In situ Raman spectroscopic measurement of the 13C/12C ratio of dissolved CO2 at high temperatures and pressures: Method and implications
Year: 2025
Type: article
Abstract: • 13 CO 2 (aq) and 12 CO 2 (aq) can be distinguished by their peak position and FWHM in Raman spectrum. • The optimal likelihood curve fitting method (OLCF) proposed to decompose the overlapping bands. • A Raman in situ quantitative analysis method for 13 C/ 12 C of dissolved CO 2 was established. • The decomposition of sedimentary organic matter leads to more depleted 13 C in hydrothermal fluids, Submarine hydrothermal activities release a large amount of greenhouse gases such as CO 2 and CH 4 into the ocean, influencing the global carbon cycle. Carrying out high-temperature and high-pressure hydrothermal experiments to simulate these geochemical processes is a prerequisite for clarifying the source of carbon-containing substances in the hydrothermal fluid. In situ monitoring of the change in carbon isotope composition of CO 2 is essential for high-temperature and high-pressure simulation experiments, but it is also a great technical challenge. In recent years, laser Raman spectroscopy has attracted wide attention as a supplementary means to mass spectrometry for measuring the 13 C/ 12 C value of CO 2 . However, the existing research is limited to the Raman spectroscopy study of the carbon isotope composition of supercritical/liquid CO 2 , and there is little research on dissolved CO 2 in solution. In this study, we systematically studied the Raman spectral characteristics of dissolved 13 CO 2 and 12 CO 2 in the H 2 O ± 13 CO 2 ± 12 CO 2 system at 25–300 °C and 10–350 bar. The results show that the peak position of the Raman characteristic band of 13 CO 2 (aq) is 1367–1370 cm −1 , which is 14–17 cm −1 lower than that of 12 CO 2 (aq), and the full width at half maximum is 2–3 cm −1 smaller than that of 12 CO 2 , which indicate the 13 CO 2 (aq) and 12 CO 2 (aq) can be identified by Raman spectroscopy. On this basis, we proposed the optimal likelihood curve fitting method (OLCF) for the first time to decompose the overlapping bands and accurately obtain the peak height ratio (H 13 /H 12 ) of dissolved 13 CO 2 and 12 CO 2 . It has been shown that the G-factor ratios (G 13 /G 12 ) are significantly affected by temperature and the relative content of 13 CO 2 and 12 CO 2 . Obtaining an appropriate G-factor ratio is a prerequisite for accurately determining the 13 C/ 12 C of dissolved CO 2 . Based on the functional relationship between the H 13 /H 12 and the 13 C/ 12 C, we established an empirical equation to quickly estimate the 13 C/ 12 C value, thereby assisting in selecting an appropriate G-factor ratio. The calibrated G-factor can be well used to determine the 13 C/ 12 C of dissolved CO 2 in the hydrothermal experiments with 13 C labelled. In-situ monitoring experiments show that the phase separation of the hydrothermal fluid hardly causes changes in the carbon isotope composition of dissolved CO 2 . However, the contamination of organic matter, such as oxalic acid, will result in a higher content and a more negative δ 13 C of CO 2 in the hydrothermal fluids.
Source: Geoscience Frontiers
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
SDG: Climate action
Open Access status: hybrid
Estimated APC paid: $3,000
OpenAlex ID: W4415093933