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Chromatography with supercritical fluids unites the features of both gas chromatography and liquid chromatography, yet retains special characteristics of its own. The diffusion coefficient and particularly the viscosity of fluid phases may approach values for low‐pressure gases, while the solvent power may be similar to that of liquids. However, with supercritical fluids it is possible to control chromatographic separations very effectively by pressure programming, since the solubility increases with increasing density. Temperature programming, on the other hand, can have the opposite effect to that in gas‐ or liquid‐chromatography since the density decreases with increasing temperature at a given pressure. Supercritical fluid chromatography is primarily of interest for the separation of higher molecular weight compounds. The efficiency of this method of separation is demonstrated on several homologous series. Thus, a styrene oligomer with nominal Mw=2200 can be resolved by a pressure and temperature program into 40 species.

The chromatographic behavior of a number of low boiling eluents has been compared in supercritical fluid chromatography ( S F C ) with packed columns. Unmodified silica gel has been used as the stationary phase and a test mixture of polycyclic aromatic hydrocarbons ( P A H I S ) as the substrate. Carbon dioxide, nitrous oxide, trifluoromethane, chlorotrifluoromethane, n-pentane, n-butane, 1-butane, propane, ethane, dicthylether, and dimcthylethtr have beon investigated as mobile phases. LEYENDECKER ET ALOf the mobile phast -C O L , N 2 0 , C 2 t f 6 , C H F 3 , and C C I F J , which a l l possess low c'r? ticdl ternp(.ratur

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