Sorbents, Layers, and Precoated Plates

Silica gel, the sorbent with undoubtedly the widest range of TLC applications, is an amorphous, porous adsorbent variously referred to as silica, silicic acid, or porous glass. Silica gels used in traditional column and thin-layer chromatography are similar, except that the particle size used in the latter is much finer. Particle sizes for TLC are typically comparable to that used for modern high-performance column liquid chromatography. 

According to Felton (1979), the most nearly ideal condition for general TLC silica gel separations is considered to be at the level of 11-12% water (by weight). This amount of water leaves available the most plentiful middle-activ- 

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The activation of TLC plates is a subject open to dispute. Hahn-Deinstrop (1993) recommends heating silica gel and alumina layers at 120°C for 30 min 

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More detailed information on precoated layers and sorbents is available in book chapters written by Hauck and Moeller (1996) and Gocan (1990), the latter of which contains 810 references, and in reviews by Hauck and Jost (1990a, 1990b) and Hauck and Mack (1990). Hauck and Jost (1990b) tabulated all bulk sorbents and precoated plates commercially available worldwide in 1990. 

Manufacturers are currently supplying highly purified layers with carefully controlled sorbent particle and pore size, layer thickness, pH, fluorescent indicator, and binder. Once a plate is found that provides the desired results, reproducibility should be obtained if this same product is repeatedly ordered from the same source. These parameters are more carefully controlled in high-performance plates compared to conventional layers, and HPTLC plates should provide the most reproducible results. The quality and reproducibility of homemade plates will not be as good as with commercial precoated plates. 

Plates with 0.5- to 2-mm layer thickness are normally nsed for increased loading capacity. Layers can be self-made in the laboratory, or commercially precoated preparative plates are available with silica gel, alumina, cellulose, C-2 or C-18 bonded siliea gel, and other sorbents. Resolution is lower than on thinner analytical layers having a smaller average partiele size and particle size range. Precoated plates with a preadsorbent or eoneentrating zone faeilitate application of sample bands. 

Until now the application of different types of bnlk sorbents nsed for handmade PLC plates is rather widespread, and in most cases the mannfactnrers of these materials provide detailed instructions for the preparation of the preparative layers. However, the quality and especially the reproducibility of these handmade plates is frequently rather poor. Dne to this, the development of modem TLC and HPTLC, and precoated PLC plates also, becomes increasingly more important. 

The number of stationary phases available is essentially unlimited, although relatively few are, in fact, in common use. Chapter 3 discusses sorbents and precoated layers in detail. Originally, PTL was undertaken on plates made in the laboratories where they were used, and this led to the use of a wide variety of stationary phases. 

Commercial precoated layers on glass support are used in virtually all analyses. HPTLC uses plates that are smaller (10 x 10 or 10 x 20 cm), have a thinner (0.1-0.2 mm) layer composed of sorbent with a finer mean particle size (5-6 pm) and are developed over shorter distances (ca. 3-7 cm), as compared to classical 20 X 20 cm TLC plates which are generally 20 X 20 cm with a 0.25-mm layer and developed for 10-12 cm. HP plates provide improved resolution, shorter analysis time, higher detection sensitivity, and improved in situ quantification and are used for industrial pharmaceutical densitometric quantitative analyses. TLC plates are usually used for qualitative identification and purity studies as contained in pharmacopeias. 

In the first edition of Stahl s laboratory handbook Durmschicht-Chromatographie published in 1962, equipment for coating glass plates is illustrated, and detailed procedures for the preparation of TLC plates are described. Even today, TLC sorbents for a manual coating operation are produced in an only shghtly modified form, especially where labor costs are small and time is a minor consideration. The layers obtained often show good selectivity, but do not achieve the separation efficiency obtained by precoated layers and seldom lead to reproducible results. 

Table 3 shows the most important commercially available precoated layers and some typical examples of their use. In the choice of the hardware for TLC it cannot be assumed that nominally identical sorbents from different manufacturers will lead to equal separations [13]. It follows that trade names and item numbers used in publications or testing procedures should not be regarded simply as surreptitious advertising, but are absolutely essential if reproducible results are to be obtained. This is especially true for vahdated methods. An example is given in Fig. 6, which shows separations of peppermint oils on TLC plates whose sorbent specifications in the catalogs of different manufacturers are identical. When the development heights are equal, the variation in separation efficiency is clearly visible, and different hRf values are obtained for the same substances (see Table 4). 

When storing opened packages of precoated layers, care should be taken to place the plates with the sorbent underneath in the bottom part of the styrofoam and covered with plastic film. Components of the laboratory atmosphere otherwise soon become deposited on the active layers, and constituents of the glue on the back of the label can diffuse into the layer. 

Figure 22a shows the chromatogram on a sorbent layer which is not impregnated, and Fig. 22b shows the chromatogram on an impregnated TLC plate precoated with silica gel 60. However, a poorly performed impregnation leads to unusable TLCs, as shown in Fig. 22c. 

Although time consuming, it was the only way people interested in TLC could get the thin-layer plates for their work. Fortunately, the manufacturers of the sorbents reaHzed that supplying a precoated plate would be more cost-efficient and more reproducible for the users. In the early 1960s, the first prepared TLC plates came onto the market. 

Precoated plates for TLC have been commercially available since 1961. The sorbents may be coated on glass, plastic, or aluminum supports. Sorbents with and without binder, and with and without UV indicators, are available in a variety of layer thicknesses, ranging from 100 pm in the case of plastic plates and high-performance layers to 2311 pm for preparative layers. The largest selection is presented by glass plates, coated to a thickness of 250 pm in the case of analytical layers. ... 

Various other inorganic sorbents are used occasionally in TLC for rather specific separation problems. These sorbents are generally unavailable as precoated plates and must be prepared from aqueous or alcoholic slurries. For further details, see Rossler (1969). Magnesium oxide layers were developed with petroleum ether (30-50°C)-benzene (3 1) for the separation and quantification of alpha- and beta-carotene in lettuce and snails (Drescher et al., 1993). Zinc carbonate with a starch binder has been used to separate aldehydes, ketones, and other carbonyl groups (Rossler, 1969). Magnesium silicate (talc or Florisil) prepared as an alcoholic slurry has been used, to separate pesticides (Getz and Wheeler, 1968), fatty acids, and lanatosides (Rossler, 1969). Charcoal has been combined with silica gel for ketone separations (Rossler, 1969). Two forms of charcoal are available, polar and nonpolar, and each type has very different capabilities. Charcoal has had very limited use in TLC, partly because of the difficulty of zone detection on the layer. For a discussion of the adsorptive properties of charcoal, see Snyder (1975). 

The symbols and names given to different precoated plates (and sorbent powders for manual preparation of layers) are given in Table 3.1. 

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