The position of supercritical-fluid chromatography between gas- and liquid chromatography from a kinetic point of view

In the first part of this work the chromatographic performance of packed and open tubular columns in GC, SFC and LC is compared. Equations are given for the speed of analysis, pressure drop and plates per bar pressure drop. These equations are derived under the simplifying assumption of non-compressibility of the mobile phases. In part II the effect of the compressibility of the mobile phase is included. The effect of high pressure drop in GC is brietly discussed. The emphasis in the second part is on SFC, where the pressure drop over the column affects the chromatographic process, because a supercritical tluid is both highly compressible and highly non-ideal. With such tluids the solvating and transport properties and thus in principle the chromatographic retention and efficiency are a function of pressure and thus may vary along the column length. No exact analytical expressions exist to determine the intluence of the pressure drop across the column on retention, efficiency and speed of analysis in SFC. Fluid tlow through packed and open-tubular columns can, however, be described accurately by numerical expressions derived from the Darcy equation for laminar tlow. Due to density gradients significant variations of the capacity factor along the column can occur. In open columns, the effect of the increasing linear velocity on the plate height is effectively compensated by an increase in the diffusion coefficients. In packed columns it seems that both the increase in the linear velocity and in the capacity factor cause the plate height to increase along the column