Techniques for the Optimization of Supercritical Fluid Chromatographic Separations.

Abstract

Although supercritical fluid chromatography (SFC) is now well established and still growing rapidly, it must offer clear advantages over other techniques. Thus proper method development in SFC is a necessity. There are many diverse and synergetic parameters controlling SFC separations, resulting in complex retention behavior. This work describes the proper approach to method development, including investigations into unique mobile phases and some techniques which can be used for optimization of SFC separations. In Chapter One, the concepts of chromatography with supercritical fluids are first introduced, including method development and optimization. Chapter Two focuses on mobile phase method development, and examines the effects of adding a small amount of formic acid (0.3, 0.5, and 0.7%) to a carbon dioxide mobile phase. Chosen in part because it does not respond to the flame ionization detector (FID), the formic acid is observed to increase the solvation of polar analytes, and improve selectivity and peak shape. Thermodynamic measurements are made in order to quantitatively compare the modified and pure mobile phases. In the lower temperature and pressure ranges, preferential solvation of polar compounds by the formic acid modifier results in more dramatic thermodynamic differences for the two mobile phases. Chapter Three illustrates the usefulness of short capillary columns for the reduction of method development time and for rapid analysis of simple mixtures. In Chapters Four and Five, the simplex algorithm is evaluated for rapid optimization of SFC separations, and is a preferred technique that can be applied to any number of variables, requiring little knowledge of the sample. Chapter Six introduces simultaneous interpretive methods of optimization, in which response surfaces are generated by retention mapping. Using very few data and an appropriate model, retention surfaces are used to produce a response surface, which is then searched for the optimum. Chapter Seven summarizes SFC method development, and provides a comparison of optimization techniques. Appendix A gives useful hints in the operation of SFC instrumentation, while Appendices B and C list important computer programs for the optimization of separations in supercritical fluid chromatography.