Question

Why must the spot applied to a TLC plate be above the level of the developing solvent? What problem will ensue if the level of the developing liquid is higher than the applied spot in a TLC analysis?

Answer 1

Answer:

See explanation

Explanation:

If the spot in TLC is below the solvent front, it will be observed that the spot, instead of being separated by the solvent as expected, will just dissolve away in the solvent and zero actual separation of the mixture is achieved.

If the solute is dissolved away instead of being separated by the solvent, then the experiment fails because no actual separation of the mixture is achieved.

Hence, in TLC, the spot must be applied above the solvent front so that the capillary movement of the solvent through the plate can lead to the eventual separation of the components of the mixture since the various components of the mixture will travel at different speeds through the plate.

Also, if the solvent is above the spot, the solvent may evaporate selectively from the points above the spot while separation is ongoing.