A Unique Property of Supercritical Fluids

What is unique to supercritical fluids compared to typical hquids at ambient conditions is that the variable density allows the same fluid to have a variable solvent power - i.e., the degree of solubdity of a particular component depends upon the density of the supercritical fluid solvent. Now the chemist has three parameters to exploit density (pressure), temperature, and composition. With supercritical fluids it is common to exploit the variable solvent power afforded by adjusting the density first. This allows the same fluid to be used to dissolve famihes of components selectively just by changing a physical parameter (pressure at a given temperature). At the highest densities, the solvent power of the supercritical fluid is very nearly that of the same fluid as a liquid. At that point, it is common to exploit mixtures of typical solvents in the bulk supercritical fluid to achieve more selectivity for an additional range of solute s/analytes. 

What is gained by the chemist is the capability to replace conventional sample preparation processes of extraction and fractionation (via column chromatography or back extractions) with the processes of step-wise extracting fractions with concurrent concentration. There are many examples of fractionation using supercritical fluids. 

In another. similar example [8] France et al. used a bed of activated alumina in the extraction vessel downstream of the sample itself to scrub-out (retain) the hpids while allowing hpophilic pesticides extracted from a grain sample to pass on to the collection/reconstitution device to produce a hpid-free fraction to pass on to a GC/ECD for analysis of the pesticides. This is particularly important since hpids must normaUy be removed from pesticide-containing extracts by long and tedious manual-column methods, evaporation and solvent exchange. 

The hitter acids of hops [9] have been selectively extracted in a three-step method away from the essential oil fraction (fraction 1) and a hpid fraction (fraction 3). All of the bitter acids were contained in the middle fraction 

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