[M-ZZ42] Frontiers in geochemistry: new challenges and future prospects

[MZZ42-04] Development of mid-infrared cavity ring-down spectrometer for clumped isotope analysis of CO₂

*Hideki Tomita¹, Kota Tsuge¹, Osamu Abe², Ryu Uemura² (1.Nagoya University, Graduate School of Engineering, 2.Nagoya University, Graduate School of Environmental Studies)

Keywords:clumped isotope, stable isotope, carbon dioxide, laser spectroscopy

Carbon and oxygen have multiple stable isotopes (¹²C, ¹³C, ¹⁶O, ¹⁷O, and ¹⁸O). By precisely analysing the abundance ratios of multiply substituted CO₂ molecules, known as clumped isotopologues, it is possible to determine the source of carbon dioxide, estimate the temperature at which carbonates formed in the past (palaeohydrothermal temperature), and so on. Conventionally, isotope ratio mass spectrometry has been used to measure the isotope ratios of light elements. However, precise measurement of the abundance of trace amounts of clumped isotopologues is difficult due to isobaric interferences in the mass spectrum caused by dominant molecules. In contrast, laser absorption spectroscopy determines the concentration of target molecules based on their optical absorption properties, which depend on their isotopic composition. If the absorption lines of the target molecules are selected appropriately, quantitative analysis can be made by distinguishing between isotopic molecules, and the isotopic abundance ratio can be measured with high precision.
In this study, we developed a mid-infrared cavity ring-down spectrometer for clumped isotope analysis of CO₂ molecules. Highly sensitive laser absorption spectroscopy using an optical cavity, known as cavity ring-down spectroscopy, in the 4 μm region allows precise measurement of the even week photo-absorption or quantification of low abundance CO₂ molecules. We have built a spectrometer combined with a sample injection system. In this setup, the absorption of ¹²C¹⁶O¹⁶O, ¹³C¹⁶O¹⁶O, ¹²C¹⁶O¹⁸O, and ¹³C¹⁶O¹⁸O can be measured by frequency scanning of a single quantum cascade laser. The evaluation of the spectral sensitivity and the assessment of the basic analytical performance using test samples will be presented.

Acknowledgements: This work was partially supported by JSPS KAKENHI: Grant-in-Aid for Scientific Research (B) 22H03869, Japan.