Extraction and Related Separation Techniques

Extraction and Related Separation Techniques 105 2.2.4.1 Design of SFC Apparatus... 

The next chapter introduces chromatographic techniques for analyzing complex samples, whereby multiple analytes are separated on a column and detected as they emerge from the column. But very often, samples need to be cleaned up prior to introduction into the chromatographic colunm. The techniques of solvent extraction and solid-phase extraction and related techniques are very useful for isolating analytes from complex sample matrices prior to chromatographic analysis. Solvent extraction is also useful for spectrophotometric determination. 

There are also reports of HPLC-NMR-MS in which the separation system is coupled to both NMR and MS (39). The power and potential of LC-NMR and related hyphenated techniques are likely to be enormous, extending the scope of analytical separations and obviating the need for much time-consuming preparative scale work and reducing the risk of chemical degradation of the compounds. This will allow extraction work to concentrate on natural products that... 

Microemulsion liquid extraction is often confused with microemulsion liquid membrane research. Microemulsion liquid extraction has been studied by many groups, for example, Bauer et al. [31], Tondre and Boumezioud [32], and Paatero et al. [33]. The basic idea relies on the formation of a microemulsion phase during a normal equilibrium extraction process. The feed phase is then incorporated as dispersed droplets within the newly formed microemulsion, resulting in extremely high interfacial areas per unit volume. Although similar to microemulsion liquid membranes in its use of the increased surface area, the microemulsion liquid extraction technique incorporates a forward extraction step only and is inherently equilibrium-limited. Thus, the ability of a microemulsion liquid membrane to simultaneously extract and strip a solute is its distinguishing feature in comparison with other, related separation techniques. 

A number of studies of separations by means of liquid chromatography (HPLQ, paper chromatography, and related laboratory techniques provide useful information on the utility of various complexing extractants for polyfunctional organic solutes. From such studies it is possible to obtain distribution coefficients, effects of diluents, and information on the complexation stoichiometry and bond strength. An example of such a study is the work of Stuurman et al., who used HPLC to study complexation of phenol, hydroxybenzoic acids, and other hydroxycaiboxylic acids with TOPO in a diluent of n-hexane. 

Cyclodextrin-modified solvent extraction has been used to extract several PAHs from ether to an aqueous phase. Data evaluation shows that the degree of extraction is related to the size of the potential guest molecule and that the method successfully separates simple binary mixtures in which one component does not complex strongly with CDx. The most useful application of cyclodextrin-modified solvent extraction is for the simplification of complex mixtures. The combined use of CDx modifier and data-analysis techniques may simplify the qualitative analysis of PAH mixtures. 

In this chapter the three main modes of large-scale chromatographic operation, and combined reaction and separation. Many useful but small-scale chromatographic methods have been omitted, as well as allied separation techniques which combine aspects of chromatographic principles or practice with aspects of adsorption, extraction, sedimentation or electrophoresis. Such is the pace of invention that novel processes related to chromatography are still being developed and described in the literature. 

The world of aroma compounds is becoming more and more complex. In the early days people used aromatic products like fruit juices or fruit juice concentrates which were relatively weak and still close to the related foodstulf. Later, with more knowledge of separation techniques, infusions, extracts, oleoresins and absolutes ranging from weak to strong impact were used to impart aroma. Essential oils such as spice oils already had a very strong impact. Modern analytical technologies allowed the evaluation of the chemical compositions of extracts and essential oils, so that isolates either as powerful mixtures or even as single compounds could be obtained. 

As natural product extracts often contain a large number of closely related and thus difficult to separate compounds, this classical approach may become very tedious and time-consuming. Thus, the direct hyphenation of an efficient separation technique with a powerful spectroscopic method bears great potential in order to speed up the analytical process in general. 

The analytical extraction systems related to points 1 and 2 are pervaporation-based techniques (such as those mentioned in Sections 4.3.1 and 4.3.2). Extraction based on the membrane separation of an aqueous phase and an organic phase (point 3 above) will be dealt with in Section 4.3.3. As the system concerning point 4 is very rarely used, it will not be considered here. 

The bioseparation technique which is probably the most readily adapted to modern process control techniques is extraction. Liquid-liquid extraction is a mature unit process with application in industrial-scale protein separation.30 Control techniques used on similar systems in other industrial applications should be readily adaptable to bioprocessing, the primary difficulty being the lack of data on the partitioning and related behavior of the product. 

Separation Techniques. The complexity of the organic composition of coal-derived liquids, shale oil, and their related effluents presents a formidable challenge to the analytical chemist. Our approach to this problem has been the classical separation technique based on acid-base-neutral polarity of the organic compounds. We further subdivide the neutral fraction into aromatic and non-aromatic fractions using dimethyl-sulfoxide (DMSO) extraction. DMSO effectively removes multiringed aromatic compounds with great eflBciency (85-95%) for these complex mixtures and thus allows a straightforward analysis for polynuclear... 

The application of CCC to the separation of flavonoids has been proven to be very successful. Chloroform-methanol-water can be chosen as starting point and, by modifying the relative proportions of methanol or by replacing methanol with other solvents, it is possible finally to obtain the required distribution of sample components between the two phases. EtOA PrOH H20 and /i-C6Hi4 Et0Ac CH30H H20 also are very useful solvent systems. The technique is versatile and can be employed for the initial fractionation of crude extracts for the separation of closely related flavonoids and/or the isolation of pure products. 

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