Selection of a Supercritical Fluid

Supercritical carbon dioxide (supercritical CO2) has been employed as an extracting agent in the majority of analytical SF extractions because of its advantageous characteristics accessible critical properties (31.1°C, 72.8 bar), high purity, nontoxic, nonflammable, chemical inertness, nonpolluting and relatively inexpensive. It may be vaporized at atmospheric pressure 

Supercritical CO2 is an excellent extracting agent for lipophilic analytes (alkanes, terpenes) and suitable extracting agent for moderated polar analytes (PAHs, PCBs, organochlorides, pesticides, aldehydes and esters). The main problem is its strong apolar character, which reduces its use in polar analyte extractions. This problem may be solved, at least partially, by means of a modifier, or with small additions of polar organic solvents (i.e., methanol, ethanol...). 

When polar analytes and/or analytes strongly fixed to the matrix sample are extracted, it is necessary to increase the analyte solubility in the SF by means of a modifier. A modifier is commonly a polar organic solvent which is added in a small percentage to enhance the SF polarity. Hence, the polar organic analyte extraction efficiency is also enhanced. The most common modifiers employed are methanol, propanol, tetrahydrofuran, acetonitrile, formic acid, acetone, ethyl acetate, toluene, methyl chloride, hexane and water.  

The modifier effect, entrainer effect, is defined as the analyte solubility increase produced by adding a small amount of a second solvent to the primary one (supercritical fluid). This solubihty increase is produced by analyte-modifier interactions in the supercritical phase through the intermolecular stresses (i.e., hydrogen bonds).  

The addition of a modifier can produce changes in the SF properties and in the sample matrix. The fluid properties which may change are  

---

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

The solvating power of a supercritical fluid is manipulated by changing its density. An increase in density increases the solvating power of the supercritical fluid. In addition, the oven temperature can be varied to affect the selectivity of a supercritical fluid. The polarity of a supercritical fluid is altered by the addition of an organic modifier such as methanol or acetonitrile. Table 7.1 lists some of the commonly used organic modifiers added to carbon dioxide [2]. Supercritical fluids generally exhibit a high number of theoretical plates and selectivity when compared with HPLC or GC columns. 

Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure. This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. 

The supercritical fluid extraction of analytes from solid sorbents is controlled by a variety of factors including the affinity of the analytes for the sorbent, the tortuosity of the sorbent bed, the vapor pressure of the analytes, and the solubility and the diffusion coefficient of the analytes in the supercritical fluid. Additionally, SFE efficiencies are affected by a complex relationship between many experimental variables, several of which are listed in Table I. Although it is well established that, to a first approximation, the solvent power of a supercritical fluid is related to its density, little is known about the relative effects of many of the other controllable variables for analytical-scale SFE. A better understanding of the relative effects of controllable SFE variables will more readily allow SFE extractions to be optimized for maximum selectivity as well as maximum overall recoveries. 

Much of the research discussed in this chapter has demonstrated that sc C02 is a viable alternative solvent for free-radical reactions. Moreover, several of these studies demonstrate that the unique properties of a supercritical fluid, specifically the ability to change important solvent properties such as viscosity by varying pressure (and temperature), can be exploited to manipulate reaction yield and selectivity. Solvent viscosity is particularly important for reactions that are diffusion-controlled or those for which cage-effects are important. 

SFE is carried out above the solvent critical point, and the properties of a supercritical fluid depend on pressure and change along with its density. These criteria determine the selectivity of the extraction medium. One fluid can therefore be used to extract a whole series of compound groups (depending on the pressure in the system, the temperature, extraction medium volume flow, and extraction time) and to separate the obtained extract into appropriate fractions. Selective fractionation is used, for example, to separate olfactory and gustatory substances in the extraction of hops for beer production. 

The addition of an entrainer to a supercritical (SC) solvent can lead to a very large enhancement in the solubility of a solute (up to several hundred percent) [1-3]. This phenomenon, often called the entrainer effect, has relevance in the SC fluid technology. The addition of a small amount of entrainer can increase the solubility of a solid much more than a pressure increase of several hundred bars [4]. Because the entrainer effect depends upon the nature of the solute it can be used to enhance the selectivity of a SC fluid for certain compounds. 

