Question

6) If a mixture of compounds with a wide range of polarities has to be separated, two different eluents have to be used to isolate the chemicals. In this case, the eluent of lower polarity should be used first. Please explain why?

Answer 1

The polarity of the solvent which is passed through the column affects the relative rates at which compounds move through the column. Polar solvents can more effectively compete with the polar molecules of a mixture for the polar sites on the adsorbent surface and will also better solvate the polar constituents. Consequently, a highly polar solvent will move even highly polar molecules rapidly through the column. If a solvent is too polar, movement becomes too rapid, and little or no separation of the components of a mixture will result. If a solvent is not polar enough, no compounds will elute from the column. Proper choice of an eluting solvent is thus crucial to the successful application of column chromatography as a separation technique. Thin-Layer Chromatography (TLC) is generally used to determine the system for a column chromatography separation.

Thus often a series of increasingly polar solvent systems are used to elute a column. A less-polar solvent is first used to elute a less-polar compound. Once the less-polar compound is off the column, a more-polar solvent is added to the column to elute the more-polar compound.