Interlaboratory Application of Raman CO₂ Densimeter Equations: Experimental Procedure and Statistical Analysis Using...

Raman spectroscopy has been used extensively to calculate CO₂ fluid density in many geological environments, based on the measurement of the Fermi diad split (Δ; cm^-1) in the CO₂ spectrum. While recent research has allowed the calibration of several Raman CO₂ densimeters, there is a limit to the interlaboratory application of published equations. These calculate two classes of density values for the same measured Δ, with a deviation of 0.09 ± 0.02 g/cm³ on average. To elucidate the influence of experimental parameters on the calibration of Raman CO₂ densimeters, we propose a bottom-up approach beginning with the calibration of a new equation, to evaluate a possible instrument-dependent variability induced by experimental conditions. Then, we develop bootstrapped confidence intervals for density estimate of existing equations to move the statistical analysis from a sample-specific to a population level. We find that Raman densimeter equations calibrated based on spectra acquired with similar spectral resolution calculate CO₂ density values lying within standard errors of equations and are suitable for the interlaboratory application. The statistical analysis confirms that equations calibrated at similar spectral resolution calculate CO₂ densities equivalent at 95% confidence, and each Raman densimeter does have a limit of applicability, statistically defined by a minimum Δ value, below which the error in calculated CO₂ densities is too high.