Supercritical fluid extraction of polar analytes

Supercritical Fluid Extraction of Polar Analytes Using Modified C02 and In Situ Chemical... 

Hawthorne, S.B., D.J. Miller, D.E. Nivens, and D.C. White. 1992. Supercritical fluid extraction of polar analytes using in situ chemical derivatization. Anal. Chem. 64 405-412. 

Jimenez-Carmona, M. M. and Luque de Castro, M. D., Reverse-micelle formation a strategy for enhancing C02-supercritical fluid extraction of polar analytes. Anal. Chim. Acta, 358, 1-4, 1998. 

Hawthorne, S. B., D. J. Miller, J. J. Langenfeld, Supercritical fluid extraction of polar analytes using modified carbon dioxide and in situ chemical derivation, in F. V. Bright and M. E. P. McNally, eds.. Supercritical Fluid Technology Theoretical and Applied Approaches to Analytical Chemistry. ACS Symposium Series, No. 488, Washington, 1992. 

Moore WN, Taylor LT. Analytical inverse supercritical-fluid extraction of polar pharmaceutical compounds from cream and ointment matrices. J Pharm Biomed Anal 1994 12 1227-1232. 

Meyer et al. [173] showed that supercritical fluid extraction results can give recoveries comparable to Soxhlet extraction methods, even for soils with high carbon contents. McNally et al. [174] have studied factors affecting the supercritical fluid extraction of soils. It was shown that soil type affects the recovery of moderately polar analytes. In general the organic carbon content of the soil governs analytical recovery. 

Messer DC, Taylor LT. Development of analytical SFE [supercritical fluid extraction] of a polar drug from an animal food matrix. J High Resolut Chromatogr 1992 15 238-241. 

Supercritical fluid extraction (SEE) is another modern separation technology usually employed to extract lipophilic compounds such as cranberry seed oil, lycopene, coumarins, and other seed oils. Anthocyanins generally and glycosylated anthocyanins in particular were considered unsuitable for SEE due to their hydrophilic properties, since SEE is applicable for non-polar analytes. However, a small amount of methanol was applied as co-solvent to increase CO2 polarity in anthocyanin extraction from grape pomace. New applications of SEE for anthocyanin purification have been reported for cosmetic applications from red fruits. ... 

Supercritical fluid extraction (SFE) is generally used for the extraction of selected analytes from solid sample matrices, but applications have been reported for aqueous samples. In one study, recoveries of 87-100% were obtained for simazine, propazine, and trietazine at the 0.05 ug mL concentration level using methanol-modified CO2 (10%, v/v) to extract the analytes, previously preconcentrated on a C-18 Empore extraction disk. The analysis was performed using LC/UV detection. Freeze-dried water samples were subjected to SFE for atrazine and simazine, and the optimum recoveries were obtained using the mildest conditions studied (50 °C, 20 MPa, and 30 mL of CO2). In some cases when using LEE and LC analysis, co-extracted humic substances created interference for the more polar metabolites when compared with SFE for the preparation of the same water sample. ... 

Principles and Characteristics In an attempt to develop a unified sample preparation system for extraction of various matrix/analyte combinations Ashraf-Khorassani et al. [498] have described a hybrid supercritical fluid extraction/enhanced solvent extraction (SFE/ESE ) system to remove both polar and nonpolar analytes from various matrices. The idea is that a single instrument that can perform extractions via pure C02 solvent, and all gradients thereof affords... 

Supercritical fluid extraction uses a supercritical fluid (Box 25-2) as the extraction solvent.20 C02 is the most common supercritical fluid because it is inexpensive and it eliminates the need for costly disposal of waste organic solvents. Addition of a second solvent such as methanol increases the solubility of polar analytes. Nonpolar substances, such as petroleum hydrocarbons, can be extracted with supercritical argon.21 The extraction process can be monitored by infrared spectroscopy because Ar has no infrared absorption. 

