The GC/MS analysis was performed using a CP-3800 gas chromatograph (VarianInc.) coupled to a Saturn 2000 ion trap mass spectrometer (VarianInc.). The gas chromatograph was equipped with an electron impact ion source, split–splitless injector and an autosampler. A high polarity capillary column with across-linked and bonded polyethylene glycol (PEG) phase (HP-INNOWax, 30 m * 0.25 mm i.d coated with 0.25 μm film thickness, Agilent Technologies Inc., Denmark) was used for separation of the SCFA. GC/MS conditions for determining both the isotope enrichment and concentration were as follows: positive electron impact mode; injector temperature, 285 °C; helium constant flow,1 ml/min; initial column temperature was 100 °C and hold for 1 min, then increased at the rate of 10 °C/min to 120 °C and hold for 5 min, and finally increased at the rate of 30 °C/min to 220 °C and kept at 220 °C for 3 min; solvent delay time, 3 min; total run time, 14.3 min. The volume injected was 1 mL in 1:10 split mode. All the samples were analyzed in full scan mode with a mass range of m/z 40–300. Finally, the following extracted ions were used for determining the concentration and isotope ratio of SCFA: m/z 60 for unlabeled C2, C4, i-C5 and C5; m/z 61 for [1-13C]acetate/[2-13C]acetate; m/z 62 for [U-13C]acetate; m/z 57 for t-C5; m/z 73 for C3 and i-C4. The total concentration of acetate was determined from sum of ion currents of m/z 60, 61and 62. Data acquisition and analysis was done with Saturn GC/MS workstation software (VarianInc.)