Determination of short chain fatty acid (SCFA) concentrations

SCFA concentrations (μM) of the gut-model fluid samples in vessel 3 were quantified at 20 representative timepoints spanning the entire experimental period by GC-MS. Standard solutions of acetic acid, propanoic acid, butyric acid, isobutyric acid, valeric acid and isovaleric acid were prepared at different concentrations (100, 200, 400 and 800 μM). Injection was made in splitless mode with an injection volume of 1 μL. The volatiles were transferred onto a new Zebron ZB-FFAP column (30 m × 0.25 mm × 0.25 μm film thickness; Phenomenex, UK) and chromatogram with helium as the carrier gas at 18 psi. The GC starting temperature was 60°C, held for 1 min and ramped to 180°C at 8°C/min. All the compounds were detected with a DSQ mass spectrometer (Thermo Scientific) in scan mode, 20–150 mass:charge ratio). Identification of SCFAs was based on the retention time of standard compounds and with the assistance of the data library of the National Institute of Standards and Technology.