Mobile phases for supercritical fluid chromatography

Carbon dioxide (CO2) is by far the most commonly used mobile phase in SFC. It has a number of advantages. It has a low critical temperature and is highly inert (except when used with amines) it is non-toxic, non-explosive. 

There is, however, one major problem with CO2 as a mobile phase and that is its low polarity. Thus only relatively non-polar analytes can be dissolved in CO2. Moreover, in columns packed with silica-based material there are always residual adsorptive sites. In reversed-phase HPLC the mobile phase deactivates these sites, but the CO2 is not polar enough to do that. As a consequence, the more polar analytes are adsorbed and these are then eluted as severely tailing peaks or are not eluted at all. It should be mentioned here that reports on more inert packings have been published (Li, Malik and Lee, 1994). There are some supercritical mobile phases other than CO2 that can be used, but those that are realistic to use are all non-polar. The only alternatives are the freons, of which chlorine-free freons are considered to be less harmful to the environment (Blackwell and Schallinger, 1994). 

Mobile phases for argentation chromatography. It is necessary to add acetonitrile to the CO2 to facilitate elution of TG. Acetonitrile modifies the interactions between the analytes and the stationary phase and also improves the solubility of the analytes in the mobile phase. Although the solubility of TG in supercritical CO2 is, in general, good, the presence of a polar modifier may diminish the risk of analyte precipitation as the pressure is released in the restrictor. 

In order to achieve a mobile phase that is homogeneous under all conditions applied, a minor amount of 2-propanol is added to the mobile phase. Typically, a mobile phase for the elution, on an argentation column, of TG from a vegetable oil consists of carbon dioxide-acetonitrile-2-propanol (92.8 6.5 0.7 mol%) (Demirbiiker and Blomberg, 1991 Demirbiiker, Hag-glund and Blomberg, 1990 Demirbiiker et al., 1992). 

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The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

Asche, W., Mobile phases for supercritical fluid chromatography, Chromatographia, 11, 411, 1978. 

French S B, Novotny M 1986 Xenon, a uique mobile phase for supercritical fluid chromatography. Anal Chem 58 164-166... 

For volatile materials vapor phase chromatography (gas chromatography) permits equilibration between the gas phase and immobilized liquids at relatively high temperatures. Tire formation of volatile derivatives, e.g., methyl esters or trimethylsilyl derivatives of sugars, extends the usefulness of the method.103104 A method which makes use of neither a gas nor a liquid as the mobile phase is supercritical fluid chromatography.105 A gas above but close to its critical pressure and temperature serves as the solvent. The technique has advantages of high resolution, low temperatures, and ease of recovery of products. Carbon dioxide, N20, and xenon are suitable solvents. 

The utilization of supercritical fluids in conjunction with adsorbents and active solids is well documented in the technical literature. The most frequently cited applications involve the use of dense gases for the regeneration of adsorbents (1) and as mobile phases in supercritical fluid chromatography (2). Numerous... 

Pyo, D. Ju, D. Simple method for the preparation of water-modified or methanol-modified carbon dioxide as the mobile phase in supercritical fluid chromatography. Anal. Sci. 1994, 0(1), 171-174. 

Supercritical fluid chromatography (SFC) is a column chromatographic technique in which a supercritical fluid is used as a mobile phase. A supercritical fluid is a gas or liquid brought to a temperature and a pressure above its critical point. The first report of SFC dates back to 1962 when Kesper et al. [1] used supercritical fluid chlorofluorocarbons as a mobile phase for the separation of metal porphyrins. It was not until the early 1980s that an important breakthrough of the technique occurred. This was the introduction of capillary SFC and the availability of commercial instrumentation. These became major factors in the recent rise in popularity of SFC. According to the latest estimation, approximately 100 SFC articles are published in major journals every year. 

There are two general approaches for the separation of enantiomers [1-4,28-32]. The direct method is based on the formation of transient diastereomer association complexes with a chiral selector immobilized in the stationary phase, or added to the mobile phase. The former approach requires the use of special stationary phases (section 10.4) while the later uses conventional stationary phases with special additives included in the mobile phase (section 10.5). When preparative applications are contemplated the use of immobilized chiral selectors is the more common approach. Method selection also depends on the choice of the separation mode. Table 10.2. While chiral stationary phases are the only choice for gas chromatography [16,28,33-38], and are used almost exclusively for supercritical fluid chromatography [39-43] and capillary electrochromatography [44-47], they also dominate the practice of liquid chromatography... 

Nizery, D. Thiebaut, D. Caude, M. Rosset, R. Lafosse, M. Dreux, M. Improved evaporative Ught-scattering detection for supercritical fluid chromatography with carbon dioxide-methanol mobile phases. J. Chromatogr. 1989, 467,49-60. 

Jentoft R E, Gouw T H 1972 Apparatus for supercritical fluid chromatography with carbon dioxide as the mobile phase. Anal Chem 44 681-686... 

The chiral columns used for liquid chromatography may also be used for supercritical fluid chromatography (SFC). SFC offers important advantages over HPLC and GC in the separation of enantiomers. First, SFC provides a higher resolution per unit of time than does LC, because the diffusion rates in the mobile phase and linear velocities are higher. Second, SFC chromatography is carried out at temperatures well below those used in GC. LC and GC detectors, such as FID (flame iruiized detectors) and mass spectrometry (MS), may also be applied to SFC (Chamberlain et al. 1998). 

Supercritical fluid chromatography (SFC) refers to the use of mobile phases at temperatures and pressures above the critical point (supercritical) or just below (sub-critical). SFC shows several features that can be advantageous for its application to large-scale separations [132-135]. One of the most interesting properties of this technique is the low viscosity of the solvents used that, combined with high diffusion coefficients for solutes, leads to a higher efficiency and a shorter analysis time than in HPLC. 

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