Comparison of Raman spectral characteristics and quantitative methods between 13CH4 and 12CH4 from 25 to 400 °C and 50 to 400 bar

Yuzhou Ge; Lianfu Li; Shichuan Xi; Yitong Zhang; Zhendong Luan; Xin Zhang

doi:10.1016/j.saa.2023.123380

Comparison of Raman spectral characteristics and quantitative methods between ¹³CH₄ and ¹²CH₄ from 25 to 400 °C and 50 to 400 bar

Spectrochim Acta A Mol Biomol Spectrosc. 2024 Jan 5:304:123380. doi: 10.1016/j.saa.2023.123380. Epub 2023 Sep 9.

Authors

Yuzhou Ge¹, Lianfu Li², Shichuan Xi², Yitong Zhang¹, Zhendong Luan¹, Xin Zhang³

Affiliations

¹ CAS Key Laboratory of Marine Geology and Environment & Center of Deep Sea Research, Institute of Oceanology, Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China; Laboratory for Marine Geology, LaoShan Laboratory, Qingdao 266061, China; University of Chinese Academy of Sciences, Beijing 100049, China.
² CAS Key Laboratory of Marine Geology and Environment & Center of Deep Sea Research, Institute of Oceanology, Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China; University of Chinese Academy of Sciences, Beijing 100049, China.
³ CAS Key Laboratory of Marine Geology and Environment & Center of Deep Sea Research, Institute of Oceanology, Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China; Laboratory for Marine Geology, LaoShan Laboratory, Qingdao 266061, China; University of Chinese Academy of Sciences, Beijing 100049, China. Electronic address: xzhang@qdio.ac.cn.

PMID: 37716044
DOI: 10.1016/j.saa.2023.123380

Abstract

In recent years, increasing attention has been given to quantifying the isotopic compositions of gases by Raman spectroscopy. However, related research on the carbon isotopes of CH₄ is still lacking. In this study, the Raman spectral characteristics of ¹³CH₄ and ¹²CH₄ in the pure CH₄ system and in the CH₄-H₂O system are comprehensively studied at temperatures ranging from 25 to 400 °C and pressures ranging from 50 to 400 bar. For the pure CH₄ system, the peak positions of the symmetric stretching band (ν₁) of ¹³CH₄ and ¹²CH₄ both shift to a higher wavenumber with increasing temperature and decreasing pressure, and their full width at half maximum (FWHM) values can reach 4.7 cm^-1 and 5.5 cm^-1, respectively, with increasing temperature and density. The peak position of the ν₁ band of ¹³CH₄ is always 4.6-5.1 cm^-1 lower than that of ¹²CH₄ under the same conditions, indicating that ¹³CH₄ and ¹²CH₄ can be distinguished by peak position and FWHM in the Raman spectra. Although the calculated Raman shifts (ν₀) at zero-density for ¹³CH₄ and ¹²CH₄ are different, the unified equation for determining the density of CH₄-rich fluid inclusions is still applicable, as long as to bracket the sample measurement with measurements of reference standards. For the CH₄-H₂O system, the peak position of the ν₁ band of dissolved ¹³CH₄ is approximately 2.5 cm^-1 lower than that of ¹²CH₄, while their FWHM values can be more than 12.2 cm^-1, causing more difficulty in distinguishing them in aqueous solutions. In addition, Raman quantitative calibration models of ¹²CH₄ and ¹³CH₄ in the CH₄-H₂O system are established. The difference in the Raman scattering cross section (RSCS) of dissolved ¹²CH₄ and ¹³CH₄, rather than changes in the molar density or RSCS of water is the main reason leading to the difference in the Raman quantitative calibration models of CH₄ concentrations between the ¹²CH₄-H₂O system and the ¹³CH₄-H₂O system. Our work provides a good reference for the in situ identification and quantification of the carbon isotopic composition of CH₄ in high-temperature and high-pressure environments.

Keywords: (12)CH(4); (13)CH(4); Carbon isotope; In situ; Quantitative analysis; Raman spectroscopy.