Thin-layer chromatography phase adsorption

Figure 9-20. Diagrammatic representation of 7 /values as a function of the volume fraction ns of nonsolvent in thin-layer chromatography. A, adsorption D, desorption P, phase separation F, precipitation.

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. 

Let s start with two components, A and B again, and follow their path through an adsorption column. Well, if A and B are different, they are going to stick on the adsorbant to different degrees and spend more or less time flying in the carrier gas. Eventually, one will get ahead of the other. Aha Separation—Just like column and thin-layer chromatography. Only here the samples are vaporized, and it s called vapor-phase chromatography (VPC). 

T. Cserhati, E. Forgacs and J. Hollo, Separation of color pigments of Capsicum annuum by adsorption and reversed phase thin layer chromatography. J. Planar Chromatogr.—Mod. TLC 6 (1993) 472 175. 

The removal of sterols, vitamin E vitamers, carotenoids, and other interfering material from the unsaponifiable fraction of food samples has been achieved using one or more of the following techniques coprecipitation of sterols with digitonin (91), precipitation of sterols from a methano-lic solution (195,209), adsorption chromatography on open columns of alumina (70,91,96), thin-layer chromatography on silica plates (209), and solid-phase extraction on silica (68,100) and reversed-phase (210) cartridges. 

Liquid/Solid Chromatography (LSC) is adsorption chromatography. Adsorbents such as alumina and silica gel are packed in a column and the sample components are displaced by a mobile phase. Thin layer chromatography and most open column chromatography are considered liquid/solid chromatography. 

The history of thin-layer chromatography has been the subject of a book.158 The first separations on thin layers were performed in 1938. A gas was first used as the mobile phase in adsorption chromatography by Erica Cremer in Innsbruck in 1946. Using hydrogen as carrier gas, she and her student, Fritz Prior, successfully separated air and carbon dioxide using charcoal as the adsorbent. A newly-opened branch of the Deutches Museum in Bonn, devoted to post 1945 developments, has a display featuring the work of Cremer and Prior, with a model of their original apparatus.159... 

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. 

Ion pair chromatography represents a valuable variant in thin layer chromatography. Reversed-phase layers are particular suitable as these layers have no or only a few adsorptive properties. Chromatography is possible with simple systems, e.g. mixtures of methanol or acetonitrile with water. In contrast to HPLC it is possible to use mobile phases with pH > 8. 

The principle of adsorption chromatography (normal-phase chromatography) is known from classical column and thin-layer chromatography. A relatively polar material with a high specific surface area is used as the stationary phase, silica being the most popular, but alumina and magnesium oxide are also often used. The mobile phase is relatively nonpolar (heptane to tetrahydrofuran). The different extents to which the various types of molecules in the mixture are adsorbed on the stationary phase provide the separation effect. A nonpolar solvent such as hexane elutes more slowly than a medium-polar solvent such as ether. 

The sample mixture should contain no strongly retained components and thin-layer chromatography can be used to check this (no trace should remain at the starting point). The sample may be cleaned by adsorptive hltration (suction hlter with a coarse stationary phase) or column chromatography if required. This is of special importance in the case of reaction mixtures or samples of natural origin. A guard column is essential if impurities or decomposition in the column cannot be fully ruled out. Losses can be avoided by ensuring that the sample loop is not hlled to capacity (see Section 4.6). 

Thin layer chromatography is very similar to paper chromatography. Thin layer chromatography has a wide variety of possibilities depending on the choice of the stationary phase. Adsorption, distribution and ion chromatography can be carried out in thin layer chromatography. 

The stationary phase is a solid on which the sample components are adsorbed. The mobile phase may be a liquid liquid—solid chromatography) or a gas gas-solid chromatography) the components distribute between the two phases through a combination of sorption and desorption processes. Thin-layer chromatography (TLC) is a special example of adsorption chromatography in which the stationary phase is a plane, in the form of a solid supported on an inert plate, and the mobile phase is a liquid. 

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