As stated in the introduction, the variable solvent power of a supercritical fluid is a nearly linear function of the density of the fluid. It is instructive to compare this way of selecting a solvent power to the more familiar operation used with ordinary liquid solvents. Figure 2 is a schematic of solute solubility as a function of solvent power where the solvent power ranges from that of non-polar hexane to polar water a multitude of solvents lies in between, only a few of which are shown. Consider a hypo-... 

Solubility and selectivity in a supercritical fluid are strong functions of temperature and pressure. For nearly total extraction of solutes by supercritical... 

COMMERCIAL PROCESS. A practical example of a supercritical fluid extraction process is the decaffeination of coffee. Coffee beans are first soaked in water to make the extraction more selective and then are loaded into an extraction vessel through which supercritical CO2 is circulated to dissolve the caffeine. In a separate scrubbing vessel the caffeine is transferred from the CO2 to water, also at high pressure. Extraction is continued until the caffeine content of the beans, originally... 

Some of the areas in which supercritical CO2 (SCCO2) is commercially important are summarized in Figure 8.9. Extraction processes in the food, tobacco (nicotine extraction) and pharmaceutical industries dominate. Supercritical CO2 is a selective extracting agent for caffeine, and its use in the decaffeination of coffee and tea was the first commercial application of a supercritical fluid, followed by the extraction of hops in the brewing industry. Solvent extractions can be carried out by batch processes, or by a continuous process in which the CO2 is recycled as shown schematically below ... 

The slight variation in r(371°) can be explained as follows. The rate constants for 1°, 2°, or 3° hydrogen abstractions by Cl from alkanes are nearly diffusion-controlled in conventional solvents. Consequently, the intrinsic selectivity of Cl is diminished in conventional solvents because of the onset of diffusion control. In the gas phase, selectivity is slightly higher because the barrier imposed by diffusion is eliminated. The viscosity of a supercritical fluid (a) lies between that of conventional fluid solvent and the gas phase and (b) varies with pressure. Because of the low viscosity of supercritical fluids, bimolecular rate constants greater than the 10 ° M" s diffusion-controlled limit can be realized in SCF and, as a consequence, enhanced selectivity is achieved. Consistent with this interpretation is the observation that the plot of r(371°) versus inverse viscosity is approximately linear (Figure 4.4-7) [51]. 

The examples discussed demonstrate that the unique nature of SCFs provides a means of dialing up the selectivity of a chemical process in a manner that is impossible using conventional solvents (i.e. by manipulation of temperature and pressure). SCF solvents such as CO2 and H2O, the latter of which has not yet been exploited as a solvent for organic photochemistry, are especially attractive as they are environmentally benign alternatives to a number of classical solvents which pose hazards to either health or the environment. Coupled with the tunable properties of a supercritical fluid, these solvents emerge not only as viable alternatives to conventional organic solvents, but in some cases at least, superior alternatives. Finally, a point which has not been fully emphasized in this chapter is that SCF solvents are superb tools for probing... 

A logical extension of supercritical fluid extraction in chemical analysis is to combine the process with a chromatographic method. The variable solvating power of a supercritical fluid provides the mechanism for the selective extraction of the components of interest... 

The mobile phase plays different roles in GC, LC, and SFC. Ordinarily, in GC the mobile phase serves but one purpose — zone movement. As we have seen in Chapter 28. in LC the mobile phase provides not only transport of solute molecules but also interactions with solutes that influence selectivity factors (or values). When a molecule dissolves in a supercritical medium, the process resembles volatilization but at a much lower temperature than would normally be used in GC. Thus, at a given temperature, the vapor pressure for a large molecule in a supercritical fluid may be 10 ° times greater than in the absence of the fluid. Because of this, high-molecular-mass compounds, thermally unstable species, polymers, and large biological molecules can be eluted from a column at relatively low temperatures. Interactions between solute molecules and the molecules of a supercritical fluid must occur to account for their solubility in these media. The... 

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... 

---