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Quantification of carbon isotopic composition (δ¹³C) of CO₂ can trace the source of carbon, which is of great significance for studying the evolution process of geological fluids, constraining the global carbon budget, and clarifying the mechanism of biological metabolism.

At present, the commonly used isotope measurement technology (mass spectrometry) requires long-time sample pretreatment before testing, resulting in low detection efficiency. In addition, due to the limitation of testing conditions, it can not be directly applied to in situ detection in high-temperature and high-pressure environments.

Recently, the research team led by Prof. ZHANG Xin from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) reported a rapid in situ quantitative analysis method for determining the carbon isotopic composition of CO₂ using Raman spectroscopy.

The study was published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy on Mar. 23.

Researchers constructed a high-temperature and high-pressure experimental microsimulation device, based on confocal micro-Raman technology and capillary high-pressure optical cell (HPOC). The Raman spectral characteristics of high-purity ¹³CO₂, ¹²CO₂, and their binary mixtures at high temperatures and pressure (50-450 ℃, 50-400 bar) were studied. The study shows that the full width at half maximum (FWHM) of the characteristic peaks of ¹³CO₂ and ¹²CO₂ both are 2-5 cm^-1, while the difference in peak position between them is more than 15 cm^-1 in the experimental range of temperature and pressure, indicating that the ¹³CO₂ and ¹²CO₂ can be well distinguished by the peak position of characteristic peaks in Raman spectra.

Furthermore, by studying the ¹³CO₂-¹²CO₂ binary mixtures with different proportions, researchers discussed the relationship between the peak intensity ratio, and content ratio under different temperatures and pressures (corresponding to density). Finally, the peak intensity ratio of ¹³CO₂ to ¹²CO₂ with a reference of the upper Raman characteristic peak (I+¹³/I+¹²) or the lower Raman characteristic peak (I-¹³/I-¹²) was selected as quantitative indexes to respectively establish Raman quantitative calibration models, which can be used for the in situ determination of carbon isotopic composition (¹³CO₂/¹²CO₂) of CO₂ over a wide δ¹³C under high-temperature and high-pressure environments.

"The establishment of this Raman quantitative method provides an effective detection method for in situ rapid monitoring of the carbon isotope composition of CO₂ in high-temperature and high-pressure experiment system with isotope labeling," said GE Yuzhou, first author of the study.

"With the further improvement in the accuracy and detection limits of the model, it is expected to be applied to in situ measurement in natural systems with a very low ¹³CO₂ content, such as the fluid inclusions and the deep-sea hydrothermal fluids," said Prof. ZHANG, corresponding author of the study.

This study was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of Chinese Academy of Sciences, etc.

Fig. 1 Schematic diagram of high-temperature and high-pressure experimental microsimulation device.

 Fig. 2 Variation in the peak position of the Raman characteristic of ¹³CO₂ at different temperatures and pressure (corresponding to density).

Fig. 3 Raman spectra of ¹³CO₂-¹²CO₂ binary mixtures with different proportions.

Fig. 4 Relationships between the peak intensity ratio of ¹³CO₂ and ¹²CO₂, the content ratio, and density.

Ge, Y., Li, L., Xi, S., Ma, L., Luan, Z., and Zhang, X*., 2023, Raman spectral characteristics of ¹²CO₂/¹³CO₂ and quantitative measurements of carbon isotopic compositions from 50 to 450 ℃ and 50 to 400 bar. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, v. 296.

(Text by GE Yuzhou)

Media Contact:

ZHANG Yiyi

Institute of Oceanology

E-mail: zhangyiyi@qdio.ac.cn 

(Editor: ZHANG Yiyi)