Helium head pressure carbon dioxide in supercritical fluid extraction and chromatography: Thermodynamic analysis of the effects of helium

The Peng-Robinson equation of state (EOS) is employed to quantify the effects of entrained helium when helium head pressure carbon dioxide (HHPCO2) is used in supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). Naphthalene serves as a model analyte, and the unlike interaction energy parameters of the EOS are obtained from the respective second cross-virial coefficients. The Peng-Robinson EOS provides a reasonable description of the vapor-liquid equilibrium in HHPCO2 storage tanks at room temperature. Variations in the content of helium in HHPCO2 within the relevant range (between 0 and 5 mol %) are predicted to produce significant changes in the density and solubility parameter of HHPCO2, in the solubility of naphthalene in HHPCO2, and in the chromatographic retention of naphthalene if HHPCO2 serves as the mobile-phase fluid. These results provide a computational indication that the use of HHPCO2 in SFE and SFC should be avoided.