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 (HHPC[O.sub.2]) 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 HHPC[O.sub.2] storage tanks at room temperature. Variations in the content of helium in HHPC[O.sub.2] within the relevant range (between 0 and 5 mol %) are predicted to produce significant changes in the density and solubility parameter of HHPC[O.sub.2], in the solubility of naphthalene in HHPC[O.sub.2], and in the chromatographic retention of naphthalene if HHPC[O.sub.2] serves as the mobile-phase fluid. These results provide a computational indication that the use of HHPC[O.sub.2] in SFE and SFC should be avoided.