Analytical SFE

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 

Various models of SFE have been published, which aim at understanding the kinetics of the processes. For many dynamic extractions of compounds from solid matrices, e.g. for additives in polymers, the analytes are present in small amounts in the matrix and during extraction their concentration in the SCF is well below the solubility limit. The rate of extraction is then not determined principally by solubility, but by the rate of mass transfer out of the matrix. Supercritical gas extraction usually falls very clearly into the class of purely diffusional operations. Gere et al. [285] have reported the physico-chemical principles that are the foundation of theory and practice of SCF analytical techniques. The authors stress in particular the use of intrinsic solubility parameters (such as the Hildebrand solubility parameter 5), in relation to the solubility of analytes in SCFs and optimisation of SFE conditions. 

Bartle et al. [286] described a simple model for diffusion-limited extractions from spherical particles (the so-called hot-ball model). The model was extended to cover polymer films and a nonuniform distribution of the extractant [287]. Also the effect of solubility on extraction was incorporated [288] and the effects of pressure and flow-rate on extraction have been rationalised [289]. In this idealised scheme the matrix is supposed to contain small quantities of extractable materials, such that the extraction is not solubility limited. The model is that of diffusion out of a homogeneous spherical particle into a medium in which the extracted species is infinitely dilute. The ratio of mass remaining (m ) in the particle of radius r at time t to the initial amount (mo) is given by  

After Bartle et al. [286], Reproduced from Journal of Supercritical Fluids, 3, K.D. Bartle et al., 143-149, Copyright (1990), with permission from Elsevier. 

In solubility limited extractions the rate is reduced at the beginning of the extraction. Models for SFE predict that these extractions should be carried out at as high a temperature and pressure as possible. However, the polymer will eventually soften and melt, and the particles will coalesce. 

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SFE instrument development has greatly been stimulated by the desire of the Environmental Protection Agency (EPA) to replace many of their traditional liquid-solvent extraction methods by SFE with carbon dioxide. In the regulatory environment, EPA and FDA approved SFE and SFC applications are now becoming available. Yet, further development requires interlaboratory validation of methods. Several reviews describe analytical SFE applied to polymer additives [89,92,324]. 

A real breakthrough of analytical SFE for in-polymer analysis is still uncertain. The expectations and needs of industrial researchers and routine laboratories have not been fulfilled. SFE presents some severe drawbacks (optimisation, quantification, coupling, and constraints as to polarity of the extractable analytes), which cannot easily be overcome by instrumental breakthroughs but... 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

Current practices in analytical SFE are organized into off-line and on-line procedures, despite their common physicochemical basis. Off-line SFE, the current method in fashion, offers more flexibility with respect to extracting different sample sizes and types, as well as in the choice of the final analytical method. On-line procedures are usually combinations of SFE with ancillary techniques such as GC, LC, supercritical fluid, or gel permeation chromatography. 

Commercial SFE systems have been on tlie market for only the past 5 years. Currently available systems are not perceived as being ready to perform routine extraction work. It is believed, however, that although no single analytical technique can hope to solve the diversity of sample preparation problems confronting analysts, analytical SFE will eventually take its rightful place among other sample preparation methods. 

M. E. P. McNally, Advances in Environmental SFE, AnaL Chem. 1995,67, 308A L. T. Taylor, Strategies for Analytical SFE, Anal. Chem. 1995,67,364A. 

Feller, F.J. and King, J.W. 1996. Determination of fat content in foods by analytical SFE. Semin. Food Anal. 1 145-162. 

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 can be performed effectively with very simple systems. Figure 5 displays the basic components of an effective analytical SFE device. There are relatively few commercial suppliers of dedicated supercritical fluid extraction instrumentation. Table 7 shows the companies that promote SFE instrumentation as of the writing of this chapter. Some of the more traditional instrument manufacturers such as Hewlett-Packard (7680T SFE), Dionex (SFE 723), and Supelco (SFE-400) have discontinued their SFE lines. Dionex has invested quite heavily into high-temperature/high-pressure solvent extraction devices, and this will be described in the next section. For most purposes, inexpensive and efficient extraction units can be assembled using the basic components shown in Figure 5. 

Taylor LT. Strategies for analytical SFE. Anal Chem 1995 67 364A-370A. Bartle KD. In Smith RM, ed. Supercritical Fluid Chromatography. Cambridge Royal Society of Chemistry, 1988 2-4. 

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. 

With the advent of automated analytical SFE equipment, it has become possible to rapidly ascertain what extraction or fractionation conditions would be most relevant in scaling up the process. In the United States, analytical SFE instrumentation is produced by such firms as Isco, Applied Separations, Leco, and Jasco. In Europe, analytical-scale SFC equipment is available from Berger Instruments, Thar Designs, Jasco, and Sensar. The equipment is obtained from these vendors can be, if needed, slightly modified to study the conditions that are amenable to processing nutraceuticals. King (34) has provided a interesting review of how lab-constructed equipment can be used for both analytical and process development purposes. 

Two examples will be cited that show how analytical SFE instrumentation can be used to obtain information related to the isolation of nutraceu-... 

Sample preparation is often more difficult and time-consuming than the actual analysis procedure. Furthermore, extraction of analytes from the matrix is generally the most time-consuming step of sample preparation and it can lead to relatively inefficient analyte recoveries. Off-line supercritical fluid extraction provides an alternative to traditional Soxhlet or ultrasonic liquid extraction methods. Several recent studies have shown analytical SFE provides comparable or better extraction efficiencies than Soxhlet... 

Complex Mixture Extraction. Analytical SFE can also be used for complex mixture sample preparation. Typical examples using hazardous waste samples are described below. Sample A was a soil boring contaminated with coal gasification residuals and sample B was from a waste stream from a treatment facility. The major objective of these studies was to compare the extraction abilities (e.g., amount of material extracted) of three different fluid systems using approximately four-gram aliquots of the samples. The specific fluid systems, the extraction conditions, and the percentage of the total mass of material extracted from each sample are listed in Table II. 

Taylor, L. T. (1995) Strategies for analytical SFE. Anal. Chem., 67, 364A-70A. 

The off-line SFE-SFC analysis of the extractables is used to determine the lipid and cholesterol content and to guide the process parameter study. Analytical SFE-SFC instruments and methods also provide such data using on-site analyzers for CtA/QC or in-plant operations. More efficient monitoring of high performance materials is achieved, relative to traditional multistep Soxhiet extraction operations, as well as meeting compliance to regulatory environmental and/or quality standards within the industries. 

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