Supercritical Fluid Extraction Applications
Environmental Applications
Pharmaceutical Applications
Polymeric Applications
Natural Product Applications
Food Applications

The true test of the usefulness of an analytical technique is its ability to solve real-world problems. Practical analytical applications of SFE have appeared only during the last 10 years. However, recent advances in instrumentation, such as variable electronically controlled restrictors and in-line modifier addition are ex­pected to facilitate the rapid growth of SFE. Since 1989, American proponents of SFE have consistently stressed its potential in the environmental market. More recently, SFE has been evaluated in such areas as pharmaceutical analysis, pesti­cide residue analysis, "truth in labeling" of foods, polymer additive screening, and municipal drinking water assessment. It would be impossible to cover in de­tail all applications of SFE. Consequently, selected samples have been grouped into different categories. Obviously, such a discussion reflects the interests of the author. A number of references to both original articles and reviews of the area are noted. This chapter is intended to give the reader insights into both the num­ber of applications and the current limitations of the technology.

Before reviewing individual applications by area, the different sample matri­ces shall be discussed. Supercritical fluid extraction can suffer from a wide range of unnoticed matrix effects which can produce erratic and perhaps erroneous resuits. Matrices was grouped into three categories:

  • dry inorgan­ic/organic samples;
  • wet food/plant/environmental samples, which comprise about 35% of all samples; and
  • aqueous/biological tissue samples.

Supercriti­cal fluid extraction in environmental labs is most successful with dry soil matri­ces. Industrial labs have had success with solid polymers, synthetic chemicals, and other well-defined solid matrices such as dry feeds and oil-bearing rocks. Organic trapping materials—Tenax and XAD (which are polymeric adsorbents), polyurethane foam (PUF) plugs (which are often used for adsorption of chemi­cals from air), and SPE media—also can successfully be desorbed by SFE. The extracted characteristics of these solid matrices are:

0 No sample mechanical mobility or plugging of the SFE system. Matrix components stay in the extraction vessel.

0 The sample is relatively dry. Restrictor freezing from entrained water is eliminated.

0 The matrix is insoluble in the SF, providing a clean extract.

Water causes problems in SFE. Since SF CO 2 is a nonpolar, gas-like fluid, water has low solubility, on the order of 0.1% (w/w) depending on the density and temperature of CO 2. High water content in samples sometimes prevents SF penetration of the matrix and can preferentially entrap some analytes. Although water can be a SF, current analytical instruments cannot achieve the operating temperature required to take water to its critical point. However, subcritical water (7"< 7*c), readily extracts nonpolar organic compounds from environmental ma­trices. In addition, restrictor plugging occurs as entrained water freezes out dur­ing CO 2 expansion to a gas. Semisolids and syrups can plug or pass through the extraction vessel frits, contaminating the SFE plumbing. To improve SFE recov­eries, the following techniques are under investigation:

O Vacuum/heat or microwave drying to remove water. However, this may leave the sample with a crusty surface which may show reduced perme­ability of SFs. Partial drying is compatible with SFE, but variable water content in a matrix can reduce extraction precision.

0 Addition of dispersants, such as celite, which increase surface area and re­duce stickiness of substances such as peanut butter. Dispersants provide a free-flowing powder which is easier to load into the extraction vessel and will not migrate out of the vessel. Dispersants must be selected carefully to avoid introducing impurities to the sample or encouraging retention of the desired analytes.

0 Addition of desiccants which remove most of the water and also act as dis­persants. Some of the agents being investigated include Hydromatrix (di-atomaceous earth mixed with silica), Celite, sodium sulfate, molecular sieves, silica, reverse-phase LC packings, activated charcoal, and lipophilic membrane disks.

0 Introduction of reagents that react with water to remove it without degrad­ing the sample. Some examples are thionyl chloride and calcium oxide.

Gelatinous, complex tissues pose the greatest challenge for SFE. Govern­ment, drinking/wastewater agencies, biopharmaceutical firms, clinics, and cos­metic and food companies all want to analyze biological matrices. Traditionally liquid-liquid extractions have been used for these samples; however, there is in­creasing acceptance of SPE in this area. For SFE on the other hand to replace liq­uid-liquid extraction, strong desiccants or intermediate water-removal protocols must be developed. Current evaluations point to a large opportunity in these sample types, which may represent up to 40% of all samples if technical ad­vances continue in SFE and SPE.

Supercritical fluid extraction has the potential to change the current extrac­tion procedures in analytical laboratories. Table 48 lists areas where this technol­ogy has been utilized or is being considered. Many of the studies reported to date have not been quantitative, have involved high parts-per-million analyte levels, or are fortified-sample, inert matrices.

Certainly more investigations of a qualitative or processing nature have been reported. (Table 49) shows the applications involving quantitative methodology up to 1990 (i.e., greater than 90% extraction and recovery of the target analytes has been achieved using off-line SFE). Application of this technique in areas such as ma­terials synthesis and processing, control of organic and nuclear waste, and bio-catalytic transformations also appears promising, but beyond the scope of this treatise. As more useful applications of analytical SFE are discovered, the need for a better theoretical understanding of sample preparation prior to SFE be comes imperative. Such an understanding will lead, no doubt, to more advanced applications of SFE.

Table 48 Some Applications to which SFE has been applied

Table 49 Representative Applications of Off-Line SFE for Quantitative Analysis

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