TLC

[walker8476]

If you want to separate two compounds, and you run TLC a few times with different solvent systems, how do you determine from the TLC plate which is the best solvent system to separate the two compounds from each other using column chromatography?

Do I measure the Rf values of both compounds on each plate? I've read that an Rf of about 0.30 is good, but both spots can't have an rf of 0.30 otherwise they won't separate properly.

Or do I determine which plate gives the best Rf value for each compound. Say plate three gives the best Rf for compound A and plate one gives the best Rf for compound B. Then use the solvents system for plate three to elute compound A and then switch to the solvent system used for plate one to elute compound B?

[JMB]

With silica gel TLC plates, find a solvent (e.g. starting at EtOAc/Hexane, 1:5 v/v) that just moves products above baseline. Adjust mobile phase, if necessary, so that products have Rf values in range ~0.1-0.6 after 4-5 developments of the same plate. Make sure plate is thoroughly air-dried between sequential developments. The idea is that multiple developments represent APPROXIMATELY the behavior to be expected on a column.

[walker8476]

With silica gel TLC plates, find a solvent (e.g. starting at EtOAc/Hexane, 1:5 v/v) that just moves products above baseline. Adjust mobile phase, if necessary, so that products have Rf values in range ~0.1-0.6 after 4-5 developments of the same plate. Make sure plate is thoroughly air-dried between sequential developments. The idea is that multiple developments represent APPROXIMATELY the behavior to be expected on a column.

But how do you know what's the optimum separation between spots? Because you could have two spots with Rf values in the range of 0.1-0.6, but one spot has an Rf of 0.3 and the other is 0.4

[Gerhard Kratz]

Why do you not go directly with HPLC? You will waste time doing TLC runs and afterwards you need again to find time for the starting conditions with HPLC.

[Peter Apps]

If you want to separate two compounds, and you run TLC a few times with different solvent systems, how do you determine from the TLC plate which is the best solvent system to separate the two compounds from each other using column chromatography?

Do I measure the Rf values of both compounds on each plate? I've read that an Rf of about 0.30 is good, but both spots can't have an rf of 0.30 otherwise they won't separate properly.

Or do I determine which plate gives the best Rf value for each compound. Say plate three gives the best Rf for compound A and plate one gives the best Rf for compound B. Then use the solvents system for plate three to elute compound A and then switch to the solvent system used for plate one to elute compound B?

What is it that you want to optimise ?

Do you want to get an acceptable separation in the least possible time ? Do you have lots of time and need a very clean separation ? Does column length figure in you optimum ?

When you say "column chromatography" do you mean HPLC or low pressure, and how will you be detecting your compounds when you go to the column stage ?

Peter

[walker8476]

If you want to separate two compounds, and you run TLC a few times with different solvent systems, how do you determine from the TLC plate which is the best solvent system to separate the two compounds from each other using column chromatography?

Do I measure the Rf values of both compounds on each plate? I've read that an Rf of about 0.30 is good, but both spots can't have an rf of 0.30 otherwise they won't separate properly.

Or do I determine which plate gives the best Rf value for each compound. Say plate three gives the best Rf for compound A and plate one gives the best Rf for compound B. Then use the solvents system for plate three to elute compound A and then switch to the solvent system used for plate one to elute compound B?

What is it that you want to optimise ?

Do you want to get an acceptable separation in the least possible time ? Do you have lots of time and need a very clean separation ? Does column length figure in you optimum ?

When you say "column chromatography" do you mean HPLC or low pressure, and how will you be detecting your compounds when you go to the column stage ?

Peter

I want to use column chromatography, not HPLC, so the glass tube etc. I want to collect the compounds as they elute.

I want to optimize the separation between two compounds, in a reasonable time.

[carlo.annaratone]

You obviously need two different Rf for two differnet compounds, otherwise you'll never have separation! Therefore you need to have values AROUND 0.30. For two compounds, ideally one above and one below; for three one above, one at 0.3 and one below; and so on. If you have many peaks, there might be no optimal solution; you might need to run two separate columns, or switch to a stronger solvent after the first series of peaks

[Fernando]

Hi

If you have to perform a TLC separation I recommend the book by Egon Stahl
"Thin Layer Chromatography" wich has thousands of examples of compounds.

Good luck!

[JMB]

While I encourage reading about separation techniques (as you obviously have done), parameters given (optimum Rf at 0.3) are usually idealized. You should not consider them as the only values that work. If you were to give us the the actual Rf values achieved with one or two different elution mixtures, then we would probably be able to provide more specific advice.

The technique that I described worked well for me with carbohydrates that had been selectively or only partially derivatized. Bear in mind that TLC is planar DRY chromatography---I performed my subsequent column chromatography in the DRY column mode.

JMB

I should note that I used lab. prepared TLC plates made with microscope slides; this enabled scouting experiment to be performed very quickly.