The Road to Unified Chromatography: The Importance of Phase Behavior Knowledge in Supercritical Fluid Chromatography and Related Techniques, and a Look at Unification

Abstract

In supercritical fluid chromatography (SFC) and related techniques, and in chromatography in general, knowledge of phase behavior is essential to understand mass transfer throughout the process. Complete three-dimensional phase diagrams of binary fluids including CO2are both rare and difficult to use. However, much of the information necessary for chromatography is contained in two-dimensional projections of the critical loci of binary mixtures. When CO2is one of the fluid components, we are able to quickly produce these projections in the laboratory using a peak-shape-sensitive flow-injection procedure based on SFC equipment. The importance of the phase behavior knowledge gained is illustrated with examples of sample injection in open-tubular SFC and detector interfacing in packed-column SFC when the detector is operated at a different temperature and pressure than the column outlet. Knowledge of the phase behavior of chromatographic mobile phases also leads us to new possibilities for control of selectivity and shortening of analysis times by making use of temperatures and pressures that are off limits in conventional gas (GC) and liquid (LC) chromatography. This leads us to a unified view of chromatography in which parameters can be varied continuously as long as we remain in the one-phase region of the phase diagram. In this view, conventional GC and LC become limiting cases of the more general description.