Critical temperatures chromatography

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. 

Figure 7.5 The Enhanced Fluidity Liquid Chromatography range. This occupies the volume in the phase diagram below the locus of critical temperatures, above and below the locus of critical pressures, and is composed mostly of the less volatile mobile-phase component. Reproduced by permission of the American Chemical Society.

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

IUPAC defines supercritical chromatography as a separation technique in which the mobile phase is kept above (or relatively close to) its critical temperature and pressure. 

A variety of modern instrumental analytical techniques have attracted considerable attention in the last decades as alternative separation and analysis methods with respect to HPLC. This includes, in particular, supercritical fluid chromatography (SFC), which utilizes condensed carbon dioxide (above or near its critical temperature of... 

Supercritical fluid chromatography is a form of chromatography in which the system is held near the critical temperature of the mobile phase and pressure utilized to effect solvency and hence migration. 

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. 

Supercritical Fluid. To be useful as a mobile phase in chromatography, a supercritical fluid must have a relatively low critical temperature and pressure, and a relatively high density/solvating power at experimentally accessible pressures and temperatures. The former criterion excludes water and most common organic solvents, whereas the latter excludes such low-boiling substances as helium, hydrogen, and methane. Commonly used fluids are listed in Table I. 

Figure 7.6 The Supercritical Fluid Chromatography range, above both the critical temperature and pressure at all compositions. (Reprinted with permission from reference 17. Copyright 1997 American Chemical Society.) Reproduced by permission of the American Chemical Society.

Supercritical fluid chromatography-thermal energy analyser Supercritical fluids are produced by heating a gas above its critical temperature or compressing a liquid above its critical pressure. In SFC, the sample is carried through a separating column by a supercritical fluid (typically C02) that is used as mobile... 

Supercritical fluid chromatography (SFC) is a GC method of analysis of compounds in systems where normal GC presents resolution difficulties (Lee and Markides, 1987). A supercritical fluid has properties at a critical temperature intermediate between a liquid and a gas. At and above this critical temperature, a gas cannot be compressed into a liquid, irrespective of the pressure, but it solvates solid matter as if it were a liquid. A supercritical fluid diffuses freely into and out of adsorbent pores with a minimum of resistance. A major advantage of SFC chromatography is its ability to effect separation of oligomers without derivatization. 

The mobile phases that have been used most extensively to date are n-pentane and carbon dioxide. Pentane has the advantage that it is a liquid under ambient conditions, so that it can be handled and pumped in the same way as mobile phases for liquid chromatography. On the other hand, its critical temperature is relatively high (almost 200 °C) and it is a highly inflammable compound. 

The key step in developing a reduced coordinate system lies in identifying characteristic scaling parameters. In the case of gases, these characteristic parameters are the critical pressure, the critical volume, and the critical temperature. We seek similar scaling parameters for chromatography. 

Klesper, E., Corwin, A.H., and Turner, D.A. 1962. High pressure gas chromatography above critical temperature. Journal of Organic Chemistry, 27 700. 

Over the past twenty years, fluids above their critical temperatures and pressures have been used for a variety of analytical purposes first as mobile phases in chromatography [1,2], thus giving rise to supercritical fluid chromatography, and then as solvents for the selective extraction of species — from solid samples in most cases [3-5]. 

The advantages of using supercritical mobile phases in chromatography were recognized in the 1950s by Kles-per et al. [1], among others. Carbon dioxide is the most frequently used supercritical mobile phase, due to its moderate critical temperature and pressure, almost... 

To control and maintain the critical temperature of the mobile phase (CO2), the column is installed in an oven, similar to those used for GC or high-performance liquid chromatography (HPLC), depending on the type of column used (Fig. 2). 

---