Supercritical fluid extraction sample collection

Removing an analyte from a matrix using supercritical fluid extraction (SEE) requires knowledge about the solubiUty of the solute, the rate of transfer of the solute from the soHd to the solvent phase, and interaction of the solvent phase with the matrix (36). These factors collectively control the effectiveness of the SEE process, if not of the extraction process in general. The range of samples for which SEE has been appHed continues to broaden. Apphcations have been in the environment, food, and polymers (37). 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

The instrumental requirements for supercritical fluid extraction are quite simple. A pump is essential to generate the extraction pressure in a themostated extraction vessel. The soluble sample components are then swept from the vessel through a flow restrictor into a collection device that is normally at ambient pressure. The fluid used for supercritical fluid... 

To date most of the work which has been done with supercritical fluid extraction has concentrated on the extraction of analytes from solid matrices or liquids supported on an inert solid carrier matrix. The extraction of aqueous matrices presents particular problems [276-278]. The co-extraction of water causes problems with restrictor plugging, column deterioration, and phase separation if a nonpolar solvent is used for sample collection. Also, carbon dioxide isay have limited extraction efficiency for many water soluble compounds. 

Figure 28-14a shows how a supercritical fluid extraction can be carried out. Pressurized fluid is pumped through a heated extraction vessel. Fluid can be left in contact with the sample for some time or it can be pumped through continuously. At the outlet of the extraction vessel, the fluid flows through a capillary tube to release pressure. Exiting C02 evaporates, leaving extracted analyte in the collection vessel. Alternatively, the C02 can be bubbled through a solvent in the collection vessel to leave a solution of analyte. 

In this paper, the supercritical fluid extraction (SFE) of organic compounds from sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and from a standard reference material certified for 13 polynuclear aromatic hydrocarbons (PAH), dibenzofuran, and pentachlorophenol was examined using a two- and a four-vessel extractor. Although the results achieved by SFE for the sand and the standard reference soil samples were very encouraging, previous data obtained in our laboratory on the standard reference soil and a few other standard reference marine sediments were less favorable. It was therefore decided that an investigation of seven variables for their influence on the analyte recoveries from the standard soil sample would be useful. Two tests were conducted in which these variables were investigated. In Test 1, the seven variables selected were pressure, temperature, moisture content, cell volume, sample size, extraction time, and modifier volume. In Test 2, the seven variables were pressure, temperature, volume of toluene added to the matrix, volume of solvent in the collection vessel,... 

Bjorklund, E., L. Mathiasson, P. Persson, et al. 2001. Collection capacity of a solid phase trap in supercritical fluid extraction for the extraction of lipids from a model fat sample. J. liquid Chromatogr. Rel. Technol. 24 2133-2143. 

Supercritical fluid extraction has been applied to extract phenolic acids from a variety of plant samples. It uses high-pressure to force carbon dioxide to be a mixture of liquid and gas phases, which is called a supercritical fluid. The liquid and gas phase mixture of carbon dioxide can more readily permeate the sample matrix than only the gas phase of carbon dioxide. The compounds solublized in the liquid phase of carbon dioxide are extracted from the sample matrix and collected after they elute from the outlet of the system. The biggest advantage of supercritical fluid extraction is that there is no, or less, organic solvent involved in the extraction, due to the use of carbon dioxide supercritical fluid as the major solvent.. The carbon dioxide readily evaporates as gas phase at the system outlet. Thus, unlike other solvent extraction methods, there is no evaporation step for the extraction, making this an environmentally friendly method. However, the system is much more expensive and delicate than the other novel technology extraction... 

Supercritical fluid extraction coupled to SFC has been used for the extraction, separation and identification of PAHs from coal. The supercritical extract was expanded with the aid of a frit restrictor accommodated in the sample cavity of a cooled micro-injector, the analytes being deposited by condensation while CO, was sent to waste through a vent valve. Subsequently, the loop contents were connected on-line to the mobile phase of the capillary chromatograph. The extracted analytes were detected by off-line FTIR spectroscopy following collection on a KBr disc and evaporation of the solvent [104]. 

Chlorinated phenolic compounds in air-dried sediments collected downstream of chlorine-bleaching mills were treated with acetic anhydride in the presence of triethylamine. The acetylated derivatives were removed from the matrix by supercritical fluid extraction (SEE) using carbon dioxide. The best overall recovery for the phenolics was obtained at 110°C and 37 MPa pressure. Two SEE steps had to be carried out on the same sample for quantitative recovery of the phenolics in weathered sediments. The SEE unit was coupled downstream with a GC for end analysis . Off-line SEE followed by capillary GC was applied in the determination of phenol in polymeric matrices . The sonication method recommended by EPA for extraction of pollutants from soil is inferior to both MAP and SEE techniques in the case of phenol, o-cresol, m-cresol and p-cresol spiked on soil containing various proportions of activated charcoal. MAP afforded the highest recoveries (>80%), except for o-cresol in a soil containing more than 5% of activated carbon. The SEE method was inefficient for the four phenols tested however, in situ derivatization of the analytes significantly improved the performance . 

