Thin-layer chromatography, like the principle of other chromatographic techniques, is a technique that separates the various constituents of a sample by their different physical properties. These physical properties include the size, shape, charge, volatility, solubility, and adsorption of the molecule. Thin layer chromatography separations are generally the result of a combination of several separation mechanisms, most of which are adsorption and partitioning, as well as ion exchange or gel permeation. In the process of thin-layer chromatography, the characteristics of the stationary phase and the mobile phase vary greatly depending on the separation principle. Therefore, for the convenience of description, the basic principles of each type of chromatography and the related stationary phase and mobile phase technology are respectively introduced. A brief introduction is as follows.
Section 1 Adsorption Chromatography
First, the principle of adsorption chromatography
The single component of the mixture dissolved in one phase by adsorption chromatography exhibits a concentration change at the interface of the other phase, and the concentration of the component often occurs on the surface of the other phase. This phenomenon is called adsorption. Adsorption thin-layer chromatography is to uniformly deposit the adsorbent on a smooth surface such as glass, metal or plastic, and then place a sample on it to spread the mobile phase, so that the components are continuously adsorbed. The agent is adsorbed, dissolved by the mobile phase, desorbed and moved forward. Since the adsorbent has different adsorption capacities for different components, the mobile phase also has different desorption capacities, and the components which are more closely combined with the adsorbent are more difficult to be desorbed by the developing agent, and the components which are more loosely combined with the adsorbent are easier. Desorbed by the developer. Therefore, in the process of the mobile phase flowing forward, the different components move at different distances, and the original mixture can be separated. The degree of separation is generally expressed by the value of the shift value Rf, and the value can be separated from the origin by the distance from the center of the spot of the separated component and the origin of the developer.
Third, the requirements of mobile phase in adsorption chromatography
The choice of mobile phase must be considered in combination with the nature of the material being separated and the nature of the adsorbent chosen. When using a polar adsorbent for chromatographic separation, when the substance to be separated is a weakly polar substance, a weakly polar solvent is generally selected as a developing solvent; when the separated substance is a highly polar component, a polar solvent is required to flow. phase. If a less adsorbent adsorbent (such as diatomaceous earth or talc is used instead of silica gel) is used for a polar substance, the polarity of the mobile phase must also be reduced accordingly.
In thin-layer chromatography, when the activity of the adsorbent is constant (such as grade II or grade III), the satisfactory separation of the multi-component sample depends on the choice of the developer. The selection principle of the developing agent is mainly to select a developing agent of a corresponding polarity according to the polarity of the separated substance.
Micellar electrokinetic capillary chromatography is a new type of chromatography based on micellar solubilization and electrophoretic mobility. When the sample enters the capillary, it is distributed in the aqueous phase and the micellar phase. The components that are distributed more in the aqueous phase move faster, and the components that are more distributed in the micellar phase move slowly. Therefore, the various components can be separated depending on their distribution in the two phases. Micellar electrokinetic capillary chromatography is characterized by its inadequacy of the inability of ordinary electrophoresis to separate neutral compounds, and can be used for the separation and determination of many electro-neutral drugs.
Third, reversed phase thin layer chromatography
Reversed-phase thin-layer chromatography uses a chemical bonding reaction to bond a hydrocarbyl silane to the surface of a silica gel to form a non-polar stationary phase. This method can be used for the separation of polar, polar compounds and even the separation of water-soluble compounds. The solvent system can be selected according to theory and expected method because of the linear relationship between the Rf value and the organic modifier content in the mobile phase. The method has high recovery rate and good reproducibility, and the sheet can be reused.
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