Environmental applications of SFE appear to be the most widespread in the literature. A typical example is the comparison of extraction efficiency for 2,3,7,8 -tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from sediment samples using supercritical fluid extraction and five individual mobile phases with Soxhlet extraction was made (101). The mobile phases, carbon dioxide, nitrous oxide, pure and modified with 2% methanol as well as sulfur hexafluoride were examined. Pure nitrous oxide, modified carbon dioxide and modified nitrous oxide systems gave the recoveries in the acceptable range of 80 to 100%. Carbon dioxide and sulfur hexafluoride showed recoveries of less than 50% under identical conditions. Classical Soxhlet recoveries by comparison illustrated the poorest precision with average extraction efficiencies of less than 65%. Mobile phase choice, still as yet a major question in the science of supercritical fluid extraction, seems to be dependent upon several factors polarity of the solute of interest, stearic interactions, as well as those between the matrix and the mobile phase. Physical parameters of the solute of interest, as suggested by King, must also be considered. Presently, the science behind the extraction of analytes of interest from complex matrices is not completely understood. 

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. 

U. Ullsten and K. E. Markides, Automated on-line solid phase adsorption/supercritical fluid extraction/supercritical fluid chromatography of analytes from polar solvents , J. Micmcolumn Sep. 6 385-393 (1994). 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

In more recent years, supercritical fluid extraction has been found to be useful for the extraction of low to moderately polar compounds. The requirements in any particular case depend on the characteristics of the matrix, the drug, and the drug metabolites. The concentration range of the drug is also obviously important, as it will determine the methods selected for the separation and analytical procedures. 

Hills et al. (1991) applied simultaneous supercritical fluid extraction to roasted coffee beans. This technique can be used with or without a derivatizing reagent In a dissociative mechanism, the adsorbed analyte must first desorb from a matrix active site and be dissolved in the supercritical fluid and then react to form the less polar derivative, which favors solvation in the supercritical carbon dioxide. In the associative mechanism, derivatization occurs while the analyte is adsorbed on the active site of the matrix. Reaction with the adsorbed analyte results in the desorption of the non-polar derivative into the supercritical fluid . Thus 2-hexenedioic acid (E.49) was identified for the first time as a native compound (without the use of a derivatizing agent). Benzenic and furanic compounds and caffeine were also identified. 

One limitation of carbon dioxide as an extractant is its polarity. In its supercritical state and at low densities, CO2 has a polarity close to that of hexane. Even at extremely high pressures the solubility parameter may not approach that which is required to solubilize and extract polar analytes. This limitation can be overcome by the use of another extraction fluid, which is more polar, or by adding a polar modifier to the CO2. The most commonly used modifier with CO2 has been methanol. Increased solubilities and recoveries of polar analytes have been reported when a polar modifier is added to a less polar supercritical fluid (66-68). The ability of the supercritical fluid to dissolve a particular analyte is not the only factor, which affects extraction efficiency. The degree to which the analyte partitions into the supercritical fluid fi om the solid-sample matrix depends greatly on the sorptive and active sites on the solid matrix and the polarity of the solute (64,69). The addition of a polar modifier or entrainer, such as methanol, to a supercritical fluid such as CO2, not only increases the solubility of polar analytes in the supercritical fluid, but also may help block sorptive sites on the surface of the sample matrix. 

An automatic method for the separation and determination of RF vitamin in food samples (chicken liver, tablet, and powder milk) is proposed by Zougagh and Rios [2], The method is based on the online coupling of supercritical fluid extraction (SFE) with a continuous flow-CE system with guided optical fiber fluorometric detection (CE-CE-ED). The whole SFE-CF-CE-FD arrangement allowed the automatic treatment of food samples (cleanup of the sample followed by the extraction of the analytes), and the direct introduction of a small volume of the extracted material to the CE-ED system for the determination of RF vitamins. Fluorescence detection introduced an acceptable sensitivity and contributed to avoidance of interferences by nonfluorescent polar compounds coming from the matrix samples in the extracted material. Electrophoretic responses were linear within the 0.05-1 pg/g range, whereas the detection limits of RE vitamins were in the 0.036-0.042 pg/g range. 

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