Selectivity in supercritical fluid

Analytical supercritical fluid extraction 7.7.1. Selectivity in supercritical fluid extraction... 

Retention and selectivity in supercritical fluid chromatography (SFC) are a complex function of many experimental variables and are not as easily rationalized as in the case of gas and liquid chromatography. Retention in SFC is dependent on temperature, density (and pressure drop), stationary-phase composition, and the mobile-phase composition. Many of these variables are interactive and do not change in a simple or easily predicted manner [1]. 

Solubilities and selectivities in supercritical fluid mixtures near critical end... 

The column pressure practically does not influence the separation selectivity in gas and liquid chromatography. It, however, substantially influences the separation selectivity in supercritical fluid chromatography (SFC)." ... 

Smith and Udseth [154] first described SFE-MS in 1983. Direct fluid injection (DFT) mass spectrometry (DFT-MS, DFI-MS/MS) utilises supercritical fluids for solvation and transfer of materials to a mass-spectrometer chemical ionisation (Cl) source. Extraction with scC02 is compatible with a variety of Cl reagents, which allow a sensitive and selective means for ionising the solute classes of interest. If the interfering effects of the sample matrix cannot be overcome by selective ionisation, techniques based on tandem mass spectrometry can be used [7]. In these cases, a cheaper and more attractive alternative is often to perform some form of chromatography between extraction and detection. In SFE-MS, on-line fractionation using pressure can be used to control SCF solubility to a limited extent. The main features of on-line SFE-MS are summarised in Table 7.20. It appears that the direct introduction into a mass spectrometer of analytes dissolved in supercritical fluids without on-line chromatography has not actively been pursued. 

In analytical chemistry there is an ever-increasing demand for rapid, sensitive, low-cost, and selective detection methods. When POCL has been employed as a detection method in combination with separation techniques, it has been shown to meet many of these requirements. Since 1977, when the first application dealing with detection of fluorophores was published [60], numerous articles have appeared in the literature [6-8], However, significant problems are still encountered with derivatization reactions, as outlined earlier. Consequently, improvements in the efficiency of labeling reactions will ultimately lead to significant improvements in the detection of these analytes by the POCL reaction. A promising trend is to apply this sensitive chemistry in other techniques, e.g., in supercritical fluid chromatography [186] and capillary electrophoresis [56-59], An alter-... 

M. Buback, Kinetics and selectivity of chemical processes in fluid phases, in Supercritical Fluids Fundamentals for Application, E. Kiran and J. M. H. Levelt-Sengers, eds., Kluwer, Dordrecht, 1994. 

In order to overcome this lack of selectivity, photodimerizations have been performed in micelles,16,17 in supercritical fluids,18 in inclusion compounds19 and in the solid state.20 Nevertheless, such reactions are difficult to run on a preparative scale, and better results can be obtained by careful choice of an appropriate solvent. Enantioselcctive gas chromatography combined with GC/MS analysis proved to be a very efficient tool for the direct assignment of constitution and configuration of the photocyclodimers formed.21 In this manner, /ram-1,2-di-vinylcyclobutane has been prepared by sensitized irradiation of buta-1,3-diene.22... 

The use of enzymes in supercritical fluids presents the problem that the number of parameters influencing the stability of the enzymes increases dramatically. This is the reason why, up to now, no prediction can be made of whether an enzyme is stable under supercritical conditions and if the equivalent of even higher activity and selectivity is available compared with reactions in organic solvents. In the following chapters the influence of parameters on the enzyme stability will be given. Although much research has been done [2-5] it is very difficult to compare these results because non-standard methods have been applied. 

Hitzler, M. G., Smail, F. R., Ross, S. K., Poliakoff, M. Selective Catalytic Hydrogenation of Organic Compounds in Supercritical Fluids as a Continuous Process. Organic Process Research Development. 1998, 2, 137 - 146. 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

Selective and Complete Hydrogenation of Vegetable Oils and Free Fatty Acids in Supercritical Fluids... 

A novel fluidized-bed coating process using the rapid expansion of supercritical solutions (RESS) is described for the encapsulation of fine particles [2,3]. This process exploits the capability of supercritical fluids to act as a selective solvent. Supercritical fluids are noteworthy in that their... 

Mixtures of C02 and methanol were selected for the initial investigation of the solvatochromic behavior in supercritical fluid systems. This combination is of interest as it combines the low critical temperature and pressure of carbon dioxide with a polar, less volatile modifier. This system exhibits relatively simple Type I phase behavior and several groups have published measurements of mixture critical points (19-21). At intermediate compositions the critical pressure for this fluid is much higher than that of either pure C02 or pure methanol, reaching a maximum of approximately 2400 psi (20). 

In addition to chemicals, biological catalysts such as enzymes can be used to catalyze reactions in SC CO2. Since the first attempt to operate reactions in supercritical fluids published by Randolph et al. [34], various type of enzymes were studied lipase, oxidase, decarboxylase, dehydrogenase, proteinase, etc. [33,35-37]. The effect of different parameters was extensively reported by Ballesteros et al. [35]. Enzyme activity and stability in supercritical conditions as well as the benefits of using supercritical fluids for enzymatic reactions (improved reaction rates, control of selectivity, etc.) have been demonstrated [36]. 

Selectivity Factors Affecting, in Supercritical Fluid Chromatography... 

St. Ktippers, The use of temperature variation in supercritical fluid extraction of polymers for the selective extraction of low molecular weight components from poly(ethylene terephthalate), Chromatographia 33 434-440 (1992). 

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