Improving isotope ratio precision in Raman-based isotope ratio analysis of CO2 via fiber-diameter-controlled instrumental broadening.

Raman-based isotope ratio analysis (RIRA) has emerged as a promising non-destructive analytical method for determining carbon and oxygen isotope ratios of microscopic CO2 fluids trapped in minerals. Recent theoretical studies have demonstrated that the isotope ratio precision of RIRA is principally governed by Raman signal intensity, spectral bandwidth, and detector pixel resolution. Among these factors, the influence of spectral bandwidth has remained experimentally underexplored because changing instrumental broadening simultaneously increases photon collection efficiency and spectral overlap between adjacent isotopologue bands, introducing competing effects on isotope-ratio precision that are difficult to evaluate independently. In this study, we investigated how controlled instrumental broadening achieved by varying pickup fiber diameter influences the isotope ratio precision of RIRA for CO2. Raman spectra of 10 MPa CO2 fluid were acquired using pickup fibers with four different core diameters (50, 100, 200, and 400 μm). Spectral deconvolution of the four isotopologue bands (18ν+F.D., 13ν+F.D., ν+F.D., and ν+H.B.) was performed within the 1350-1450 cm-1 region. Reliable deconvolution was achieved for the 50, 100, and 200 μm pickup fibers, whereas severe overlap between the 18ν+F.D. and 13ν+F.D. bands prevented stable peak separation for the 400 μm pickup fiber. Among the three fiber diameters that allowed reliable spectral deconvolution, the highest isotope ratio precisions were achieved using the 200 μm pickup fiber under the 180 s exposure condition, yielding analytical precisions of 1.58 ± 0.34‰ (intensity ratio) and 1.62 ± 0.40‰ (area ratio) for 13C/12C, and 7.25 ± 0.87‰ (intensity ratio) and 7.38 ± 0.81‰ (area ratio) for 18O/16O. These values correspond to approximately twofold improvements relative to those obtained using the 50 μm pickup fiber. The experimentally observed precision improvements were broadly consistent with theoretical predictions based on Raman peak area scaling, indicating that photon-count statistics govern the primary dependence of isotope ratio precision on Raman signal intensity. The remaining differences can be attributed to pickup-fiber-dependent instrumental broadening, which influences isotopologue-band profiles and the stability of spectral deconvolution. Moderate instrumental broadening improved isotope ratio precision, whereas excessive broadening degraded isotopologue-band separability because of severe band overlap. These results demonstrate that appropriate selection of pickup fiber diameter provides a practical means of controlling instrumental broadening and thereby improving isotope ratio precision in RIRA without modification of the spectrometer hardware.