Extraction Protocol
Factors Determining Extractability
Extraction Scenarios
Extraction Profiles
Mechanics of Analytical SFE
Kinetic Models of Supercritical Fluid Extraction
General equations
Solute mass balance for supercritical phase
Solute transfer across the interface (sink-source term)
External mass transfer
Internal mass transfer
Overall mass transfer resistance
Desorption kinetics
Instantaneous solute transfer
Local equilibrium concept
Initial and boundary conditions
Modeling results and discussion - Static stage
Fluid displacement models for full SFE static recovery
Modeling analytical SFE tests (CSTR)
New equilibrium model for analytical SFE tests
Establishing new SFE models by analogy
Extraction Aids
Matrix Problems
Modifier Introduction
Developing a Method
Quantification in SFE

Supercritical fluid extraction can be accomplished using a static, dynamic, or coupled static/dynamic mode. In static extraction, a fixed amount of SF interacts with the analyte/matrix (e.g., tea bag + cup of water). Normally, the extraction vessel containing the matrix is pressurized with the chosen fluid at a certain tem計erature. The high diffusivity of the SF is then utilized to access the analyte/ma負rix. Alternatively, a pump may be used to recycle the limited amount of SF through the matrix. After the extraction is completed, a valve at the outlet of the cell is opened to allow analyte to be swept from the cell via decompression into the trap. Frequently, a static extraction is followed by several minutes of dynamic extraction to enhance removal of the extracted analytes from the extraction ves貞el. Rather than trap the analytes offline, some analysts have essentially sampled either the SF headspace or the recycled SF by withdrawing an aliquot for analy貞is. In this case, the concentration of analyte in the aliquot will have to be rela負ively high for a successful analysis because there will have been no trapping or concentration of the analyte. Without a recycling pump, thorough mixing of the SF phase with the matrix may not be possible in this mode.

The static mode conserves SF and is often used when modifiers and derivatizing reagents are employed. For example, the liquid polar modifier or derivatizing reagent can simply be added to the cell prior to pressurization, rather than being premixed with the fluid phase. A static extraction, however, may not be exhaus負ive if insufficient fluid has been used. On the other hand, fluid contamination is seldom a problem in a static extraction, unless the analyte is present at trace levels.

A dynamic extraction employs fresh SF which is continuously passed over and/or through the sample matrix (e.g., coffee maker). A dynamic extraction can be more exhaustive than a static one; however, impurities in the SF become a concern when using large amounts of fluid during an extraction. The contami要ants in the SF will ultimately arrive at the collection device and become con苞entrated, and may interfere with the extract analysis. For example, an extraction with CO 2 that contains 1.0 ppb nonvolatile hydrocarbon impurity will yield 0.1 u.g of impurity at the trap if the dynamic mode required 100 g of CO 2 for extrac負ion. The quality of CO 2 and its packaging is extremely high in the United States and CO 2 impurity is seldom an issue, but in other parts of the world less pure SF is quite common. Another experimental problem with dynamic extraction is an enhanced probability for coextraction of matrix components—the use of more SF should in turn remove more marginally extractable components. A long dy要amic extraction also risks the unwanted physical movement of matrix compo要ents to the trapping device. Removal of analytes from the trap also becomes more probable as dynamic extraction time increases. In spite of these problems, dynamic extraction is the favored strategy for at least 90% of all reported appli苞ations of SFE.

A combination of an initial static period followed by a dynamic one is gaining popularity, especially for situations where solvated analyte must diffuse to the matrix surface to be extracted. The extraction starts in a static mode with no net flow through the system. When the extraction has proceeded for a given amount of time, the system is put into a dynamic mode by switching the valves. Fresh SF enters the vessel, replacing the original SF which has exited through the restric-tor to the trap. Multiple combinations of static/dynamic cycles have been em計loyed recently for the quantitative removal of a drug from a crushed tablet, for example,121 via this extraction strategy. A single dynamic extraction yielded ap計roximately 90% recovery; the static/dynamic protocol gave 99% recovery of the drug.

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