Question

Chromograhy

1) Why should a TLC plate be removed from the solvent before the solvent front reaches the top of the plate?

2) Why should the developing chamber for a TLC plate not be open to the atmosphere?

Answer 1

1)

If TLC plate is left in, the solvent will keep flowing up the plate, and it will carry the samples with it. Once it reaches the top, it will either remain there or evaporate, but the solvent will keep depleting from the bottom, meaning the samples will start to get pushed together, until they become indistinguishable again.

2)

You do not want the eluting solvent to evaporate as this will make the apparent solvent front be inaccurate leading to the wrong R_f. In fact, if the solvent evaporates from the plate, new solvent will keep entering the plate from the bottom, and, eventually, your spots will move all the way to the top of the plate rather than stopping at an R_f characteristic of some chemical property of the material being analyzed.