[blaisem]

Hello everyone.

I ran a TLC plate on acetylferrocene (Rf value 0.5) and ferrocene (Rf value 0.95).

Then I purified my product via column chromatography and checked the efficiency of the column by running TLC plate on the eluent collected.

My results yielded Rf values of 0.43.

Given the difference between 0.43 and 0.5, is it ever possible that a minor amount of impurity can have an impact on the Rf value, or should I always expect total separation, and, therefore, highly consistent Rf values (ie. I made a measurement error on the Rf value).

Thank you to anyone for their advice!

[discodermolide]

Hello everyone.

I ran a TLC plate on acetylferrocene (Rf value 0.5) and ferrocene (Rf value 0.95).

Then I purified my product via column chromatography and checked the efficiency of the column by running TLC plate on the eluent collected.

My results yielded Rf values of 0.43.

Given the difference between 0.43 and 0.5, is it ever possible that a minor amount of impurity can have an impact on the Rf value, or should I always expect total separation, and, therefore, highly consistent Rf values (ie. I made a measurement error on the Rf value).

Thank you to anyone for their advice!

Surely this has to do with the quality of the silica, what grade did you use for the column?

[blaisem]

Unfortunately, I never even thought to check that.  It was my first time doing column chromatography.  It was in a lab at school, so I assume the instructor found something more or less suitable.

Are you saying the silica was carried with the solution out of the column, and then effected the elution of the sample on the TLC plate?  I didn't know this was possible.

[discodermolide]

Unfortunately, I never even thought to check that.  It was my first time doing column chromatography.  It was in a lab at school, so I assume the instructor found something more or less suitable.

Are you saying the silica was carried with the solution out of the column, and then effected the elution of the sample on the TLC plate?  I didn't know this was possible.

No, I am saying that the grade of silica used in TLC is or can be very different from column grade silica. Thus the retention/absorption properties can be very different. Look at an Aldrich Catalogue.

[w4rlock]

How long are your TLC sheets? Could you have spotted the TLC at different heights?
I don't think a minor impurity is going to cause a significant change of the Rf value. I think it's more likely that you had a slightly different composition of mobile phase for the second TLC.

This only makes sense if you are using relatively short TLC sheets, if the difference in Rf is significant then that's another matter.

[fledarmus]

What was your solvent system? And how large is your solvent reservoir?

I have found that with very small chambers and volatile mixed solvents, you can see a lot of drift from plate to plate just due to differential evaporation of the solvents. Dichloromethane is very bad about this. If you have larger chambers with no filter paper on the sides, it is possible that your vapor isn't always in equilibrium with your liquid, and you can see evaporation from the plate as it elutes which will also appear as a change in the R_f.

The best practice would be to always spot your product spots against a sample of the crude material - that way you can tell if the spots are just running differently under your conditions, or if you truly have a change on the silica gel.

Impurities shouldn't affect your R_f unless they are really changing the plate or the solvent properties. Traces of DMSO, DMF, water, or TEA can cause substantial streaking and spot movement, for example. Impurities from the reaction rarely do; they just show as an extra spot.

[blaisem]

I was really looking for some more information on the TLC; not expecting too much in the responses.  I am pleasantly surprised by all the offer of advice to fix the issue.  This definitely gave me a lot of information on what to consider in this process.  I thank you all for providing me with this.

It was for a lab in inorganic, though, so I can't really go back and fix any of it.  Also, the materials used were provided on a counter, and I didn't write down information like the type of silica or column used.  Unfortunately, I can't pursue or resolve any of the ideas from these comments further.  I just wanted to post my thanks for the time you took to make these helpful responses