Ph buffer for organic acid analysis

The concentration of VFA was determined using high-performance liquid chromatography (Shimadzu Corp., Kyoto, Japan) after diluting the supernatant 3 times with distilled water. Briefly, the analytical specifications were as follows: column, Shim-pak SCR-102H (7 mm, i.d. 8.0 mm × 300 mm, Shimadzu Corp., Kyoto, Japan); eluent flow rate and mobile phase for organic acid analysis (Shimadzu Corp., Kyoto, Japan) at 0.8 mL/min; column temperature, 40 °C; reaction reagent and flow rate, pH buffer for organic acid analysis (Shimadzu Corp., Kyoto, Japan) at 0.8 mL/min; conductivity detector (CDD-10AVP, Shimadzu Corp., Kyoto, Japan). Quantification of the VFA concentration was performed using an external standard quantitation method [14 (link)].
The NH₃-N concentration was measured by diluting samples 50 times with 0.1 M phosphate buffer (pH 5.5) and then they were analyzed following the procedure of the modified Fujii—Okuda method [19 (link)] using an NH₃ kit (FUJIFILM Wako Pure Chemical Corp, Osaka, Japan). The plate was read by a microplate reader (SH-1000 Lab, Corona Electric Co., Ltd., Japan) at an optical density of 630 nm.

The headspace gas samples were analyzed by injection of 1 ml of the gas using Hamilton gastight syringe (Hamilton Company, Reno, Nevada, USA) in a gas chromatograph (GC-8A, Shimadzu Corp., Kyoto, Japan) as described previously (22 (link)).
VFA were analyzed using high-pressure liquid chromatography (HPLC) (23 (link)). Briefly, the analytical specifications were as follows: column, Shim-pak SCR-102H (7 mm, i.d. 8.0 mm × 300 mm, Shimadzu Corp., Kyoto, Japan); eluent flow rate and mobile phase for organic acid analysis (Shimadzu Corp., Kyoto, Japan) at 0.8 ml/min; column temperature, 40°C; reaction reagent and flow rate, pH buffer for organic acid analysis (Shimadzu Corp., Kyoto, Japan) at 0.8 ml/min; conductivity detector (CDD-10AVP, Shimadzu Corp., Kyoto, Japan). Quantification of VFA concentration was performed using the external standard quantitation method.
For measuring the concentration of NH₃-N, samples were diluted 100 times using 0.1 M phosphate buffer (pH 5.5) and then analyzed according to the Modified Fujii-Okuda method (24 (link)) using NH₃ kit (FUJIFILM Wako Pure Chemical Corp., Osaka, Japan).