Question on RF-TLC

[ruffian]

Hello guys, I have a sample which can be seperated pefectly on analytic reverse phase TLC plates. But It doesnt work on preparative Reverse Phase TLC plates with same solvent system. It's hard to seperate and even kind of mess on RF TLC plates. First I thought those plates have some problem. But I use two different kind which one is 1000 micron and other is 250 micron. All of them are same. SO I have no idea now.

Anybody can give me any suggestiion ? I am appreciate it .

[tom jupille]

One possibility: TLC results can vary tremendously depending on the pre-equilibration of the plates with the solvent system. Thicker plates take longer to equilibrate.

Another possibility: development of a TLC plate typically involves a certain amount of "demixing" of the mobile phase (certainly true with normal-phase plates; less of a problem with reversed-phase), which means that you are running the equivalent of gradient conditions. Look carefully at your analytical plates as they develop; if you see any evidence of "secondary" fronts or if any of your spots get compressed, then you have significant demixing. You may have to tweak your solvent composition and/or equilibration time to get the amount of demixing to match on the two thicknesses.

[ruffian]

Thank you so much. I really appreciate it.

So that's mean there nothing wrong with plates? But weird thing is when I pre-elute those Reverse Phase plates with pure methanol. It looks like something in plates. The solvent didnt develop evenly not because of "edging effect". They more like wave. Is that also because of demxiing ? It's hard to describle. I'll post some pic tomorrow. Thank you very much.

[tom jupille]

when I pre-elute those Reverse Phase plates with pure methanol. It looks like something in plates

Not unusual for plates to adsorb crud out of the atmosphere; that's one reason for pre-washing the plates.

The solvent didnt develop evenly not because of "edging effect". They more like wave. Is that also because of demxiing

That's exactly what you would expect to see from demixing in a reversed-phase system. The methanol is preferentially sorbed to the stationary phase surface, so the leading edge of the solvent becomes depeleted of methanol. At some point, you get "dewetting", where the water "beads up" on the hydrophobic surface.