C hromatographic methods are usually classified according to the physical state of the mobile phase-for example, GC, supercritical fluid chromatography (SFC), and LC. This admittedly oversimplified classification scheme exists partly because of the similarity of most chromatographic stationary phases, which are usually either polymers or solid adsorbents and are often deactivated by some chemically bonded moiety. Because of the banality of stationary phases, any attempt to unify the various forms of chromatography will naturally center on the mobile phase (1-4). To date, many researchers consider the most promising approach to unification to be the use of CO 2 as the mobile phase because of the "tunable" solvent strength of this fluid.