Type 5975 c

VOCs were analyzed using the industrial standard methods for air and particulate monitoring^{77 ,78} . The VOC concentration was measured using a thermal desorbed instrument (Tekmar 6000/6016) interfaced with a gas chromatograph (HP 7890 B) and mass selective detector (HP 5977 A, AMA Co., Germany). The working conditions of the TDS were as follows: gas pressure of 20 kPa, inlet temperature of 250 °C desorption temperature of 250 °C for 10 min; cold trap temperature held at 120 C for 3 min, followed by a rapid increase to 260 °C. An HP-5MS column (50 m, i.d. 0.25 mm, and film thickness 0.25 m) was used for chromatographic separation. The temperature program was 40 °C for 3 min followed by 10 °C/min up to 250 °C for 3 min and then an increase to 270 °C. The ion energy of the MS (type 5975 C, Agilent) was 70 eV; the ion source temperature was 230 °C; the Quadrupole temperature and the interface temperature were 150 and 280 °C, respectively; and the mass spectrometry scanning mass ranged from 28 to 450 m/z. The retrieval and qualitative analysis of the mass spectrometry data were accomplished using the NIST 2008 library, which was housed in the computer of the temperature instrument. In addition, the chromatographic peak area normalization method was used for the calculation of the relative concentrations of the mass spectrometry data.

Stool SCFAs were measured using a gas chromatography-mass spectrometry (GC-MS; Gas chromatograph (type 7890A, Agilent, USA) equipped with a mass selective detector (type 5975C, Agilent, USA). The measurement details were described in our previous study (30 (link)) and Supplementary File 1. The standards, including acetate-d3, propionate-d5, butyrate-d7, valerate-d9, isobutyric acid, 2-methylbutyric acid, and 3-methylbutyric acid (isovaleric acid), were purchased from Sigma (St. Louis, MO, USA). A set of diluted standards was prepared (1,000 μg/mL, 100 μg/mL, 10 μg/mL, 1 μg/mL, 0.1 μg/mL, and 0.01 μg/mL) with ethyl acetate before analysis. Data analysis was performed with ChemStation (Agilent, CA, USA, v E.02.02.1432) and Chroma TOF software (LECO, St. Joseph, MI, USA, v 4.34). SCFA concentrations in fecal samples were reported in milligrams per gram (mg/g) of feces.