Preconcentration supercritical fluid extraction

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. ... 

Analytes can be separated from complex matrices by sample preparation techniques that include liquid extraction, supercritical fluid extraction, and solid-phase extraction. Dilute ionic analytes can be preconcentrated by adsorption onto an ion-exchange resin. Nonionic analytes can be concentrated by solid-phase extraction. Derivatization transforms the analyte into a more easily detected or separated form. 

Supercritical fluid extraction [153,154], accelerated solvent extraction [68] and subcritical fluid extraction [107,155] have been studied. To reduce the equipment cost and the analysis time in the extraction process and sample preconcentration, a solid-phase microextraction method was proposed by Pawliszyn and coworkers [156-158]. 

Efforts are being made to reduce both the use of organic solvents and the time-consuming clean-up and preconcentration steps. One such approach is to apply supercritical fluid extraction (SFE) [ 164] with carbon dioxide as the extraction medium for sample preparation. 

Supercritical fluid extraction (SFE), usually with carbon dioxide and, often, with a modifier, has become of increasing interest in the last few years because of its selectivity, preconcentration effect, efficiency, simplicity, rapidity, cleanness, and safety, mainly concerning the extraction of organic compounds prior to separation and detection by chromatographic techniques. It has several advantages over classical solvent extractions, in comparison with recent extraction techniques. Approaches to obtain quantitative extractions, including fluid choice, extraction flow rate, modifiers, pressure, and temperature, are presented, as well as the potential for SFE to extract polynuclear aromatic hydrocarbons (PAHs) from soils, sediments, and biota. Improvements and new environmental applications are also reported. 

Examples of basic studies of offline extraction and preconcentration of pesticide residues using other techniques, such as online dialysis, steam distillation, supercritical fluid extraction, pressurized liquid extraction, cloud point extraction, or liquid-liquid membranes, have been reported. The large amounts of matrix coextractives and the need for clean extracts in CE/ultraviolet (UV) analysis are the main reasons for their scarce application. 

Supercritical fluid extraction (SEE) using supercritical carbon dioxide (SC-CO2) has been successfully used for isolation of volatile nitrosamines from different matrices such as tobacco and food products. This technique presents several advantages with respect to other extraction methods (e.g., mineral oil distillation or low-temperature vacuum distillation) currently used. Thus, SEE minimizes sample handling, provides fairly clean extracts, expedites sample preparation, and reduces the use of environmentally toxic solvents. Good results have also been obtained with the use of SPE in the analysis of food matrices combining extraction with Extrelut sorbent and purification with Florisil. This method is applicable for the analysis of a range of the most widely encountered volatile N-nitrosamines, including the poorly volatile NDBA, NDBzA, and N-nitroso-N-methylaniline in various food products. Active carbon is suitable for this preconcentration step due its low cost, versatility, and easy application. 

Given the variety of explosives, it is no surprise that chromatography is central to their analysis. Techniques that are or have been used include TLC, GC, HPLC, ion chromatography, CE, MEKC, supercritical fluid extraction, and size exclusion chromatography. Samples can be prepared for chromatographic analysis with simple solvents using water, acetone, and the like. SPME is also used to preconcentrate samples, particularly when the explosives must be extracted from an aqueous environment. SPME can be adapted to HPLC to avoid thermal degradation issues that commonly arise in explosives. Another approach is to derivatize the explosives directly on the fiber such that the derived substance can be analyzed by means of GC. ... 

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-... 

In spite of the significance of the publications cited the technique of preparative SFC has received relatively little attention when compared with analytical SFC. The method has the potential capability of replacing normal phase preparative HPLC because when coupled with supercritical fluid extraction (SFE) it carries out the extraction, preconcentration and chromatographic fractionation in a single nm according to Saito et al. [13]. 

More recently, some studies have reported the use of supercritical fluid chromatography (SEC) [479,480], Coupling SEC with SEE, sample extraction, preconcentration, and quantification can be performed in a single step. The mobile phase, carbon dioxide, can be modified by adding different... 

Wai, C.M. Lin, Y. Ji, M. Toews, K.L. Smart, N.G. "Extraction and Separation of Uranium and Lanthanides with Supercritical Fluids, Chapter 23, pp 390-400 in "Metal Ion Separation and Preconcentration Progress and Opportunities"-, A. H. Bond and M.L. Dietz Eds ACS Symposium Series 716 American Chemical Society Washington, DC, 1998 Oxford University Press 1999. 

CM Wai, Y Lin, M Ji, KL Toews, NG Smart. Extraction and separation of uranium and lanthanides with supercritical fluids. In AH Bond, ML Dietz, RD Rogers, eds. ACS Symposium Series 716 Progress in Metal Ion Separation and Preconcentration. Washington, DC Am Chem Soc, 1999, Chapter 21, pp. 390-400. 

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