A high-precision measurement system for carbon and hydrogen isotopic ratios of atmospheric methane and its application to air samples collected in the western Pacific region

Abstract

In order to study temporal and spatial variations of atmospheric CH4 quantitatively, we originally improved a measurement system for carbon and hydrogen isotopic ratios (δ13C and δD) of CH4 to attain high-precision measurements. By analyzing 100 mL aliquots of an ambient air sample, the precision of our system is 0.08%0 for δ13C and 2.2%0 for δD (1σ), which are one of the highest precisions reported so far. The system consists mainly of a CH4 preconcentration device and a continuous-flow gas chromatograph isotope ratio mass spectrometer equipped with a combustion furnace and a pyrolysis furnace for measurements of δ13C and δD. The preconcentration trap temperature was maintained at -130 ± 1°C during collection of CH4 from the air sample by passing it through the trap, then at -83 ± 1°C while remaining air components such as N2 and O2 except for CH4 escaped, and finally at 100 ± 1°C for CH4 elusion. The isotopic values are measured on a mass spectrometer, relative to respective reference gases. For this study, the δ13C and δD values of the reference gases were calibrated against our primary standards provided by the IAEA: our δ13C primary standard is NBS18, whereas our δD primary standards are V-SMOW and SLAP. To ensure the long-term stability and reproducibility of our measure-ment system, a calibrated whole air stored in a high-pressure cylinder, which was called "test gas," was measured at least twice on each day when sample measurements were made. To measure small air samples, such as those extracted from ice cores, we also examined the relation between the sample size and the measured value of δ13C and δD: gradual enrichment of the δ13C occurred with decreasing CH4 content less than 8 nmol whereas no such effect could be seen for the δD. Furthermore, preliminary results of latitudinal distributions of δ13C and δD were discussed along with CH4 concentrations obtained by our shipboard air-sampling program. © 2009, Meteorological Society of Japan.