Quantitative Measurements of Composition, Pressure, and Density of Microvolumes of CO2-N2 Gas Mixtures by Raman Spectroscopy.

Quantitative analysis of gases by Raman spectroscopy is based on relative Raman scattering cross sections (RRSCS) and the evolution of different spectral parameters (peak position, peak area, peak intensity, etc.). However, most of the calibration data were established at low pressure (low density) and without evaluating the effect of the composition. Using these data may lead to considerable errors, especially when applied to gas mixtures at high pressure as found in natural fluid inclusions. The aim of this study is to reevaluate the RRSCS of CO2 and to establish new calibration data based on the variation of CO2 Fermi diad splitting as a function of pressure (density) and composition over a pressure range of 5-600 bar at 22 and 32 °C. A high-pressure optical cell system (HPOC) and a heating-cooling stage were used for Raman in situ analyses at controlled PTX conditions. Our experimental results show that the RRSCS of CO2 varies slightly with pressure but can be considered constant over the studied pressure range. It can be used to measure the proportion of CO2 in gas mixtures with an uncertainty of about ±0.5 mol%. Different polynomial equations were provided to calculate pressure and density of CO2-N2 gas mixtures with an uncertainty of ±20 bar or 0.01 g·cm-3. A comparison of PVTX properties of natural CO2-N2 fluid inclusions hosted in quartz from the Central Alps (Switzerland) obtained by Raman measurement and as derived from phase transition temperatures by microthermometry experiments shows comparable values.