The impacts of elevated temperature and mNaCl for in situ Raman quantitative calibration of dissolved gas species

Abstract In the past ten years, Raman spectroscopy analysis has been widely applied to the quantification of the components of hydrothermal fluids and fluid inclusions. However, the accuracy of the quantitative results of Raman spectroscopy is highly dependent on the quantitative calibration model. How temperature and salinity affect quantitative calibration models remains an unsolved question, which severely hinders the accurate geochemical characterization of hydrothermal fluids and fluid inclusions. In this study, the Raman spectra of the symmetric stretching mode of methane (~ 2917 cm−1) in homogeneous CH4-containing aqueous solutions were obtained at temperatures from 25 to 300 °C, pressures from 0.1 to 40 MPa, and mNaCl from 0 to 5.0 mol/kg to research the effects of elevated temperature and salinity on the Raman spectroscopy quantitative measurement of gas species, and two Raman calibration models of CH4 that reference OH bending and stretching bands were established. Both models show high dependence on salinity. Large measurement errors, causing underestimation of methane concentrations by up to 57.5%, occur if the salinity of the fluids is not taken into account during the calibration. The contributions of the Raman scattering cross section (RSCS) and molar density to the change of quantification factors (QFs) were quantified, and the results showed that the variation of the RSCS is the main factor determining QF values. The significant discrepancy between the measured and predicted QFs demonstrates that changes in the peak areas of both H2O and CH4 contribute to the dependence of quantitative calibration models on temperature and salinity.