Combustion and reduction method gas sample for radiocarbon measurement using an automatic reduction system

Full text: Methane has been a major sample for AMS radiocarbon measurement since the gas is known one of causes of the global warming [1-4]. Not only atmospheric methane, but also biproduct methane of bioindustries. For the atmospheric methane measurement, large size of air should be treated to obtain proper amount of carbon for AMS measurement with reasonable statistic precision. In many cases, atmospheric methane samples are measured using gas ion source with CO₂ from after combustion process because of the limitation of carbon size. However, since the industrial methane samples are mostly high purity methane, sufficient carbon can be obtained from small size of sample. And graphite is preferred rather than gas sample. A simple and fast procedure to convert methane to graphite was developed using an automatic reduction system in KIGAM AMS Lab [5]. Biproduct methane gas was collected from a production process of a food industry, in which starch is produced from corn. The methane sample was purified cryogenically, and mixture gas of 1 mg of the methane and oxygen was reacted at 600°C in a quartz reaction tube with the volume of 8.72 ml of the reduction system. The combustion process was completed within roughly three hours. CO₂ gas was purified cryogenically, and reduced to graphite with H₂ gas and Fe catalyst in the same tube. Finally, 0.69 mg of graphite was obtained with reduction yield of 91 %. The graphite was measured with 1 MV AMS at KIGAM. The raw material, corn, and methane were measured to be 101.07 ± 0.44 pMC and 99.24 ± 0.44 pMC, respectively. For the methane measurement with high content from industrial process, graphite could be obtained with the simple treatment process. The precision of the measurement was better than gas measurement.