Layers chemically bonded plates

C. Reversed-phase chemically bonded plates Whatman KC,g plates consist of a layer of sorbent produced by reacting octadecyl monochlorosilane with the surface silanol groups of a special silica gel [30]. This presents a nonpolar hydrocarbon surface to the solutes for separations by reversed-phase TLC. The advantage of this chemically bonded medium compared to conventional impregnated nonpolar phases... 

Quality of the adsorbent layer. Layers for HPTLC are prepared using specially purified silica gel with average particle diameter of 5-15 /mi and a narrow particle size distribution. The silica gel may be modified if necessary, e.g. chemically bonded layers are available commercially as reverse-phase plates. Layers prepared using these improved adsorbents give up to about 5000 theoretical plates and so provide a much improved performance over conventional TLC this enables more difficult separations to be effected using HPTLC, and also enables separations to be achieved in much shorter times. 

The extent of fluorescence quenching often depends on the sorbent medium and is generally more severe for silica gel than for chemically bonded sorbents [183]. In many cases the emission signal can be enhanced by application of a viscous liquid to the layer before scanning the plate. Common fluorescence enhancing... 

Precoated plates are also available for reversed-phase liquid-liquid partition thin-layer chromatography. Here the silica gel has been treated with an octadecyl silylating reagent thus coating the particles with a non-polar chemically-bonded thin film. The solvent employed is more polar than the film and chromatographic development results from partition between these two phases. 

Cadet J, Voituriez L, Berger M (1983) Separation of nucleic acid components and their radiation-induced degradation products on chemically bonded C12 reversed-phase thin-layer plates. J Chromatogr 259 111-119... 

C. A. T. Brinkman and G. de Vries, Thin-layer chromatography on chemically bonded phases a comparison of precoated plates, J. Chromatogr., 258 43 (1983). 

Native and microcrystalline cellulose precoated plates are used in the life sciences for the separation of polar compounds (e.g. carbohydrates, carboxylic acids, amino acids, nucleic acid derivatives, phosphates, etc) [85]. These layers are unsuitable for the separation of compounds of low water solubility unless first modified, for example, by acetylation. Several chemically bonded layers have been described for the separation of enantiomers (section 10.5.3). Polyamide and polymeric ion-exchange resins are available in a low performance grade only for the preparation of laboratory-made layers [82]. Polyamide layers are useful for the reversed-phase separation and qualitative analysis of phenols, amino acid derivatives, heterocyclic nitrogen compounds, and carboxylic and sulfonic acids. Ion-exchange layers prepared from poly(ethyleneimine), functionalized poly(styrene-divinylbenzene) and diethylaminoethyl cellulose resins and powders and are used primarily for the separation of inorganic ions and biopolymers. 

The absolute Rp value and its reproducibility on inorganic oxide layers depends on the layer activity. This is controlled by the adsorption of reagents, most notably water, through the gas phase [100]. Physically adsorbed water is removed from silica gel layers by heating at about 120°C for 30 minutes. Afterwards the plates are stored over a drying agent in a grease-free desiccator. Heat activation is not usually required for chemically bonded layers. Deactivation of silica gel layers by exposure to the atmosphere is extremely rapid. In modern environment-controlled laboratories, layers achieve a consistent level of activity almost instantaneously, that should provide sufficient reproducibility for most separations. Indeed, brief exposure of activated layers to the laboratory atmosphere may render the activation process a waste of effort. 

The stationary phase in RP-TLC can be C8 or C18 chemically bonded silica gel, or a sUica gel layer impregnated with paraffin or silicone oil. Better correlations are usually obtained for the Pm values vs. log P on C18 reversed phase, probably because the secondary interactions between the compound and the free silanol groups on the silica surface are stronger on C8 phases than on Cl8. The samples are usually detected by fluorescence quenching or by spraying the plate with specific derivatization reagents. The measurements of Pm values on silica layers impregnated with oils have lower reproducibiUty than those on silica C8 or C18 bonded phase. 

Solvent mixtures containing greater than 35-60% water are impossible to use with some chemically bonded reversed-phase layers, the exact amount depending on the brand and type of plate and the chain length of the bonded moiety. Addition of salt to the water may allow mobile phases with higher water content to be used. 

Sherma, J., and Latta, M. (1978). Reversed phase thin layer chromatography of chloroplast pigments on chemically bonded C g plates. J. Chromatogr. 154 73-75. 

CYANO PHASE. Silica gel chemically bonded with a CN functional group through a hydrophobic spacer. The layer has polarity between amino and di-methylsiloxane reversed-phase plates and can function with normal- or re-versed-phase mechanisms, depending on the mobile phase. 

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