Capillary columns containing microparticulate adsorbents were used with CO as the mobile phase. By operating at selected temperatures, the CO 2 2 mobile phase changed from liquid or supercritical fluid to gas between the column inlet and outlet. The column efficiency was examined for both situations. Similar Ž . minimum-reduced plate heights h ; 1.5 were obtained for both situations, how-Ž . ever, higher optimum mobile phase linear velocity u was observed when using opt a mobile phase that changed from supercritical fluid to gas than from liquid to gas. Ž Silica and alumina adsorbents provided high selectivities for unsaturated olefinic . and aromatic hydrocarbons. Silica adsorbents were more suitable than alumina for the separation of highly unsaturated compounds such as aromatic compounds, while alumina-packed capillary columns yielded lower resistance to CO mobile 2 phase flow and allowed faster separations. Chromatographic separations can be w optimized by combining various modes of chromatography i.e., liquid chromatog-Ž . Ž . raphy LC , supercritical fluid chromatography SFC , and gas chromatography Ž .x GC along the column length.