Use of isopycnic plots in designing operations of supercritical fluid chromatography: II. The isopycnic plots and the selection of the operating pressure–temperature zone in supercritical fluid chromatography

In SFC, the key chromatographic parameters, the retention factors and the column efficiency, strongly depend on the density of the mobile phase. This indicates that the isodensity or isopycnic plots, drawn on the pressure–temperature plane, can provide an effective tool to help analyzing how the chromatograms obtained in SFC evolve, when the experimental conditions, the inlet and outlet pressures and the column temperature, are changed. In a companion paper, we analyzed the role of density in controlling the physical properties of the mobile phase, which in turn controls solute retentions and column efficiencies. In this report, we analyze the operating conditions in SFC with reference to the isopycnic plots of carbon dioxide. This analysis clarifies the differences and similarities between the operating conditions selected in the subcritical zone and those located in the supercritical zone. It also sets out an operational map illustrating how retention factors vary with respect to the operating temperatures and pressures. This study is focused on the use of pure carbon dioxide as the mobile phase, but the same method of investigation is also applicable when the mobile phase contains a modifier.