Increased effort has been directed to the application of supercritical fluids for extraction of environmental samples. The range of fluids that has been examined, details of the extraction and collection procedures, and a comparison with conventional procedures for extraction have been given (Bowadt and Hawthorne 1995). An extensive review that covers both supercritical fluid chromatography and supercritical fluid extraction has been given (Chester et al. 1998) and includes 533 references. The following is therefore a highly selective and compromised summary. 

McDowell and Metcalfe proposed the use of supercritical fluid extraction (SEE) with ultra high purity CO2 as supercritical fluid for the extraction of phthalate esters from sediment samples. Extracts were collected by bubbling the vented gas through 12 ml hexane, and solvent... 

Some techniques that combine the properties of extraction and cleanup are supercritical fluid extraction (SEE) and matrix solid-phase dispersion (MSPD). Supercritical fluids, i.e., at a temperature and pressure in excess of their critical point, have unique properties for selective extraction of analytes from a sample. Solid samples are mixed with an inert dispersant, such as hydromatrix, and the mixture packed into the cell of the SEE apparatus. The sample is extracted with supercritical CO2, with or without addition of organic modifier, and the extracted analytes may be collected inline or offline on suitable adsorbents (Figure 3). Further cleanup of the sample extract may be performed using SPE. MSPD is based on intimate mixing of animal tissue sample with a bonded silica, such as Cig, and packing of the blended material into a column from which interferences can be eluted by washing with solvents and the analytes eluted using a selective solvent. 

The procedure is described as follows supercritical fluid extiactions were performed with an automated ISCO SFX m 3560 instrument using 6 mL extraction vessels. The extraction vessels were filled with 100 mg of dried plant samples mixed with anhydrous sodium sulfate. The extracted analytes were collected into 10 mL of methanol. The internal standard was added to the collection vials immediately after extraction. The collection temperature was 5 °C. The SFE instrument was equipped with a 260 mL syringe pump for the addition of carbon dioxide at a flow rate of 1.5 mL/min and a manually controlled Jasco PU-980 HPLC pump for addition of the modifier (methanol) at flow rates of 0.04-0.1 mL/min (2.6-6.6 %). The restrictor temperature was set at 60 °C in all extractions [11]. In this paper, the extracts from SFE were found to be much cleaner in comparismi with those obtained by solid-liquid extractions or Soxhlet extractions. The results showed that supercritical fluid extraction is a valuable alternative technique to traditional extraction methods of Catharanthus alkaloids from dried leaves. 

On a semipreparative scale, SFC has shown its value for the isolation of tocopherols and tocotrienols from wheat germ oil, a process which may be of industrial interest (162,166). It consists of four steps, i.e., supercritical fluid extraction (SEE) of the oil, preconcentration, chromatography on a silica column with UV detection at 290 nm, and fractionation. Tocopherols were considerably enriched (up to 70% to 85% purity) in the collected fractions relative to the crude samples, in which they represented only minor constituents among an excess of triglycer-... 

Supercritical fluid extraction (SFE) is the most widespread of these methodologies. SFE is based on the use of a fluid at temperatures and pressures near the critical point. It comprises an extraction phase where the analyte is extracted from the sample matrix, followed by collection or trapping of the analytes, which might be coupled online into an analytical instrument, usually a liquid chromatograph. Off-line collection of the analytes can also be achieved after depressurizing of the supercritical fluid (SF) into a collection device such as an empty vessel, a vessel containing a small volume of solvent, solid-phase or solid-liquid phase traps, or a cryogenicaUy cooled capillary (reviewed by Turner etal. ). 

By changing the density of the snpercritical flnid, different fractions may be selectively extracted from the complex mixtnre or simple matrix. On decompression, the extracted solntes are precipitated and may be collected from injection into a gas chromatography or SFC for analysis. Figure 7.2 shows a simple apparatus for on-line supercritical fluid extraction (SFE)/SFC solutes extracted from the sample matrix are deposited from the end of a restrictor into the internal loop of the microinjection valve of the capillary SFC. The valve loop contents are subsequently switched into the SFC column by means of liquid or supercritical carbon dioxide. 

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