In situ quantification of minor compounds in pressurized carbon dioxide using Raman spectroscopy

In this work, we present an optical approach that allows quantification of compounds dissolved down to a sub 10 ppm (molar) level in supercritical carbon dioxide (scCO2) by in situ Raman spectroscopy. The quantification strategy is based on the consideration of Raman signal intensity ratios and of weak Raman signals of CO2. The first measure makes the quantification strategy independent of the refractive index, laser power fluctuations and changes in alignment, and is thus robust against variations of the operation conditions. The second measure circumvents the classical challenges related to the dynamic range limit of detectors, which is inherent to the quantification of minor compounds. Example solubilities of the wax alkyl ketene dimer (AKD) in CO2 were quantified at 333 K and a range of pressures from 10 MPa to 18 MPa. The solubilities are between 0.1 mg g−1 and 1.5 mg g−1 corresponding to ∼9 ppm and ∼132 ppm (molar), respectively.