General aspects on the properties of supercritical media

Carbon dioxide has a critical temperature, Tc, of 31.3°C, and a critical pressure, Pq, of 72.9 atm. When carbon dioxide is compressed at a temperature and a pressure above critical it does not liquefy but forms a dense gas (Fig. 2.1). The mobile phase in SFC is thus gaseous and solvating. Such a 

The mobile phases may be characterized by three key factors density, diffusivity and viscosity. Typical values of these properties regarding gases, supercritical fluids and liquids are given in Table 2.1. Let us first consider the densities. These reflect the solubilities of the analytes. The mobile phase in GC does not dissolve the analytes and as a consequence the analytes will have to pass through the column in a gaseous state, and that is the inherent limitation of GC. Nowadays there are GC columns that can withstand up to 600°C, but there are few analytes that are stable at those temperatures. In liquid chromatography (LC) the situation is different because here the mobile phase, by virtue of its density, transports the analytes through the separation column and there is no immediate need for elevated temperatures. However, there is a price to pay for the increase in analyte solubility, and that is a drastically reduced diffusivity (Table 2.1). 

Source van Wasen, U., Swaid, I. and Schneider, G. M., Physicochemical principles and applications of supercritical fluid chromatography, Angew. Chem. Int. Ed. Engl, 19, 575-87, 1980 

As a consequence the chromatographic processes are quite slow in LC compared with those in GC. Here SFC comes in as an alternative, providing relatively high densities and higher diffusivities than observed in conventional liquids (Blomberg, 1988 Blomberg et al, 1994). 

SFC has been performed with packed as well as with open tubular columns. In the former case, the aim is to provide a substitute for packed column HPLC and in the latter case it is to provide a substitute for open tubular GC. 

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