Question

A TLC plate showed two spots with Rf values of 0.25 and 0.26. The plate was removed from the developing chamber, the residual solvent was allowed to evaporate from the plate, and then the plate was returned to the developing chamber. What would you expect to see after the second development was complete?

Answer 1

From the first addition, we have a Rf difference of 0.01.

Let's say that the overall distance from baseline to solvent front was 10 cm long. Then the two spots traveled 2.5 and 2.6 cm.

Now, consider what happens if we run the plate again. When the solvent hits the first spot, it begins picking it up, and the spot will travel 0.25 * 7.5 cm (the distance from the spot to the end of the plate). So this new spot will be at 2.5 + .25*7.5 = 4.375 cm.

Similarly, when the solvent hits 2.6 cm, it will start picking up the second spot. The second spot will move 2.6 cm + .26*7.4 = 4.524 cm.

The new Rfs are then 0.4375 and 0.4524, for an Rf difference of 0.0149.

Thus, if you develop the plate a second time, you would expect to see increased separation between the two spots!