Modifiers, in supercritical

In this table, we provide solubility parameters for some liquid solvents that can be used as modifiers in supercritical fluid extraction and chromatography. The solubility parameters (in MPa1/2) were obtained from reference 3, and those in cal1/2cm 3/2 were obtained by application of Equation 4.1 for consistency. It should be noted that other tabulations exist in which these values are slightly different, since they were calculated from different measured data or models. Therefore, the reader is cautioned that these numbers are for trend analysis and separation design only. For other applications of cohesive parameter calculations, it may be more advisable to consult a specific compilation. This table should be used along with the table on modifier decomposition, since many of these liquids show chemical instability, especially in contact with active surfaces. 

Levy JM, Ritchey WM. Investigations of the uses of modifiers in supercritical-fluid chromatography. J Chromatogr Sci 1986 24 242-248. 

Ashraf-Khorassani, M. Levy, J.M. Addition of modifier in supercritical fluid chromatography using a microhore reciprocating pump. Chromatographia, 1995, 40, 78-84... 

L. J. Mulcahey, Applications of Modifiers in Supercritical Fluid Extraction and Chromatography, PhD dissertion, Virginia Tech, Blacksburg, VA, 1991. 

A gradient that runs with 30-80% methanol or acetonitrile is not uncommon. This amount of modifier is generally not needed in supercritical fluid chromatography to affect the same separation. Typical modifier composition in SFC is 1.0-10% and would achieve higher Hildebrand Solubility Parameter adjustment overall than the broader gradients found in LC. 

Current work with supercritical fluids can also illustrate the importance of cosolvents. Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. A correlation suggests that both physical and chemical forces are important in the solvation process in polar cosolvent supercritical CO2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermody-namic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide (Ting et al., 1993). 

Extraction of the amine surfactant using pure CO2 at 65°C and 150 bar has been attempted, but it has been found from TGA that there is completely no extraction at all. In 1985, Dandge et al. [4] have reported that the solubility of amines in supercritical CO2 generally decreases with increasing basicity (Kb). The threshold basicity, above which the compound becomes insoluble in CO2, was found to be about 10 9. As such, it is a foregone conclusion that the surfactant dodecylamine (Kb 10 4) is insoluble in pure supercritical CO2. This explains the zero extraction observed and thereby justifies the need of a modifier (methanol in this case). 

The use of supercritical fluid chromatography for carotene separation has been examined and optimized, especially in regard to temperature, pressure, and organic modifiers in the supercritical fluid (71). With an RP column it was possible to resolve an a-carotene-cis isomer from an all-trans carotene as well as two cis isomers of /3-carotene from an all-trans /3-carotene. As with HPLC, only polymeric C,8 columns were able to resolve the cis isomers of a- and /3-carotene from the all-trans isomers. Supercritical fluid chromatography offers the advantage not only of an efficient separation but also of fast analysis. Indeed, the use of SFC with ODS-based columns for the analysis of carotenoid pigments affords a threefold reduction of analysis time compared to HPLC (72). The elution order of carotenoids and their cis isomers was found to be the same as in RP-HPLC. The selectivity of the system could further be increased by adding modifiers (e.g.,... 

Supercritical fluid extraction conditions were investigated in terms of mobile phase modifier, pressure, temperature and flow rate to improve extraction efficiency (104). High extraction efficiencies, up to 100%, in short times were reported. Relationships between extraction efficiency in supercritical fluid extraction and chromatographic retention in SFC were proposed. The effects of pressure and temperature as well as the advantages of static versus dynamic extraction were explored for PCB extraction in environmental analysis (105). High resolution GC was coupled with SFE in these experiments. 

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). 

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... 

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

Modified Mobile Phases. In addition to pure supercritical fluids, much research has been performed on the use of modifiers with supercritical fluids. That is, rather than switching to a completely different supercritical fluid for the mobile phase, a small percentage of a secondary solvent can be added to modify the mobile phase while (hopefully) maintaining the mild critical parameters of the primary fluid. 

TABLE I Modifiers That Have Been Used in Supercritical Fluid Technology With Carbon Dioxide as the Primary Supercritical Fluid... 

In addition to varying modifier identities to achieve different extraction efficiencies in SFE, another parameter that can be varied is the concentration of the modifier that is added to the primary supercritical fluid. In many cases as the concentration of the respective modifier in the primary supercritical fluid increases the critical temperature needs to be adjusted to accommodate the change. The adjustment of the corrected critical temperature is normally done on a mole fraction basis (moles of CO2 versus moles modifier). 

Since the early days of SFC, there always has been a desire to extend the useful range of the technique to more polar molecules. A similar type of desire exists in SFE. The hope for achieving efficient extractions of polar molecules from polar as well as non-polar substrates can only be realized with the use of more polar primary supercritical fluids or by the use of modifiers. Many of the more primary supercritical fluids that exists namely, ammonia or water, are not effectively usable in the analytical laboratory due to instrumental as well as safety restrictions, therefore, the need to do more research on the use of modifiers in SFE is greatly necessitated. Based upon the limited study that was done within the scope of this chapter, a few conclusions can be drawn. These conclusions are summarized in Figure 16. 

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