The effect of dissolved helium on the density and solvation power of supercritical carbon dioxide

Carbon dioxide (CO2) cylinders pressurized with helium in their headspace are widely used in supercritical fluid chromatography and extraction (SFC and SFE). A few percent of helium are dissolved in the liquid CO2 phase of the cylinder, which can result in such problems as retention time shift and poor reproducibility in SFC as well as reduced solubility and extraction rate in SFE. In this study, a high precision density meter and a gas chromatograph equipped with a thermal conductivity detector are used to monitor the density and composition of the supercritical fluid generated from a helium headspace CO2 cylinder over the duration of its use. These measurements are related to the solubility of soybean oil and cholesterol in the fluid. The density measurements (accurate to 10-4 g/mL) show that the density of the fluid is linearly proportional to the helium content of the CO2. However, the significant drop of solute solubility in helium-CO2 mixtures cannot be explained by the reduction in fluid density alone and may involve a disruption of the solvent (CO2) shell around the dissolved solute.