFA standards used in this study were as follows: acetic acid, glacial (# AX0073, EDM Millipore, Burlington, MA, USA), propionic acid (# 94425), butyric acid (# 19215), isobutyric acid (# 46935-U), valeric acid (# 75054), isovaleric acid (# 78651), 2-methylbutyric acid (# 49659), acetic acid (1,2-13C₂, 99%) (# CLM-113, Cambridge Isotope Laboratories, Inc (Andover, MA, USA)), propionic acid-13C₃ (99 atom % ¹³C) (# 589586), butyric acid-1,2-13C₂ (99 atom % ¹³C) (# 491993). Deuterated internal standards isobutyric-d7 acid (# I789183), valericd9 acid (# V091417) and isovaleric-d9 acid (# I917572) were purchased from Toronto Research Chemicals (Toronto ON, Canada). Aniline (# 242284) and N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) (# E6383) were obtained from Sigma-Aldrich.