Sciencemadness Discussion Board

TLC challenge

Pumukli - 3-11-2022 at 11:41

I'm playing with something (an organic preparation) nowadays and made a TLC sheet in the morning.
I was surprised by the outcome of this run and thought I would post it here and ask your opinion!

"If you had this TLC sheet what kind of information could you decipher from the look of it?" This is the "challange"! :)

A little help:
- The sheet is "Alufolien Kieselgel 60 F254", Merck, at least 30 years old.
- Eluent was heptane/acetone 4:1
- The sheet was developed in iodine vapours.
- The leftmost ("empty looking") spot is the starting compound.
- The other spots are a "time series" of sampled (then chloroform extracted) reaction mixture spots.
- The time elapsed since reaction start is 2 minutes, 18 minutes and 60 minutes, in this order, from left to right.

What does this sheet tells you? :)
timesequence.jpg - 462kB

Right after I dried the "plate" but before the iodine vapour treatment the sheet looked like this:

IMG_20221103_101916847.jpg - 560kB

Fulmen - 3-11-2022 at 12:31

Wrong eluent?

Pumukli - 3-11-2022 at 12:46

Are you asking me? I thought that I ask and you answer! ;)

But you are right, the eluent system was not too good, the Rf-s are obviously too high. You can not change this though. Try to decipher more info! :)

By the way, the expected end product of the reaction is less polar than the starting material. ;)

Fulmen - 3-11-2022 at 13:36

Your starting material could have reacted quickly to an intermediate compound that doesn't show up before slowly reacting to the final product. Or perhaps you spilled coffee on some paper and are pulling our legs :-)

Texium - 3-11-2022 at 13:42

It tells me you need to re-run it with a less polar eluent, and maybe invest in a 254 nm UV lamp if these compounds are UV active.

CharlieA - 3-11-2022 at 14:29

Just out of curiosity, did you pre-treat the plate in any way, e.g., by heating it for a period? I don't really know how its age would have affected things, but perhaps the long age and/or no pre-treatment (activation) were a factor.

Pumukli - 3-11-2022 at 22:36

Fulmen: Good point! (Not the coffee thing, ha-ha.) Hm, I may try next time a more controlled sampling/extraction/spotting. (The different size of the spots may be (partly) explained by variable volumes of samples and extracting solvent.)

Texium: yes, I plan to use a different eluent next time. Something with more "differentiating power" of targeted compounds. On the other hand I don't have a proper 250 nm UV light so I used what I found at home: a 360 nm one. The spots were faint but at the exact same position as iodine showed later.

CharlieA: The pre-treatment meant a quick drying with a hair drier. Just to remove absorbed water.
The age I think shows up on the upper side, right below the front of the solvent front as that "zig-zag" pattern.
Next time I will "pre-run" the empty sheets in pure acetone. This way I may be able to sweep everything "of last 30 year's wear and tear" up with the front, out of way.

Thanks for the contributions guys!

Keep ideas coming! :)

Mateo_swe - 4-11-2022 at 11:31

Maybe make a DIY TLC lamp?

Cheap DIY TLC lamp (SM thread)