Operating parameters in SFC mobile phases

The partition coefficient of a solute between mobile and stationary phases in SFC and hence the value of the retention parameter k depends on the density. 

The analogue of temperature programming in GC or gradient elution in HPLC is density programming [25] the pressure is changed so that a linear or asymptotic density profile is achieved according to the above polynomial. From eqn (9.3) it can be shown that there is a threshold density which all members of the homologous series co-elute. 

Supercritical fluid chromatography is developing along two lines that differ in the nature of the column used packed-column SFC and capillary-column SFC. 

Early work in SFC was carried out using absorbants such as alumina or silica, or stationary phases insoluble in supercritical CO2, such as polyethylene glycol. Now, bonded non-extractable stationary phases such as octadecyl-silane and aminopropyl-bonded silicas, are usually used in packed columns. 

Wall-coated capillary columns are therefore increasingly used in SFC [30]. These have the great advantage of high efficiency because of greater length consequent on the permeability of capillaries to the flow of low-viscosity supercritical fluids. The low volume flow rates of capillary columns facilitate the use of toxic, corrosive, inflammable, or expensive fluids. Capillary columns are also compatible with a variety of detectors (section 9.3), permit maximum 

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