Keywords:carbon dioxide, high precision, triple oxygen isotope
The triple oxygen isotopic compositions (Δ17O) of tropospheric carbon dioxide (CO2) can be a useful tracer to quantify carbon cycle in terrestrial environments. Traditionally, CO2 must be converted to O2 to determine Δ17O precisely. However, toxic and dangerous reagents such as fluoride must be needed for the reactions to convert CO2 to O2. Alternative safer, more simple, and more easy techniques should be needed for the routine measurements on the Δ17O of tropospheric CO2. In this study, I developed an alternative new method for the high precision measurements on the Δ17O of CO2 using Cavity Ring-Down Spectroscopy (CRDS) for H2O. First of all, atmospheric sample with CO2 was introduced into pre-evacuated line and separated from atmospheric nitrogen (N2), oxygen (O2) and water (H2O) using the differences in the boiling points. Then, N2O having the similar molecular weight and the similar boiling point with CO2 was separated from CO2 using a PorapakPS column (3m long, 0.31cm i.d.) under -70 °C. By reacting the purified CO2 with H2 at 600 °C under the existence of the nickel catalysts, CO2 was converted to methane (CH4) and water (H2O). Subsequent to purify H2O from CH4, Δ17O of H2O was measured in CRDS. In this study, I developed this new vacuum line to purify atmospheric CO2 and to convert CO2 into H2O. Then, I determined the optimum conditions for the purification of CO2, the reduction of CO2, and the analysis of H2O.The new system developed in this study has several advantages over the conventional methods, such as (1) safe and easy operations, (2) less than 50 min for the analysis, and (3) comparable precisions with previous methods. By using the new system developed in this study, we can determine Δ17O of tropospheric CO2 with precision better than 0.015 ‰ (1σ). By using the new system developed in this study, I determined the difference in the Δ17O values between tropospheric CO2 in Nagoya and that in car exhaust, and found that tropospheric CO2 in Nagoya was 0.22‰ higher in Δ17O than that in car exhaust.