Use of isopycnic plots in designing operations of supercritical fluid chromatography: I. The critical role of density in determining the characteristics of the mobile phase in supercritical fluid chromatography

In Supercritical Fluid Chromatography (SFC), the key chromatographic parameters of any compound, its retention and efficiency, are known to strongly depend on the density of the mobile phase. This indicates that iso-density, also called isopycnic, plots drawn on the pressure–temperature plane can provide an effective tool to analyze how SFC systems may operate under different combinations of inlet and outlet pressures and column temperature. To effectively use these isopycnic plots in designing the operations of SFC systems, however, a deeper understanding of the factors behind the dependence of the performance of these systems on the mobile phase density is required. The nature of this density dependence is explored with reference to the key physical properties of the mobile phase, its viscosity, diffusivity and solubility. This study is focused on the use of pure carbon dioxide as the mobile phase, but this method of investigation is applicable for other mobile phase combinations as well.