Precoated chemically modified

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. 

Particularly in the beginning of thin-layer chromatography, but also today, sorbents without chemically modified surfaces are of most importance. In former times hand-made thin-layer plates were used nearly exclusively, but today the trend has moved in the direction of industrial precoated layers. In addition to glass plates, plastic and aluminium sheets are also offered as supports for the precoated layers. To stabilize the precoated layers mechanically, special binders are added that do not or nearly do not interfere with the chromatographic properties. To enlarge the possibilities of detection, added indicators can be mixed homogeneously with sorbents. Different types of silica gel are by far the most versatile and therefore the most frequently used stationary phases in the case of bulk sorbents as well as for application to precoated layers. 

III. MODIFIED SORBENTS AND PRECOATED PLATES A. Chemically Modified Sorbents... 

Cations, detection and identification of, 519-520 Cellulose, 108-109,925 detection of peptides and proteins on, 435-437 enantiomeric separations on triacetylcellulose thin-layer plates, 630-631 for separation of carbohydrates, 485-487 Centrifugal layer chromatography (CLC), 79,172 Cephalosporins, 452-457 Chemically modified sorbents, 110-119 hydrophillic modified phases, 110-112 hydrophillic modified precoated layers, 113-1 IS... 

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. 

Chapter 3 through 6 deal with the commercial and noncommercial stationary phases used for the direct and indirect enantioseparations by means of TLC and with the chiral modifiers of mobile phases, which are used exclusively in direct separations. Chapter 3 describes the commercial chiral and nonchiral sorbent materials and commercial precoated layers used in chiral separations. Thus, it deals with silica gel native and esterified cellulose chiral plates (reversed phase plates impregnated with a chiral selector) and C-18, C-18W, diol, diphenyl, and C-2 chemically bonded silica gel. At the end of this chapter, the author discusses the quantification of enantiomers by using densitometry, depending on the type of the stationary phase employed. 

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