Subcritical and Supercritical Fluid Chromatography

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. 

As a matter of fact, the main advantage in comparison with HPLC is the reduction of solvent consumption, which is limited to the organic modifiers, and that will be nonexistent when no modifier is used. Usually, one of the drawbacks of HPLC applied at large scale is that the product must be recovered from dilute solution and the solvent recycled in order to make the process less expensive. In that sense, SFC can be advantageous because it requires fewer manipulations of the sample after the chromatographic process. This facilitates recovery of the products after the separation. Although SFC is usually superior to HPLC with respect to enantioselectivity, efficiency and time of analysis [136], its use is limited to compounds which are soluble in nonpolar solvents (carbon dioxide, CO,). This represents a major drawback, as many of the chemical and pharmaceutical products of interest are relatively polar. 

Although some applications for preparative-scale separations have already been reported [132] and the first commercial systems are being developed [137, 138], examples in the field of the resolution of enantiomers are still rare. The first preparative chiral separation published was performed with a CSP derived from (S -N-(3,5-dinitrobenzoyl)tyrosine covalently bonded to y-mercaptopropyl silica gel [21]. A productivity of 510 mg/h with an enantiomeric excess higher than 95% was achieved for 6 (Fig. 1-3). 

Examples with other Pirkle-type CSPs have also been described [139, 140]. In relation to polysaccharides coated onto silica gel, they have shown long-term stability in this operation mode [141, 142], and thus are also potentially good chiral selectors for preparative SFC [21]. In that context, the separation of racemic gliben-clamide analogues (7, Fig. 1-3) on cellulose- and amylose-derived CSPs was described [143]. 

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TABLE 11 The Enantiomeric Resolution of Racemic Compounds on Cyclodextrin-Based CSPs by Means of Subcritical and Supercritical Fluid Chromatography... 

Chiral Chromatography by Subcritical and Supercritical Fluid Chromatography... 

Cantrell, G.O. Stringham, R.W. Blackwell, J.A. Weckwerth, J.D. Carr, P.W. Effect of various modifiers on selectivity in packed-column subcritical and supercritical fluid chromatography. Anal. Chem. 1996, 68 (20), 3645-3650. 

Toward this end, we have investigated tandem or coupled processes that embodied the use of pressurized fluids, namely carbon dioxide, for both extraction, fiactionation and reaction. Related exanqrles to the work described here are coupling supercritical fluid extraction (SFE) with production scale supercritical fluid chromatography (SFC) for the enrichment of high value tocopherols from natural botanical sources (/0), or subcritical water hydrolysis of vegetable oils (77) followed 1 partition into dense carbon dioxide to produce industrially-useM mixtures of tty acids. 

Supercritical fluid chromatography (SEC) refers to the use of mobile phases at temperatures and pressures above the critical point (supercritical). SEC uses carbon dioxide as a main component in the mobile phase because its critical point (31.3°C, 7.39 MPa) is easy to reach. However, carbon dioxide is similar to alkanes in solvent strength and therefore unsuitable for the elution of polar compounds. This character is corrected by the addition of a significant amount of polar solvents, mainly alcohols, to increase the polarity of the mobile phase. In such conditions, the supercritical state is not actually reached. Often temperatures lower than the critical and pressure above the critical are applied. These are designated as subcritical conditions. Nevertheless, separations are performed indistinctively in super- or subcritical conditions. 

Supercritical fluids can also be used as mobile phases in chromatography [20, 21). Stationary phases used in both GC and LC can be employed. The sample is normally injected into a mobile phase which is in the subcritical liquid state. Subsequently it is converted into a supercritical fluid by raising the temperature above the critical point. 

Analysis of plants normally involves a sample preparation stage such as extraction or distillation followed by analysis with gas chromatography or liquid chromatography. The common methods used currently for the isolation of essential oils from natural products are steam distillation and solvent extraction (Ozel Kaymaz, 2004). Losses of some volatile compounds, low extraction efficiency, degradation of xmsaturated compounds through thermal or hydrolytic effects, and toxic solvent residue in the extract may be encountered with these extraction methods. Recently, more efficient extraction methods, such as supercritical fluid extraction (SFE) (Simandi et al., 1998) and accelerated solvent extraction (ASE) (Schafer, 1998) have been used for the isolation of organic compounds from various plants. Subcritical or superheated water extraction (SWE) is non-toxic, readily available, cheap, safe, non-flammable and is a recyclable option. 

Mourier PA, EUot E, Caude MH, Rosset RH, Tambute AG. Supercritical and subcritical fluid chromatography on a chiral stationary phase for the resolution of phosphine oxide enantiomers. Anal Chem 1985 57 2819-23. 

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