Stationary phase in thin-layer chromatography

The techniques of chromatography are used mainly for the separation of materials that are normally difficult to separate and are present in very small quantities in mixtures. There are various methods of chromatography, but the principles are similar. They all depend upon a material for the sample to stick to and a solvent (liquid or gas) to push the sample along. The absorbent surface can be paper or a thin layer of a suitable unreactive powder, like alumina. This is called the stationary phase. In thin layer chromatography (TLC), the material can be chosen for ease of separation. 

A review of all sorbents used as stationary phases in thin-layer chromatography (TLC) is reported. The specific apphcation field of aU sorbents is described according to tbeir relative chemical physical properties. New materials have been developed for the high-performance thin-layer chromatography (HPTLC) technique to offer both high-efficiency separations and high-sensibility analysis. In particular, sUanized sihca gel bas been extensively used as stationary phase in reversed-phase (RP) chromatography for its hydrophobic properties. 

Gocan, S. (1990). Stationary phases in thin layer chromatography. In Modem Thin Layer Chromatography, N. Grinberg (Ed.). Marcel Dekker, New York, pp. 5-137. 

Malinowska, I., and Rozylo, J. K. (1991). Chitin, chitosan, and their derivatives as stationary phases in thin layer chromatography.7. Planar Chromatogr.—Mod. TLC 4 138-141. 

The use of aluminas as stationary phases or supports for liquid stationary phases in thin-layer chromatography is of importance for some fields of application, but it is less widespread compared with the use of silica gels. 

Because of the similarities in the theory and practice of these two procedures, they will be considered together. Both are examples of partition chromatography. In paper chromatography, the cellulose support is extensively hydrated, so distribution of the solutes occurs between the immobilized water (stationary phase) and the mobile developing solvent. The initial stationary liquid phase in thin-layer chromatography (TLC) is the solvent used to prepare the thin layer of adsorbent. However, as developing solvent molecules move through the stationary phase, polar solvent molecules may bind to the immobilized support and become the stationary phase. 

Thin layer chromatography differs from all other types of chromatography discussed before by the simplicity of the technique used. Thin layer chromatograph has a stationary phase and a mobile phase just like every other chromatographic method. However, the stationary phase for thin layer chromatography is not located in the column as in gas or liquid chromatography. Instead, it is fixed on a glass, aluminum or plastic plate as a thin layer. 

In contrast to HPLC, stationary phases for thin-layer chromatography (TLC) have not improved substantially with time. However, the reasons for using TLC include parallel separation of samples, high-throughput screening, static and sequential detection for identification, and integrity of the total sample. Moreover, TLC promises future prospects for improved separation performance. 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

The difference between this technique and GC or HPLC is that the separation process occurs on a flat essentially two-dimensional surface. The separated components are not usually eluted from the surface but are examined in situ. Alternatively, they can be removed mechanically for further analysis. In thin-layer chromatography (TLC), the stationary phase is usually a polar solid such as silica gel or alumina which is coated onto a sheet of glass, plastic, or aluminium. Although some moisture is retained by the stationary phase, the separation process is predominantly one of surface adsorption. Thin layers are sometimes made from ion-exchange or gelpermeation materials. In these cases the sorption process would be ion-exchange or exclusion. 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

Modification of the Activity of Alumina by Addition of Water Stationary and Mobile Phases Typical Stationary and Mobile Phase Systems Used in the Separation of Various Inorganic Ions Spray Reagents in Thin-Layer Chromatography Protocol for Reagent Preparation... 

In thin-layer chromatography (TLC), the stationary phase is a uniform layer of a finely divided powder that has been coated on the surface of a glass or plastic sheet and that is held in place by a binder. The capacity of the system is dependent on the thickness of the layer, which may range from 0.1 to 2.0 mm. The thinner layers are used primarily for analytical separations, while the thicker layers, because of their greater sample-handling ability, are useful for preparative work. 

In Thin-Layer Chromatography (TLC), the a-cyclodextrin (12-14) and the soluble 6-cyclodextrin polymers have been thoroughly studied as components of the mobile phase. The y-cyclodextrin, because of its excellent solubility, is also promising, especially for larger molecules, but has not yet been studied. Several attempts have demonstrated that TLC-plates coated by insoluble finely powdered cyclodextrin-polymers (i.e. cyclodextrin in stationary phase) can separate isomers, but this possibility has not yet been fully exploited. 

CD s dissolved in the mobile phase solutions have been used in thin-layer chromatography with polyamide stationary phase (29,30) and 2) CD complexation equilibria of ionic compounds were studied by a chromatographic method using an ion exchanger as stationary phase and mobile phase solutions containing CD s in various concentrations (31). 

Background and principles Thin-layer chromatography is the other most commonly used form of planar chromatography and uses a very similar experimental approach to paper chromatography. The principal difference is that this technique relies on the separation of biomolecules from a mixture on the basis of partition and/or adsorption. There is a distinct difference between the process of adsorption and a/isorption, and they are not interchangeable terms Whereas molecules that are a/isorbed are taken up into , those that are adsorbed stick to a surface. So, in thin-layer chromatography, the mobile phase is adsorbed (sticks to) and subsequently moves along the stationary phase. The stationary phase consists of an adsorbent (sticky) layer on a flat plate or sheet. The most commonly encountered adsorbent layers comprise silica gel, alumina (not aluminium) or cellulose, while popular solvents include hexane, acetone and alcohol. 

The progress in thin-layer chromatography was made possible by improvements in the stationary phases, optimisation of mobile phase composition, and in chromatographic instrumentation. 

The stationary and mobile phases used in thin-layer chromatography are the same as those used in liquid chromatography generally. Thus adsorbent solids, ion-... 

In thin-layer chromatography cellulose can also be put in as a stationary phase Alumina and silica geP " and mfactures... 

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