Chromatographic Separation with Supercritical Fluids

In a chromatographic separation, a mixture of substances is transported by a carrier, the mobile phase, over a surface, the stationary phase. Between the two phases mass transfer processes take place, which lead to different transport velocities along the surface of the stationary phase for different components of the mixture. The components reach the end of the stationary phase at different times and can be detected and collected separately. 

Supercritical fluid chromatography (SFC) uses supercritical gases as a mobile phase [14-17]. The solvent power of supercritical gases is determined by density (see above) which can be easily modified by pressure [18]. Furthermore, separation of the eluted compounds is easily achieved by pressure release. Liquid solvents may be only applied in minor quantities as modiflers. Therefore, SFC has good potential for scale-up to preparative and production scale chromatography [15-17,19,20]. 

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Chromatographic Separation With Supercritical Fluids (SFC) 4.1. GENERAL CONSIDERATIONS [ 11... 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

Since high solute dlffusivity, lower viscosity and excellent solvating properties can be obtained with supercritical fluids, higher chromatographic efficiencies and faster analysis time than liquid chromatography can be obtained with SFC (21). It is also possible to separate non-volatile high molecular weight compounds at relatively low temperatures. 

Infrared spectroscopy is probably the most widespread analytical spectroscopic technique for identification and characterization of organic compounds. Because of this identification capability infrared spectroscopy is desirable as a detection technique for chromatographic separations. With the advent of Fourier transform infrared spectroscopy/ the speed and sensitivity of infrared detection is greatly enhanced making such applications feasible. FT-IR detection has been widely accepted as a detector for gas chromatography (GC/FT-IR) (1) and has been applied with limited success to liquid chromatography (LC/FT-IR) (2)/ and more recently to supercritical fluid chromatography (SFC/FT-IR) (3). The recent review articles cited here provide excellent introduction and references to current state-of-the-art in these areas. 

The development of methods of analysis of tria2ines and thek hydroxy metabohtes in humic soil samples with combined chromatographic and ms techniques has been described (78). A two-way approach was used for separating interfering humic substances and for performing stmctural elucidation of the herbicide traces. Humic samples were extracted by supercritical fluid extraction and analy2ed by both hplc/particle beam ms and a new ms/ms method. The new ms /ms unit was of the tandem sector field-time-of-flight/ms type. 

Scheme 7.3 Generalised supercritical fluid methods for extraction and analysis with and without separation of interfering components. After King [5], Reproduced from the Journal of Chromatographic Science, by permission of Preston Publications, a Division of Preston Industries, Inc.

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

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