Question

Briefly describe the effect of the following procedural errors on a thin-layer chromatogram.

a) Applying TOO LARGE a sample when spotting the plate.

b) Placing the spotted plate into a developing chamber containing an eluent level ABOVE the level of the origin.

c) Allowing the plate to remain in the developing chamber until the eluent front reaches the TOP of the plate.

d) Removing the plate from the developing chamber when the eluent front has move only HALF-WAY up the plate.

e) Developing the plate in an UNCOVERED chamber.

Answer 1

a) Applying TOO LARGE a sample when spotting the plate.

if you have too much sample in the spot you will have your spots with a long tail (Tailing) which dont let you know the exact Rf value of each compounds.

Tailing occurs because Non-adsorbed solute will migrate through the stationary phase faster than expected, a phenomenon, the substance trails a line behind it like a tail.

b) Placing the spotted plate into a developing chamber containing an eluent levelABOVE the level of the origin.

the compounds will disolve in the eluents before start the "running" in the TLC.

c) Allowing the plate to remain in the developing chamber until the eluent front reaches

     the TOP of the plate.

if you wait to long after the eluent reach the top of the plate your compouns will mix again in the top of the TLC plate.

d) Removing the plate from the developing chamber when the eluent front has moveonly HALF-WAY up the plate.

If you remove the TLC plate before reaching the top of the plate, you will not have a good separation and you cannot calculate the RF value.

e) Developing the plate in an UNCOVERED chamber.

The solvent will evaporate, if you use as eluent a mixture of solvents, the most volatile solvent will evaporate and the eluente will have a diferente polarity than what you expected

Answer 2

a) Too large spot will spread more and won’t give a good separation

b) The spot will get spread and become wide. The spotted compound may dissolve, the plate will not develop properly and affect the good separation.

c). Allowing the plate to remain in the chamber until the eluent front reaches the top of the plate will affect the rf calculation.

d) It will not allow the plate to fully develop and will not give a good separation. The spots won’t separate properly.

e) The eluent will evaporate . If the eluent is mixture of solvents possibilities are there for one solvent to escape more and there by change the polarity of the solvent mixture. If the solvent evaporate the spots won’t move in the TLC plate. So will lead to poor separation.