Carbon Isotope Measurement of Extremely Low Amounts of CH4: Application to Volcanic Gases from Satsuma-Iwojima, Japan

ments from volcanic gases have been carried out to identify their origins and behavior.1 Especially, knowledge of the carbon isotope ratio (13C/12C) is very important for studies performed in volcanic and geothermal areas, because of the existence of several carbon gases, such as CO2, CO, CH4, COS, CS2. Early studies2,3 showed that geothermal CH4 is enriched in 12C relative to CO2. Isotope equilibrium between CO2 and CH4 was suggested in some geothermal gases, based on the calculated fractionation factors between them.4,5 Also, the 13C exchange rate between CO2 and CH4 was also calculated.6 Previous carbon isotopic studies of CO2 and CH4 in volcanic volatiles were limited to geothermal gases of medium – low temperature. Although high-temperature volcanic gas is considered to be the best example to discuss isotopic equilibrium as well as chemical equilibrium, carbon-isotope data of CH4 is very scarce. One of the reasons for the almost no available data is that the CH4 content in high-temperature volcanic gases is usually extremely low to analyze the carbon-isotope composition. A conventional standard carbon isotope analysis without preconcentration is usually applied to more than 50 nmol CH4, which is equal to the CH4 content in 1 ml of a 1000 ppm sample. In addition, reliable carbon-isotope data of CO, whose content in high-temperature volcanic gas is usually higher than that of CH4, are also few, due to the oxidation of CO during storage of the gas sample after collection. In this study, a preconcentration system attached to a GC/C/MS (gas chromatography/combustion/mass spectrometer) was developed in order to analyze the carbonisotope composition of CH4 with extremely low content and CO in volcanic gases.