Supercritical fluid chromatography solutions

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. 

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. 

Finally, supercritical fluid chromatography, in which a supercritical fluid is used as the mobile phase, was introduced by Klesper [164-166]. SFE directly coupled to SFC provides an extremely powerful analytical tool. The efficient, fast and selective extraction capabilities of supercritical fluids allows quantitative extraction and direct transfer of the selected solutes of interest to be accomplished to the column, often without the need for further sample treatment or cleanup. Extraction selectivity is usually achieved by adjusting the pressure of the supercritical fluid at constant temperature or, less often, by changing the temperature of the supercritical fluid at constant pressure. SFE coupled with packed column SFC has found... 

Supercritical fluid chromatography provides increased speed and resolution, relative to liquid chromatography, because of increased diffusion coefficients of solutes in supercritical fluids. (However, speed and resolution are slower than those of gas chromatography.) Unlike gases, supercritical fluids can dissolve nonvolatile solutes. When the pressure on the supercritical solution is released, the solvent turns to gas. leaving the solute in the gas phase for easy detection. Carbon dioxide is the supercritical fluid of choice for chromatography because it is compatible with flame ionization and ultraviolet detectors, it has a low critical temperature. and it is nontoxic. 

Cosolvent-modifled supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16t17t20). However, distinguishing between such effects in a chromatography... 

H. Daimon and Y. Hirata, Trapping efficiency and solute focusing in on-hne supercritical fluid extraction/capillary supercritical fluid chromatography , J. Microcolumn Sep. 5 531-535(1993). 

C.M. White, D.R. Gere, D. Boyer, F. Pacholec, and L.K Wong, Analysis of pharmaceuticals and other solutes of biochemical importance by supercritical fluid chromatography, J. High Resolut. Chro-matogr., 77 94(1988). 

T.A. Berge and J. F. Deye, Correlation between column surface area and retention of polar solutes in packed-column supercritical fluid chromatography, J. Chromatogr., 594 291 (1992). 

The complicated dependence of retention in supercritical fluid chromatography as a function of temperature and pressure is examined. Simple thermodynamic relationships are derived and discussed which allow the calculation of the slope of solute retention as a function of both temperature and pressure. 

The effect of temperature, pressure and density on solute retention (k1) in supercritical fluid chromatography (SFC) has been well studied.(1-6) Retention in SFC depends upon both solute solubility in the fluid and solute interaction with the stationary phase. The functional relationship between retention and pressure at constant temperature has been described by Van Wasen and Schneider. ( 1 ) The trend in retention is seen to depend on the partial molar volume of... 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

Supercritical fluid chromatography (SFC) is very similar in principle to, and is as convenient as, high-performance liquid chromatography, but it uses as the high-pressure eluant fluid CO2 (or other fluid) above its critical point (for CO2 Tc = 31.3°C, Pq = 7.38 MPa, pc = 0.448 g cm-3). SPC can separate relatively small and/or thermally labile molecules. The analyte is introduced as a solution in methanol. Small amounts of organic solvents can be added as "modifiers." Any C02 brought out with the analyte to ambient atmosphere will evaporate harmlessly. 

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