The instrumentation developed for SFC is based on both gas and liquid chromatographs. The
supercritical fluid is delivered to the column by a modified HPLC pump. Columns are either very
narrow bore capillaries of the type used in GC with a chemically bonded stationary phase (p. 99) or
packed reverse phase HPLC columns (p. 123). They are enclosed in a constant temperature oven and,
although temperature programming is not used, pressure programming is often employed as a form of
gradient elution. Increasing the pressure throughout a separation speeds up the elution process by
increasing the density and hence the solubilizing power of the supercritical fluid. Carbon dioxide is a
non-polar mobile phase so small amounts of more polar compounds such as methanol or glycol ethers
can be added if polar mixtures are to be separated as this improves peak shapes.
The nature of a supercritical fluid enables both gas and liquid chromatographic detectors to be used in
SFC. Flame ionization (FID), nitrogen phosphorus (NPD), flame photometric (FPD) GC detectors (p.
100 et seq.) and UV and fluorescence HPLC monitors are all compatible with a supercritical fluid
mobile phase and can be adapted to operate at the required pressures (up to several hundred bar). A very
wide range of solute types can therefore be detected in SFC. In addition the coupled or hyphenated
techniques of SFC-MS and SFC-FT-IR are attractive possibilities (cf. GC-MS and GC-IR, p. 114 et
seq.).
The applications of SFC include the analysis of mixtures of hydrocarbons, triglycerides, high relative
molecular mass and thermally labile compounds. Its advantage over GC lies in its ability to separate
mixtures at much lower temperatures and over HPLC in improved efficiency due to more rapid mass
transfer (larger solute diffusion coefficients) and ease of coupling to a mass spectrometer. The wide
choice of detector and ease of controlling retention times by pressure programming are also features
that make SFC attractive but the instrumentation is currently much more expensive than either GC or
HPLC. Its future growth is therefore uncertain.
4.3.4—
Thin-layer Chromatography
Summary
Principles
Separation of mixtures in microgram quantities by movement of a solvent across a flat surface;
components migrate at different rates due to differences in solubility, adsorption, size or charge; elution
is halted when or before the solvent front reaches the opposite side of the surface and the components
examined in situ or removed for further analysis.
Apparatus and Instrumentation
Thin layers of powdered cellulose, silica gel, alumina, ion-exchange or gel-