Preparative planar chromatography

Often, planar chromatography is used as a preparative step for the isolation of single components or classes of components for further chromatographic separation or spectroscopic elucidation. Many planar chromatographic methods have been developed for the analysis of food products, bioactive compounds from plant materials, and essential oils. 

Modem planar chromatography is suitable not only for qualitative and quantitative analysis but also for preparative purposes. The separation efficiency of a thin-layer chromatographic system is independent of this intended purpose and is mainly determined by the quahty of the stationary phase, that is to say, by the applied coated layer. Therefore, progress in modem planar chromatography can be attributed not only to the development of the efficiency of the instmments but also to a large extent to the availability of high-quahty precoated layers. And today, as in the past, bulk sorbents for self production, especially of preparative layer chromatography (PLC) layers, are widely used. 

More stringent requirements, especially with regard to separation efficiency and reproducibihty in preparative planar chromatography also, led to increased application of precoated plates in this field. Figure 3.3 shows a scanning electron micrograph of a cross section through a PLC plate silica gel. 

Planar chromatography has been used for preparative purposes in the past and is still used today. The reasons for the usage of preparative layer chromatography are, first of all, its cost-effectiveness and flexibility. 

Nyiredy, Sz., Possibilities of preparative planar chromatography, in Planar Chromatography, A Retrospective View for the Third Millennium, Nyiredy, Sz., Ed., Springer Scientific, Budapest, 2001, chap. 20. 

Modihcation of the ES chamber was reported by Ruminski [8], who applied a glass rod (3) to obtain even distribution of solvent to the adsorbent layer (Eigure 6.5). This modihcation was employed for preparahve planar chromatography, especially with plates of large width. Preparative chromatograms can be developed even on 40-cm-wide plates. 

FIGURE 6.10 The cover of the prepared chromatoplate for gradient development in a fully online horizontal chamber 1 — solvent system inlet, 2 — Silcoflon cover sheet, 3 — chromatoplate, 4 — solvent system outlet, 5 — channel for solvent system. (From Nyiredy, Sz. and Benko, A., Proceedings of the International Symposium on Planar Separations, Planar Chromatography 2004, Nyiredy, Sz., Ed., Research Institute for Medicinal Plants, Budakalasz, 2004, pp. 55-60. With permission.)... 

Application of anticircular development to preparative planar chromatography is not popular in spite of the possibility of obtaining a good resolution of mixture components, especially of higher Rp values. Delivery of the mobile phase to the... 

Isocratic linear development is the most popular mode of chromatogram development in analytical and preparative planar chromatography. It can be easily performed in horizontal chambers of all types. The mobile phase in the reservoir is brought into contact with the adsorbent layer, and then the movement of the eluent front takes place. Chromatogram development is stopped when the mobile phase front reaches the desired position. Usually 20 X 20 cm and 10 X 20 cm plates are applied for preparative separations, and this makes the migration distance equal to about 18 cm. Due to the fact that the migration distance varies with time according to the equation Z, = (Z, c, and t are the distance of the solvent front traveled, constant,... 

All these modes of mnltiple chromatogram development are mainly apphed in analytical separation however, there are some examples of preparative planar chromatography [31,32]. 

Considering the principal factors affecting preparative planar chromatography, described by Nyiredy [101,102], it seems that the most important factors are the stationary and mobile phases with respect to the natural mixtures. 

Preparative planar chromatography is a very important step in the complicated procedures of isolation of group of compounds or pure substances from complex matrices. The method gives additional possibilities of using various adsorbents and eluent systems to achieve complete separation of stracmral analogs. The method also enables combining the various methods of sample application, plate development, and derivatization to achieve satisfactory separation of isolated plant extracts components. 

Nyiredy, Sz., Preparative planar chromatography, in Handbook of Thin Layer Ghro-matography. Fried, B. and Sherma, J., Eds., Marcel Dekker, New York, 1995, pp. 307-340. 

Additional details of planar chromatography—methods of descending and radial development, how to prepare TLC plates, tips on how to apply the sample, what to do if the spots are not visible—and the details of preparatory TLC, etc., are beyond our scope. 

Thin-layer chromatography (TLC), sometimes also called planar chromatography, employ a stationary phase immobilized on a glass or plastic plate and an organic mobile phase. It is a rather old technique whose application in residue analysis has been limited in the past by poor chromatographic resolution, inadequate selectivity, and insufficient sensitivity (49). This was due to inherent problems in the quality of the available stationary phase materials and in the uniformity of the layers prepared. Today, the availability of affordable, precoated plates with acceptable performance and consistency has led to the general acceptance of TLC as an efficient procedure for residue analysis (50). The method is used preferentially when analysts must process large numbers of samples in a short period of time (51). 

B. Fried and J. Sherma, Preparative thin-layer chromatography. In Planar Chromatography, Marcel Dekker, Inc., New York, 1982, p. 161. 

Various binders have been used to give mechanical stability to the layer spread on the suppon plate. Basic requirements are that the binder should not interfere with solute-sorbent interactions, with elution, and detection procedures. At the same time, these binders have to provide compact and adherent layers together with the sorbent. There are various binders that have been applied to prepare the stationary phase for planar chromatography (Table 10.7). 

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