Supercritical Subcritical Fluid Chromatography

Super- and subcritical fluid chromatography (SFC) underwent extremely rapid development in the mid-1980s, and was believed to be a technique with a great potential that combined the advantages of GC and HPLC. However, inherent fundamental and technical limitations of this technique became apparent and led to some stagnation in its further development. 

In spite of this critical note, the potential of SFC in analytical and preparative-scale enan-tioseparations has been already illustrated. Technical development in this field may open even more challenges for this technique. The advantage of SFC for preparative separations is that the high-pressure liquid carbon dioxide used as mobile phase can easily be removed from the product. In addition, carbon dioxide is non-hazardous and relatively inexpensive. On the other hand, this mobile phase creates the following problems the solubility of polar compounds is limited, and alcohols or other polar modifiers have to be used. Although this makes the technical advantage of SFC questionable, the method may offer some advantages for chiral compounds that may dissolve in SFC mobile phases. Selected examples of preparative SFC enantioseparations are summarized in Table 10 [168-171]. 

Chiral compound CSP Mobile phase Sample size, mg References 

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Camel V, Thiebaut D, Caude M, Dreux M. Packed column subcritical fluid chromatography of underivatized amino acids. J Chromatogr 1992 605 95-101. Anton K, Bach M, Geiser A. Supercritical fluid chromatography in the routine stability control of antipruritic preparations. J Chromatogr 1991 553 71-79. Giddings JC, Meyers M, Wahrhaftig AL. Int J Mass Spectrom Ion Physi 1970 4 9-20. 

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. 

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

Supercritical fluid chromatography using carbon dioxide as the eluent is often carried out subcritically at 20°C or 25°C, because the more dense eluent... 

Chiral Chromatography by Subcritical and Supercritical Fluid Chromatography... 

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. 

A, acaricide AV, avicide I, insecticide F, fungicide H, herbicide GR, growth regulator N, nematocide R, rodenticide US, ultrasonication LLE, liquid-liquid extraction CU, cleanup ImCU, immuno cleanup SPE, solid-phase extraction MSPD, matrix solid-phase dispersion SBSE, stir bar sorptive extraction SD, steam distillation FMSE, focused microwave Soxhiet extraction PFE, pressurized fluid extraction ASE, assisted solvent extraction ScFE, subcritical fluid extraction SFE, supercritical fluid extraction SEC, size-exclusion chromatography LC, liquid chromatography (fraction collection) LTLP, low temperature lipid precipitation. 

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. 

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. 

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