Supercritical fluid extraction sample preparation

Neither MAE nor ASE is currently in a configuration that would readily lead to the automation of sample preparation. Supercritical fluid extraction can be used as online system that can then be connected to the chromatographic and detection systems. Connected online with the GC/MS, SFE was successfully used for the determination of PAHs in marine sediments. Using either CO2 alone or modified with toluene or MeOH in the extraction, the PAHs were cryofocused in the accumulation cell of the GC and then directly chromatographed. For the study of PAHs in marine sediments, a new extraction technique, which consists of the combination of ASE (dynamic and static mode) and SFE (dynamic mode), was developed, with an extraction time longer than in ASE but shorter than in SFE, and... 

S. A. Westwood (Ed.), Supercritical Fluid Extraction and its Use in Chromatographic Sample Preparation, CRC Press, Boca Raton, PE (1992). 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

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). 

Separation and detection methods A survey on determination of tin species in environmental samples has been published by Leroy et al. (1998). A more detailed overview of GS-MS methodology has been published by Morabito et al. 1995) and on sample preparation using supercritical fluid extraction has been described by Bayona (1995)- The techniques are now under control, so that routine procedures are available at a relatively low cost (Leroy et al. 1998). 

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. ... 

Current interest in supercritical fluid extraction as a sample preparation technique for chromatographic analysis is intense, in spite of it receiving very little tion until the mid-1980s. Although neglected by analytical cl Hsts, during the... 

Principles and Characteristics Supercritical fluid extraction uses the principles of traditional LSE. Recently SFE has become a much studied means of analytical sample preparation, particularly for the removal of analytes of interest from solid matrices prior to chromatography. SFE has also been evaluated for its potential for extraction of in-polymer additives. In SFE three interrelated factors, solubility, diffusion and matrix, influence recovery. For successful extraction, the solute must be sufficiently soluble in the SCF. The timescale for diffusion/transport depends on the shape and dimensions of the matrix particles. Mass transfer from the polymer surface to the SCF extractant is very fast because of the high diffusivity in SCFs and the layer of stagnant SCF around the solid particles is very thin. Therefore, the rate-limiting step in SFE is either... 

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... 

L.G. RandaU, W.S. Miles, F. Rowland and C.R. Knipe, Designing a Sample Preparation Method Which Employs Supercritical Fluid Extraction (SFE), Hewlett-Packard Publ. 435091-2102E, Wilmington, DE (1994). 

A.L. Howard and L.T. Taylor, in Supercritical Fluid Extraction and Its Use in Chromatographic Sample Preparation (S.A. Westwood, ed.), Blackie, London... 

More elaborate sample preparation is often needed for complex sample matrices, e.g., lotions and creams. Many newer SP technologies such as solid-phase extraction (SPE), supercritical fluid extraction (SFE), pressurized fluid extraction, accelerated solvent extraction (ASE), and robotics are frequently utilized (see Ref. [2]). Dosage forms such as suppositories, lotions, or creams containing a preponderance of hydrophobic matrices might require more elaborate SP and sample cleanup, such as SPE or liquid-liquid extraction. 

Klesper, E. Corwin, A. H. Turner, D. A. J. Org. Chem., 1962, 27, 700. Sugiyama, K. Saito, M. Hondo, T. Seda, M. J. Chromatogr. 1985, 332, 107. Anderson, I. G. M. Supercritical fluid extraction coupled to packed column supercritical fluid chromatography. In Supercritical Fluid Extraction and Its Use in Chromatographic Sample Preparation Westwood, S. A. Ed., Blackie Academic and Professional, Chapman and Hall UK, 1993 p. 112. 

Classical sample preparation methods such as distillation, soxhlet extraction are still used [839, 840], but specific techniques such as supercritical fluid extraction (SFE) [841], and increasingly in recent years, adsorption techniques such as solid phase micro-extraction (SPME) [841a] are also being used for isolation, separation, and identification of flavor and fragrance materials. 

For sample preparation, isolation and separation traditional methods like distillation (e.g. essential oil content of raw materials) or Soxhlet extraction are still in use. Beyond that, more recent methods are employed, for example supercritical fluid extraction with liquid carbon dioxide. 

Before the extraction procedure may commence, the sample must be prepared in such a way that it is in a condition for extraction of the analyte(s). For analyzing sulfonamide residues in liquid samples such as milk, a pretreatment dilution step with water prior to direct fluorometric detection may be required (207). Dilution of milk with aqueous buffer (208) or sodium chloride solution (209) prior to sample cleanup has also been reported. For the analysis of honey a simple dissolution of the sample in water (210, 211) or aqueous buffer (212) is generally required. Semisolid samples such as muscle, kidney, and liver, require, however, more intensive sample pretreatment. The analyte(s) must be exposed to extracting solvents to ensure maximum extraction. The most popular approach for tissue break-up is through use of a mincing and/or homogenizing apparatus. Lyophilization (freeze-drying) of swine kidney has been carried out prior to supercritical-fluid extraction of trimethoprim residues (213). 

Starting with a description of the analytical challenge in Chapter 19, the third part, which is devoted to analytical attitudes, proceeds with a detailed description in Chapter 20 of modern sample preparation procedures including solid-phase extraction, matrix solid-phase dispersion, use of restricted-access media, supercritical fluid extraction, and immunoaffinity cleanup. Flexible derivatization techniques including fluorescence, ultraviolet-visible, enzymatic, and photochemical derivatization procedures are presented in Chapter 21. 

The objective of this panel was to develop protocols that could be used today with a minimum of methods development. The panel members recognized that promising research is currently underway to develop new techniques for sample preparation. However, procedures such as supercritical fluid extractions or preparative ion chromatographic separation of polar fractions are still considered to be in a research phase and not yet ready for general laboratory application. The procedures presented will be periodically reviewed and updated as the state of available technology improves. 

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