Supercritical-fluid extraction analytical-scale

A complete understanding of SFE and its relation to other extraction methods cannot be made without some knowledge of the basic properties of supercritical fluids and the basic principles of analytical SFE instrumentation. The purpose of this section is to give an introduction to the use of supercritical fluids in analytical-scale extractions while focusing on the application of SFE to pharmaceutical analysis. 

Supercritical fluid extraction (SFE) has been extensively used for the extraction of volatile components such as essential oils, flavours and aromas from plant materials on an industrial as well as an analytical scale (61). The extract thus obtained is usually analysed by GC. Off-line SFE-GC is frequently employed, but on-line SEE-GC has also been used. The direct coupling of SEE with supercritical fluid chromatography (SEC) has also been successfully caried out. Coupling SEE with SEC provides several advantages for the separation and detection of organic substances low temperatures can be used for both SEE and SEC, so they are well suited for the analysis of natural materials that contain compounds which are temperature-sensitive, such as flavours and fragrances. 

Supercritical fluid extraction (SFE) is a fluid extraction process that has been applied on both a processing scale and for analytical chemical applications. 

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. 

Supercritical fluid extraction (SFE) utilizes the unique properties of supercritical fluids to facilitate the extraction of organics from solid samples. Analytical scale SFE can be configured to operate on- or off-line. In the online configuration, SFE is coupled directly to an analytical instrument, such as a gas chromatograph, SFC, or high-performance liquid chromatograph. This offers the potential for automation, but the extract is limited to analysis by the dedicated instrument. Off-line SFE, as its name implies, is a stand-alone extraction method independent of the analytical technique to be used. Off-line SFE is more flexible and easier to perform than the online methods. It allows the analyst to focus on the extraction per se, and the extract is available for analysis by different methods. This chapter focuses on off-line SFE. 

Fahmy, T.M., Pulaitis, M.E., Johnson, D.M., McNally, M.E.P., Modifier effects in the supercritical fluid extraction of solutes from clay, soil, and plant materials. Anal. Chem., 65 (10), 1462-1469,1993. Langenfeld, J.J., Hawthorne, S.B., Miller, D.J., Pawliszyn, J., Role of modifiers for analytical scale supercritical fluid extraction of environmental samples. Anal. Chem., 66(6), 909-916,1994. Hawthorne, S.B., Methodology for off-line supercritical fluid extraction. In Supercritical Fluid Extraction and Its Use in Chromatographic Sample Preparation, Westwood S.A. (Ed.), Blackie Academic and Professional, 39-64, 1993. 

Turner, C., Eskilsson, C.S., Bjorklund, E. Collection in analytical-scale supercritical fluid extraction. J. Chromatogr. A 947, 1-22 (2002)... 

Camel, D. Tambut6, B. Caude, M. Analytical-scale supercritical fluid extraction A promising technique for the determination of pollutants in environmental matrices. J. Chromatogr., A 1993, 642 (1-2), 263 -281. 

J. J. Langenfeld, S. B. Hawthorne, D. J. Miller, J. Pawliszyn, Role of modifiers for analytical-scale supercritical fluid extraction of environmental samples. Anal. Chem., 66 (1994), 909-916. 

Hawthorne SB. Analytical-scale supercritical fluid extraction. Anal Chem 1990 62 633A-642A. 

In the next section, we will give an overview of the instrumentation generally used to carry out analytical-scale supercritical fluid extraction. 

A simple diagram outhning key elements in the process of analytical-scale supercritical fluid extraction is shown in Figure 6. As noted earlier, the process is straightforward ... 

Finally, although it is not rigorously part of supercritical fluid extraction, a last step generally afforded hy analytical-scale supercritical fluid extractors is reconstitution of the extracted components in a solution that is appropriate for the subsequent analytical instrument. In the case of gravimetric assays, the net loss of sample weight or the net weight of extract components can be measured and reconstitution of extracted components is not necessarily employed. 

In this introductory book chapter, several modem extraction techniques will be described, including supercritical fluid extraction, pressurized liquid extraction, pressurized hot Avater extraction, microwave assisted extraction, membrane-assisted solvent extraction, solid phase micro extraction and stir-bar sorptive extraction. These are techniques that meet many of today s requirements in terms of environmental sustainability, speed and automation. Basic principles of operation as well as method optimization will be discussed and compared for the different techniques. Both analytical and industrial applications will be discussed, together with commercial instruments available on the market. Key references will be given, and conclusions regarding applicability of the different techniques with respect to sample e, target-molecules and analytical vs. large-scale applications. 

King, J. W. (1995) Analytical-process supercritical fluid extraction a synergistic combination for solving analytical and laboratory scale problems. Trends Anal Chem., 14,474-81. 

Analytical methods for monitoring the compounds were developed or modified to permit the quantification of all 23 compounds of interest. As noted earlier, the compounds were initially studied in small-scale extractions by groups. This approach assured minimal interferences in the analyses conducted during the initial supercritical fluid carbon dioxide extractions. Table II summarizes the data on the recovery of organics from aqueous samples containing the compounds of interest at concentration levels listed in Table I when the sample preparation techniques and analytical methods described were used. For each experimental run, blank and spiked aqueous samples were carried through the sample prepration and analytical finish steps to ensure accurate and reproducible results. Analyses of sodium, calcium, and lead content were also conducted on selected samples by using standard atomic ab-... 

Several liquids and gases can be brought into the supercritical phase. Different solvents can be selected as extraction media for use in analytical-scale SFE. Carbon dioxide is most commonly used as an SFE medium because of its desirable properties and easy handling it is relatively inexpensive and commercially available at a purity grade acceptable for most analytical applications. Another advantage of carbon dioxide is that the polarity can easily be adjusted by adding modifiers such as methanol to the supercritical fluid or the extraction vessel. 

In analytical-scale SFE, the usual approach has been to expand the compressed mixture to nearly ambient pressure the supercritical fluid expands to ideal-gas conditions and the solutes precipitate in a collection region. This makes an SFE instrument conceptually quite simple. Contact a sample with a supercritical fluid at the designated temperature, density (controlled by pressure), and chemical composition to dissolve the analytes of choice impose a flow through the sample to remove the dissolved analytes from the sample region and expand the resultant mixtiu-e of extraction solvent plus extracted components to ambient pressure to get rid of the solvent and collect the concentrated extracts. At that point, what is left is a reconstitution step to get the extract into a solvent compatible with the target analytical instrument. 

The rest of this section will present details about various aspects of the process getting the extraction solvent from a gas cylinder to the extraction chamber, controlling the extraction conditions, separating the extracted components from the supercritical extraction fluid, and collecting and reconstituting extracted components. Note that the discussion will focus on using carbon dioxide as the primary, bulk extraction fluid since this is the most widespread fluid in use for analytical-scale SFE at this time. 

Supercritical fluids (SFs) are being used increasingly as extraction solvents due to increased restrictions on the use of traditional solvents, particularly chlorinated solvents, that are a result of environmental legislation. Carbon dioxide (CO2) has been the solvent of choice due to its low toxicity, relatively low cost, convenient critical properties, and ease of recycling. Supercritical (SC) CO2 has been found to be a particularly useful solvent for extraction of organic compounds at both analytical and process scales (1). Direct extraction of metal... 

---