| The use of non-traditional mobile phases in reversed-phase chromatography is explored. Mobile phases consisting of mixtures of carbon dioxide and an organic modifier such as methanol or acetonitrile are used to examine selectivity and retention in various regions of the phase diagram, from solvating gas to supercritical fluid, to subcritical fluid, to enhanced fluid, to liquid. Continuous trends are seen across the phase diagram as the variables of composition, density, pressure, and temperature are changed. This work supports the concept of "unified chromatography.";Shape selectivity is examined as a function of the aforementioned variables, using a variety of test solutes with differing shape or planarity. It is found that, for all mobile phase conditions, shape-selective interactions occur in the stationary phase, and that choosing mobile phase conditions that enhance stationary phase interactions enhances shape selectivity.;Methylene selectivity is examined as a function of mobile phase density. By extrapolating retention as a function of mobile phase density to a density value of zero, the stationary phase contribution to methylene selectivity is determined. A stationary phase chain length dependence of methylene selectivity was found for long-chain stationary phases, but this was not seen for short-chain phases.;The use of superheated water as a mobile phase is examined. Retention characteristics of superheated water mobile phases are determined, and the thermodynamics of retention with pure water mobile phases at ambient and superheated temperature are compared. Retention thermodynamics are also compared to values observed with mixed hydroorganic mobile phases at room temperature. Retention with pure water mobile phases at room temperature appears to be entropically driven, while at superheated temperature retention is enthalpically driven.;The effect of temperature on the phase ratio and on gradient reequilibration volume is examined. It is found that, if possible changes in the phase ratio are not accounted for, error may occur in van't Hoff analysis if the phase ratio is assumed to be constant. In addition, it is found that gradient reequilibration polar-embedded-group stationary phases is not temperature dependent, while gradient reequilibration on regular phases is temperature dependent. |
