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Stationary Phases for Thin-Layer Chromatography

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Stationary Phases for Thin-Layer Chromatography ( stationary-phases-thin-layer-chromatography )

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Journal of Chromatographic Science, Vol. 40, November/December 2002 Stationary Phases for Thin-Layer Chromatography Simion Gocan University Babes-Bolyai , Department Analytical Chemistry, Cluj-Napoca, Romania Abstract This paper presents a review of the literature concerning development of the stationary phase for thin-layer chromatography (TLC) in the last ten years. The silica gel remains the most important adsorbent for TLC separation. The kinetic properties of the silica gel thin-layer and the new TLC plates have been presented. Other materials used as stationary phase were alumina, zirconium oxide, Florisil, and ion-exchanger. Chemically new bonded stationary phase development is also discussed.The improvement of the separations of some organic mixtures by impregnation of silica gel, cellulose, or polyamide plates with transition metal ions and silver salts and their applications is presented. The impregnation of the thin-layer with organic stationary phase and inclusion complexes is another method used for the enhancement of the separation efficiences. Another modality to improve the selectivity in TLC using ion-pairing as reagent of impregnation is described as well. The actual state of chiral separation by TLC is discussed with concrete references to recent advances in chiral stationary phases. The use of nonpolar chemically bonded stationary phases impregnated with transitional metal ions is presented as chiral stationary phases. The cellulose, modified cellulose, chitin, chitosan, and their derivatives are presented; and their potential for the analysis of the racemates is discussed. The cyclodextrines and macrocyclic antibiotics were used with very good results for enantiomeric separation by TLC. A new separation approach with molecular imprinting polymers was reported as a chiral stationary phase in TLC. The examples provide a wide range of structural types that can be readily resolved enantiomerically by TLC. Introduction The success of the separation of a complex mixture by thin- layer chromatography (TLC) greatly depends on the choice of the stationary phase. Sorbents generally used in TLC include 12-μm particle size of silica gel, alumina, other mineral oxides, chemical-bonded silica gel, cellulose, polyamide, polymeric ion exchange, impregnated silica gel, and chiral phase. High-performance thin-layer chromatography (HPTLC) is dis- tinguished from conventional TLC. HPTLC involves a layer pre- pared from a particle of approximately 5 μm with a narrow particle size distribution. TLC over the latest 10 years. Information until 1990 can be found in TLC handbooks (1,2). Review Silica gel Silica gel is by far the most widely used adsorbent and remains the dominant stationary phase for TLC. The great majority of TLC analyses are carried out using normal phase (NP) silica gel layer. Because most high-performance liquid chromatography (HPLC) analyses are performed using reversed-phase (RP) sorbents, the methods are complementary for achieving separation and con- firming qualitative and quantitative results. Silica gel particles The surface of the silica gel was investigated with the best and most adequate methods for analyses. The hydrated silica gel sur- face can contain three kinds of silanol groups: free silanol, gem- inal, and associated silanols (3–5). These different silanol types are used to identify and measure magic-angle-spinning (MAS) 29Si NMR (6) and diffuse reflectance infrared spectroscopy with Fourier transform (3). Nonhydrogen-bonded free silanol is more acidic and can cause strong and deterious binding of basic solutes because of their highly acidic nature. Therefore, silica gel with a higher concen- tration of free and highly acidic silanols often shows increased retention and broad peak tailing for basic compounds. Fortunately, free silanols generally occur in a low concentration on the silica gel surface. The surface of silica gel with the highest concentration of geminal and associated silanols is favored most for the chromatography of basic compounds because these silanols are less acidic. Today few analytical laboratories prepare their own TLC plates. Precoated plates for TLC, HPTLC, and preparative TLC are com- mercially available. The silica gel used as adsorbent for HPTLC plates has a mean particle size of approximately 5 μm and, for TLC, 12 μm. The pore diameter of both is mainlyl 60 Å and the surface area is approximately 500 m2/g. The most frequently used thicknesses of thin layer for analytical purposes are approximately 200–250 μm. The thin layer for preparative purposes is available with thicknesses of up to 2 mm (7). This review utilizes the development of the stationary phase for The support for the thin layer of the adsorbent has to be an inert Downloaded from https://academic.oup.com/chRreopmrosdcuic/tiaornt(ipchloeto-caobpsyitnrga)cotf/ed4i0to/r1ia0l/c5o3n8te/n5t3o4f4th5i5s journal is prohibited without publisher’s permission. 1 by guest on 07 June 2018

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