3.4.3 Supercritical Fluid Chromatography, SFC
A liquid turns into a super critical fluid (SCF) when its temperature rises above the critical temperature—the temperature where it can no longer exist as a liquid no matter how much pressure is applied. Super critical fluids exist in a state between a liquid and a gas and have the penetration abilities of a gas and the dissolving power of a liquid. As a result, SCF is a mixture of GC and HPLC and in some cases is superior to GC or HPLC. Super critical CO2, the same matrix used to selectively extract caffeine from coffee beads and nicotine from tobacco products, can be used as a mobile phase in chromatography. Super critical fluid chromatography is a special form of HPLC where a near-identical system is used but the mobile phase, as noted above, is super critical CO2. The system is therefore pressured and temperature controlled to maintain the super critical fluid. SCF is a form of normal phase chromatography that is used for the analysis of thermally labile molecules. The same types of packed and capillary columns that are used in HPLC are utilized in SCF. Due to the nature of the super critical fluid, packed columns can actually contain more theoretical plates than capillary column. Also, the shape of the van Deemter curve is different from those observed in GC and HPLC in that a minimum plate height exists over a very broad range of linear velocities. SCF can be used for a variety of separations but it is most commonly used in the separation of chiral compounds in the pharmaceutical industry. In place of a solvent gradient the chromatographer uses pressure programming and the affinity of the stationary phases to separate complex mixtures of analytes. Pressure programming in SCF is analogous to temperature programming in GC and gradient programming in HPLC. Methods of detection include UV/Vis, mass spectrometry, FID (as in GC but unlike in HPLC) and evaporative light scattering.
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