Silica gel precoated

Silica-gel-precoated preparative TLC plates impregnated with AgNOj are widely applied in this technique [60,90]. To impregnate a silica-precoated plate with silver nitrate, the plate is dipped for 3 min in a 1% AgNOj solution in a methanol-water mixture (4 1, v v). The plate is then dried, first in air with exclusion of light, and next, it is activated at 80°C in a drying oven. If impregnated plates are not used immediately, they should be stored wrapped in black plastic film in a desiccator [90-92]. 

TLC is carried out on a 5 x 20 cm silica gel precoated plate (Merck) with a solvent system of pyridine-ethylacetate-acetic acid-water (75 25 15 30), tert-butanol-acetic acid-water (2 1 1) or n-butanol-acetic acid-water (60 15 25) (11). 

Characteristic properties of silica gel precoated layers and columns... 

TLC is used for qualitative analysis its reproducibility is relatively poor. For the analysis of dyes by TLC, extracts are spotted on areas as small as possible on the silica gel precoated plate and developed by a suitable developing solvent. Selection of developing systems depends on the chemical properties of... 

Zlatanov, L., Gonnet, C., and Marichy, M. (1986). Demixing effects in thin layer chromatography with NHj-modified silica gel precoated plates. Chromatographia21 331-... 

Retention (hRfi, hRf2) Resolution (u, Rs) Data of DL-Dansyl-Amino Acids (Dns-AA) on Silanized Silica Gel Precoated Plates (RP-18 Merck) Impregnated with the Copper (II) Complex of Various Chiral Selectors... 

An interesting application of this technique concerned the use of silica gel precoated plates impregnated with unusual chiral selectors, such as L-lactic acid... 

Estriol Dip the concentrating zone of a precoated H PTLC silica gel 60 plate in a saturated ethanolic solution of Fast Dark Blue R salt, allow the solvent to evaporate, apply the sample solution and dip once [10, 59]... 

Fig. 42 Chromatogram of polycyclic aromatic hydrocarbons on caffeine-impregnated precoated silica gel 60 HPTLC plates with concentrating zone (Merck). The following can be recognized in increasing Rf value. — 1. benzo(ghi)perylene, 2. indeno(l,2,3-cd)pyrene, 3 benzo(a)pyrene, 4. benzo(b)fluoranthene, 5. benzo(k)fluoranthene, 6. fluoranthene.

Merck precoated silica gel F254 plates, layer thickness 0.25 mm., were used. Developing with 10 1 (vjv) benzene-acetone and visualizing with ultraviolet light, the product appears at Rf 0.58 0.67. Normally the product is found in the lower third of the column, and occasionally some is found in the last fractions of eluent collected during development. However, since the exact position of this material on the column depends critically on the way in which the column is packed, a thorough check of all fractions is advisable. 

The submitters also tried to purify the crude product by distillation (130-145° at 0.005 mm.), but under these conditions decomposition took place. It is possible to substitute thick-layer chromatography for the dry column. Using Merck silica gel F254 precoated plates, layer thickness 2 mm., and developing with 10 1 (vjv) benzene-acetone, the submitters report a 73% yield of pure product. 

The literature reports [a]n +23.2° (c = l, aqueous 5N hydrochloric acid). The product was analyzed by the submitters. Analysis caleulated for C5H11NO2S C, 40.25 H, 7.43 N, 9.39 S, 21.49. Found C, 40.14 H, 7.42 N, 9.50 S, 21.52. The product was homogeneous according to thin-layer chromatograms on precoated silica gel G plates purchased from Analtech, Inc., Newark, Delaware, and developed with the following two solvent systems (solvents, volume ratios of solvents in the same order) 1-butanol-acetic acid-ethyl acetate-water, 1 1 1 1, Rf 0.49 1-butanol-acetic acid-pyridine-water, 15 3 10 12, R/0.51. 

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. 

Chapter 3 through Chapter 8 deal with the basic aspects of the practical uses of PLC. Chapter 3 describes sorbent materials and precoated layers for normal or straight phase (adsorption) chromatography (silica gel and aluminum oxide 60) and partition chromatography (silica gel, aluminum oxide 150, and cellulose), and precoated layers for reversed-phase chromatography (RP-18 or C-18). Properties of the bulk sorbents and precoated layers, a survey of commercial products, and examples of substance classes that can be separated are given. 

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. 

All preparative PLC precoated layers have glass plates as support. They are offered by a variety of manufacturers with or without fluorescent indicators and different binders. The layer thicknesses range uniformly between 0.5 and 2 mm or exhibit a gradient. Furthermore, in some cases the precoated silica gel plates are prescored to avoid cross-contamination from track to track. 

In Table 3.2 the preparative precoated silica gel plates for straight phase PLC from different mamrfacturers are summarized. Incomplete data are due to incomplete information received from the manufacturers. 

Aluminum oxides, similar to silica gels, are available as bulk materials and as precoated plates, to be used not only for straight phase adsorption chromatography, but also for partition PLC (see Table 3.3 and Table 3.4). In particular, the aluminum oxide type 150 (i.e., mean pore diameter 150 A [15 tun]) is suitable for partition chromatographic purposes. 

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

The silica gel matrix for the RP-18 PLC precoated plates consists either of a type 60 or 150 (indicating the respective mean pore diameter). The manufacturers and the properties of the PLC plates RP-18 available at present are listed in Table 3.8. 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

FIGURE 6.19 Circular chromatography of dyes on precoated silica gel high-performance TLC plate lipophilic dyes, mobile phase hexane-chloroform-NH3, 70 30. (From Ripphahn, J. and Halpaap, H., HPTLC High Performance Thin Layer Chromatography, Zlatkis, A. and Kaiser, R. E., Eds., Elsevier, Amsterdam, 1977, pp. 189-221. With permission.)... 

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