Comparison of Raman Spectral Characteristics and Quantitative Methods between 13ch4 and 12ch4 from 25 to 400 Oc and 50 to 400 Bar

Abstract

In recent years, increasing attention has been given to the use of Raman spectroscopy to quantify the isotopic compositions of gases. 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. The results show that for the pure CH4 system, and the peak positions of the symmetric stretching band (ν1) of 13CH4 and 12CH4 both move to a higher wavenumber with increasing temperature and decreasing pressure, and the full width at half maximum (FWHM) values of them can reach 4.7 cm-1 and 5.5 cm-1 respectively, with the increase in density. However, 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 at zero-density of 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 the FWHM values of them both can reach 12.2–13.9 cm-1, making it relatively difficult to distinguish them in an aqueous solution. In addition, the Raman quantitative calibration models of 12CH4 and 13CH4 in the CH4–H2O system are established respectively. The results show that it is the differences in the Raman scattering cross section (RSCS) of dissolved 12CH4 and 13CH4, rather than changes in the molar density or RSCS of water, that is the main reason leading to differences 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 in situ identification and quantification of carbon isotopic composition of CH4 in high-temperature and high-pressure environments.

Keywords: Raman spectroscopy, in situ, quantitative analysis, carbon isotope, 13CH4, 12CH4