Precoated reversed phase

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. 

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. 

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. 

D. B. Wall, S. J. Berger, J. W. Finch, S. A. Cohen, K. Richardson, R. Chapman, D. Drabble, J. Brown, and D. Gostick, Continuous sample deposition from reversed-phase liquid chromatography to tracks on a matrix-assisted laser desorption/ionization precoated target for the analysis of protein digests, Electrophoresis, 23 (2002) 3193-3204. 

Precoated layers are available with chemically bonded phases. The most interesting group is silica gel in the reversed phase (both TLC and HPTLC) silica gel. silanised and with long chain hydrocarbons. The silanisation degree may be between 50 yf and 100%. In addition these layers can also be purchased in a water-compatible form. Water-compatible reversed-pha.se TLC may be used for separation at any water content of the eluent. 

A variety of sorbents have been used as the stationary phase in TLC, including silica gel, cellulose, alumina, polyamides, ion exchangers, chemically modified silica gel, and mixed layers of two or more materials, coated on a suitable support. Currently in the pharmaceutical industry, commercially precoated high-performance TLC (HPTLC) plates with fine particle layers are commonly used for fast, efficient, and reproducible separations. The choices of mobile phase range from single component solvent systems to multiple-component solvent systems with the latter being most common. The majority of TLC applications are normal phase, which is also a complementary feature to HPLC that uses mostly reverse-phase columns. 

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. 

Analtech supplies precoated plates for reversed-phase TLC consisting of a support layer impregnated with a long-chain hydrocarbon that gives separations similar to those obtained on a Cig-bonded layer (Section Ill.C). Mobile phases... 

Most PLC is performed on layers thicker than the normal 100-250 pm used for analytical TLC so that more load can be applied. Precoated commercial silica gel layers (designated P) are available in 500, 1000, 1500, and 2000 pm thicknesses, the 1000 and 2000 pm being by far the most widely used. Preparative layers are usually formed using particles averaging 25 pm, with a range of 5-40 pm. These coarse, nonuniform particles and the increased thickness of the layer lead to lower-resolution separations compared to analytical TLC and HPTLC. PLC plates can be commercially obtained in 5 X 20-, 10 X 20-, 20 X 20-, 20 X 40-, and 20 X 100-cm sizes. The latter three are most commonly employed because of their higher loading capacity. Manufactured layers of alumina, cellulose (MN-300 and Avicel), and C2- and Cn-bonded silica gel (for reversed-phase PLC) are also available. Because silica gel is most widely used for preparative TLC, the rest of the chapter will refer mainly to this sorbent. 

A new precoated layer in the field of reversed-phase stationary phases is the TLC aluminium sheet RP-18 F254s, which combines the advantages of partially modified RP-phases with those of aluminum sheets as carriers (141). 

---