| 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 temperature. The high diffusivity of the SF is then utilized to access the analyte/matrix. 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 vessel. Rather than trap the analytes offline, some analysts have essentially sampled either the SF headspace or the recycled SF by withdrawing an aliquot for analysis. In this case, the concentration of analyte in the aliquot will have to be relatively 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 exhaustive 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 contaminants in the SF will ultimately arrive at the collection device and become concentrated, 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 extraction. 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 dynamic extraction also risks the unwanted physical movement of matrix components 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 applications 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 employed recently for the quantitative removal of a drug from a crushed tablet, for example,121 via this extraction strategy. A single dynamic extraction yielded approximately 90% recovery; the static/dynamic protocol gave 99% recovery of the drug. |
