Principles of Supercritical Fluids

1 An Overview of the Relationship of Phases A Refresher on Phase Diagrams 

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PHYSICO-CHEMICAL PRINCIPLES OF SUPERCRITICAL FLUID SCIENCE... 

Battle KD (1988) Theory and principles of supercritical fluid chromatography. Supercritical fluid chromatography. The Royal Society of Chemistry, S. R. M. London, pp 1-28 Beach S, Latham D, Sidgwick C, Hanna M, York P (1999) Control of the physical form of salmeterol... 

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). 

Principles and Characteristics Klesper et al. [14] have introduced supercritical fluid chromatography (SFC). The oil industry has been a major force in the development of many aspects of the application of supercritical fluids. Much of the pioneering development of SFC was carried out by Sie and Rijnders [231,232], who also coined the term supercritical fluid chromatography [233]. 

Although the general principles of separation processes are applicable widely across the process industries, more specialised techniques are now being developed. Reference is made in Chapter 13 to the use of supercritical fluids, such as carbon dioxide, for the extraction of components from naturally produced materials in the food industry, and to the applications of aqueous two-phase systems of low interfacial tensions for the separation of the products from bioreactors, many of which will be degraded by the action of harsh organic solvents. In many cases, biochemical separations may involve separation processes of up to ten stages, possibly with each utilising a different technique. Very often, differences in both physical and chemical properties are utilised. Frequently... 

This article treats the benefits, possibilities and drawbacks of supercritical fluid chromatography (SFC) and supercritical fluid extraction (SFE) coupled to nuclear magnetic resonance spectroscopy. After a general overview and consideration of the motivation for such techniques, the design of high-pressure flow probes, as well as the principle experimental set-ups, are described. By means of several applications and comparison to HPLC-NMR, the utility of these hyphenated techniques is demonstrated. 

We have sought to maintain this balance by probing the principles underlying supercritical fluid behavior in the first three sections and examining applications from the perspective of these principles in the last three sections. Within each section, there is also a flow from fundamentals to applications the initial chapter provides the basis and the focus for ensuing chapters. 

The first section features new approaches to investigating physicochemical properties. Its final two chapters facilitate the transition to the second section, on chemical reactions, a new topic of fundamental importance. Phase equilibria are described in the final section of principles. Here initial chapters are devoted to modeling, and the final chapters report solubility studies. The final three sections are devoted to important applications of supercritical fluids chromatography, fractionation and separation, and fuel applications. The chapters in each of these sections are also arranged so that there is a transition to more applied topics in the later chapters. 

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. 

In this chapter, we have discussed some of the basic principles underlying SFE as well as general approaches to the instrumentation used for optimal apphcation of supercritical fluids to the area of analytical-scale SFE. The latter part of this chapter dealt with the apphcation of SFE to specific, representative food product sample preparation. It is expected that these and other new applications will support some needed challenges in food analysis. 

As we see again, the principles and applications of supercritical fluid solubility phenomena were quite well understood at the time of the filing of this patent, March 1936. 

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. 

Finally, in recent years the continuing revolutions in instrument hardware have had a profound influence on some areas of lipid analysis. The development of supercritical fluid extraction of lipids now provides an alternative to wet solvent extractions, which have basically remained unchanged in principle and practice since the 1960s. Advances in mass spectrometry and analytical chromatography now allow the direct analysis and identification of complex lipids from total lipid extracts without the requirement for solvent fractionation. 

Source van Wasen, U., Swaid, I. and Schneider, G. M., Physicochemical principles and applications of supercritical fluid chromatography, Angew. Chem. Int. Ed. Engl, 19, 575-87, 1980... 

Schneider, G.M. (1992) Physico-chemical principles of supercriticcJ fluid separation processes, in B. Wenclawiak (ed.). Analysis with Supercritical Fluids Extraction and Chromatography, Springer Verlag, Berlin, pp. 9-30. 

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