Column Chromatography: Speculate on Why Compounds Came Off as Mixture

[chiefy]

A chemist started to carry out column chromatography on a Friday afternoon, got to the point at which the two compounds being separated were about three-fourths of the way down the column, and then returned on Monday to find that the compounds came off the column as a mixture. Speculate on the reason for this. The column had not run dry over the weekend.

My guess is that the beakers below the column should have been switched. Or, maybe, the polarities of the two compounds were similar.

What does everyone else think?

 

[Moonbear]

Both of your answers sound like reasonable explanations (or both actually). So, yes, they could have been very close to each other in polarity (or size...depends on what the column was made out of), so overnight, both eluted out into the same vessel if nobody was around to change it.

 

[DocToxyn]

The assumptions being made here are that the column was continuously eluted over the entire weekend and that the receiving vessel was not changed. I guess the simplest answer was the one you stated that the collection vessel should have been changed, however, I think we need more information to fully understand what happened. Were these compounds colored such that he could tell the difference between the two of them and thus could monitor their progress down the column? Also were the compounds being separated from each other (this is not stated clearly) or from some other compounds that remained on the column? Perhaps the two compounds were never techincally separated by the column interaction.

If we do somehow assume that the compounds were separated then I don't quite understand how the observer knows that the compounds came "off the column in a mixture". His only evidence is that they are both in the collection beaker. Couldn't one have come off first and then shortly thereafter the second one followed, thus they didn't elute as a mixture but rather were mixed in the beaker.

Some other speculative thoughts would be that the elution buffer was in an open vessel on top of the column and some evaporation occurred causing the characteristics of it to change and thus alter the elution dynamics of the two compounds. Temperature change shouldn't have had a great effect since it should increased the rate of elution (with increasing temp and it would probably have to be a sizable increase), but that effect should be similar for both compounds.