Precoated layers concentrating zones

The most-nsed stationary phase in PLC is sihca gel, with type 60 taking preference. In the fnture, other sorbents snch as the RP materials will also most probably be increasingly nsed. This will also be trae for the case of special PLC plates consisting of layer combinations snch as precoated plates with concentrating zones, resnlting in simphfication of sample application as well as an increase in the efficiency of separation. 

A very helpful tool for manual application can be the employment of layers with a concentrating zone. The so-called concentrating or preadsorbent zone is a small part of the plate that is covered with an inert but highly porous adsorbent such as diatomaceous earth. Various precoated preparative layers with a preadsorbent zone are commercially available. The effect of the concentrating zone is depicted elsewhere in detail (see Chapter 3, Figure 3.4). In brief, the preadsorbent zone serves as a platform for manual application of any desired performance quality. When development starts, soluble components migrate with the mobile phase front and are... 

FIGURE 12.2 Diagrammatic representation of the effect of the concentration zone in preparative separations, (a) Precoated layer without concentrating zone, (b) Precoated layer with concentrating zone. (From Nyiredy, S., Preparative layer chromatography, in Handbook of Thin Layer Chromatography, 3rd ed., Vol. 89, Sherma, J. and Fried, B., Eds., Marcel Dekker, New York, 2003, pp. 99-133. With permission.)... 

Since the introduction of commercial precoated plates in the mid-1960s, continual developments with regard to the increase of selectivity and improvement of separation efficiency were pursued [i.e., ready-to-use layers suitable for high-performance thin-layer chromatography (HPTLC), polar and hydrophobic bonded phases, plates with concentrating zones],... 

At this point, there usually follows a chapter about the pretreatment of the samples. However, in contrast to HPLC/GC, sample preparation for TLC is not considered to be quite as critical. As well as the use of precoated layers with a concentration zone (e.g. an application zone consisting of silica 50 000 and a separation zone of sihca gel 60 or RP-18 material) upon which the matrix constituents can often be held back by suitable choice of solvent system, a chromatogram that is imusable for lack of sample preparation is more rapidly rectified (use a different preparation method and a new plate ) than an irreversibly destroyed column. A detailed treatment of the subject of sample preparation would exceed the scope of the present book. In Section 9.4, rm-der the title Examples of GMP/GLP-Conforming Testing Procedures , we describe the extraction of a pharmaceutically active substance from a tablet and the working up of plant components from dry extracts. The reader is referred to other TLC textbooks [2,21] and to literature and brochures produced by manufacturers of articles for sample preparation [28, 29]. 

Hauck, H. E., and Mack, M. (1990). Precoated plates with concentrating zones a convenient tool for thin layer chromatography. LC-GC 8(2) 88, 92, 94, 96. 

All the thin-layer plates discussed above consist of a uniform sorbent layer. Precoated layers introduced in this section are, on the contrary, combinations of different types of layers. Specific advantages of these precoated layers with concentrating zones can be summarized as falling into three categories ... 

Figure 3 The effect of the concentrating zone in preparative separations, (a) Precoaled layer without concentrating zone, (b) Precoated layer with concentrating zone.

The quality of streaking is not as important for precoated preparative layers with a concentrating zone, because the sample is applied to a practically inert zone. A better separation can, nevertheless, be achieved if the sample is applied carefully across the plate. 

Precoated analytical layers with preadsorbent zone are also commercially available for linear development. This zone serves to hold the sample until development beings. Compounds soluble in the solvent system pass through the preadsorbent zone and are concentrated in a narrow band before entering the chromatographic layer proper, and this improves their resolution. 

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