Best practices for the analyses of CO2 fluids by Raman Spectroscopy

Penny E Wieser , Charlotte L DeVitre, Isabelle Susman

Raman spectroscopy is a key method for determining CO₂ densities in geological fluids, yet acquisition, calibration, and processing methodologies vary widely between laboratories. This study evaluates how these parameters affect precision and accuracy. We show that spectral non-linearity can cause a single instrument to show variable relationships between CO2 density and spectral parameters as the spectral center and drift correction routine is varied, explaining some of the diversity in published densimeters. We show that when the calibration methodology is applied to unknowns, the Raman method is robust. For example, 240 repeat measurements of a natural fluid inclusion (FI) over ~2 years gives a 1σ variation of only ~0.008 g/cm³. We also assess the influence of variations in sample and room temperature on measurements to help inform analytical strategies. We show that high laser powers in fluid and melt inclusions causes heating of the CO2 fluid, and unacceptable drops in CO₂ density. We offer practical strategies to optimize signal without raising laser power, noting that small changes in focus and X–Y position greatly affect intensity (but not depth in sample).