Modes of Migration

In standard TLC, the migration of a sample molecule is controlled by its interaction with the bed of chromatographic media and the partition of the solute into the eluting solvent. In electrically driven TLC, the solutes may be made to move in a number of different ways.

If the sample molecules are ionic in the solvent used, the application of an electric field will exert a force on them and they will migrate electrophoreti-cally. As they migrate they are also subject to chromatographic partitioning between the solvent and the stationary phase. The individual components separate from each other by migrating at different velocities, each compound having a characteristic migration velocity that reflects the conflict between elec-trophoretic migration and chromatographic retention.

If, however, the sample molecules are uncharged in the solvent, they will only migrate if the solvent is made to flow. Whereas in standard TLC this is achieved by capillary action, in electrically driven TLC, with the right solvent and adsorbent, it can be achieved through EOF. The separation that results from an electroosmotically driven TLE experiment is, in the absence of electrophoretic effects, similar to that obtained by conventional TLC, but it is obtained much more rapidly.

There is a third mode by which solvent flow can be induced through a thin layer chromatographic media during an electrochromatography experiment. When a current flows through a wetted layer of chromatographic material the layer heats up. The power which has to be dissipated from the plate depends on the current flow and hence the resistance of the wetted plate. When the solvent is unevenly distributed through the plate this leads to an uneven evaporation of solvent from the plate causing capillary solvent migration to occur as a direct result of Joule heating. This effect is more extreme with vertically mounted plates, which under gravity drain solvent to the base of the plate. In some experiments, evaporative flow can be considerably larger than that which is generated by electroosmosis. This can be falsely identified as EOF and is particularly evident in experiments carried out with vertically mounted plates. In horizontally mounted plates, with a solvent reservoir at each end of the plate, solvent is replenished least quickly at the center of the plate. If the rate of evaporation is initially assumed to be uniform across the plate, then the middle of the plate will dry out more quickly. This leads to solvent flow from both ends of the plate towards the middle being superimposed upon any EOF.

The current flow through the solvent-wetted chromatographic layer is dependent on the overall electrical resistance of the plate, which is a function of the ionic density in the solvent. These ions may originate from soluble ionic species in the chromato-graphic material, dissolved ions in the solvent or dissociated solvent molecules. The smaller the ionic density, the higher the overall plate resistance and the smaller the current. Unless adequate cooling is provided, the input of power will cause a temperature rise in the thin layer. This will lead to evaporation of the solvent, the rate of which will depend upon the rate of power influx, the volatility of the solvent and degree of external cooling. This is the main limitation controlling the magnitude of the potential that can be employed in TLE to achieve faster migration rates.

Various methods of cooling the plates have been used in order to reduce solvent evaporation. Immersion of the plate in a solvent that is immiscible with the eluting solvent has been employed in various separations, with CCl4 being the most popular coolant for aqueous eluent systems. The purpose of the solvent bath is to provide direct cooling to the plate surface. This approach was experimentally clumsy and limited the range of analytes, since analyte solubility in the 'coolant' must be considered. It was later dropped in favour of the use of cooling pads in contact with the TLE plate, achieving cooling rates in excess of 0.1 Wcm© With high-conductivity aqueous systems this arrangement allowed the applied potential to be raised to 160 V cm-1, generating migration velocities of up to 0.1 mm s©

Solvents with limited volatility, such as higher alcohols, propylene carbonate and formamides have been used to reduce evaporation. This approach did not however gain popularity owing to several experimental limitations, the most important of which is the difficulty in removing the eluting solvent from the chromatographic material following an experimental run. This is usually necessary in order to visualize the separated compounds.

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