A complete mass balance of the conversion of headspace H2 and absorbed CO2 to CH4 and acetate was conducted to quantify the bioavailable CO2 in the amine solutions. The bioavailable CO2 in the amine solutions were evaluated in a setup resembling the biocompatibility experiments. The gas to liquid volume ratio was increased by 6.4-fold to 9.9 (Vgas/Vliquid) to ensure excess volumes of H2 were readily available for conversion. The gas to liquid ratio increment was achieved by using batch bottles (borosilicate serum bottles with rubber-butyl stoppers) with a volume of 250 mL, while the total mixture volume of the amine and inoculum was scaled down to 23 mL. The pressure and headspace gas concentration were evaluated at the beginning and at the end of each experimental run to determine the required CO2 consumption for the associated quantified production of CH4.