Separation Distance

The separation distance depends on the dimensions of the plate, the mode of development, the particle size, and the size distribution. The last property cannot be influenced by the user of pre-coated chromatoplates. Because in classical PPC capillary action is effective only for plates up to 20 cm in length, the maximum separation distance is 18 cm. For circular development the separation distance is 8-9 cm; in anticircular mode this distance is 8 cm maximum. Despite the short separation distance, correct selection of mobile phase and development mode can give high resolution.

The separation pathway in CPPC can be increased by use of a sequential technique in which the mobile phase supply to the plate is fully variable in time and

Method

Number of development 2 3 4

®t, iSy, ST, ;SV] STi ;SVl ST] ;SVi ST] ;SVj
®t,'®v, ®v®vt sti;svi

ST ISV ST IS\I ST ISII ST ISii ST ISII

ST ISV ST IS\I ST ISII ST ISii ST ISII

Figure 8 Multiple development possibilities for PPC separations.

location. The principle of this technique is that the mobile phase velocity is much higher at the beginning of separation than later. After an initial separation, the layer is carefully dried, and the mobile phase applicator is placed between two separated zones, irrespective of whether the same or a different mobile phase is used. The supply of mobile phase can be stopped at any time to transfer it directly to the area of the compound zones to be separated. This always gives the highest initial velocity of the mobile phase, which substantially shortens the analysis time. The sequential technique for preparative separations can be performed with a special device—the Mobil-RF chamber.

The separation pathway in S-RPC becomes theoretically unlimited as a result of a special combination of the circular and anticircular development modes. With this technique the mobile phase can be introduced onto the plate at any desired place and time. The solvent application system works in circular mode and with the aid of capillary action against the reduced centrifugal force (anticircular mode). Generally the circular mode is used for the separation and the anticircular mode for pushing the substance zones back to centre with a strong solvent. After drying the plate at a high rotation speed, the next development with another suitable mobile phase can be started. S-RPC is selected when the separation problem cannot be solved with a single mobile phase, but optimized mobile phases are available for the separation of the individual compounds. Also the sequence technique can be employed for fast elution of previously separated compounds with a solvent of high strength.

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