Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: uglepik on April 20, 2011, 09:27:59 AM
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I have a compound that I can pretty much dissolve only by addition of acid with pka < 3-4. As I need to purify this stuff, I'd like to run a column using silica as stationary phase and an eluant containing TFA (possibly 1-2 %), and possibly also water (same, about 1-2%).
(reverse-phase purification, HPLC and such, is not possible since the compound contains a large very lipophilic moiety).
So the question is... Does any of you have experience with using TFA on silica. Some people have proposed that it might dissolve the silica?! Or is it all good... thanks!
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I have a compound that I can pretty much dissolve only by addition of acid with pka < 3-4. As I need to purify this stuff, I'd like to run a column using silica as stationary phase and an eluant containing TFA (possibly 1-2 %), and possibly also water (same, about 1-2%).
(reverse-phase purification, HPLC and such, is not possible since the compound contains a large very lipophilic moiety).
So the question is... Does any of you have experience with using TFA on silica. Some people have proposed that it might dissolve the silica?! Or is it all good... thanks!
Use reverse phase silica-gel
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As I wrote, reverse phase is not a good idea with these compounds. ;)
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As I wrote, reverse phase is not a good idea with these compounds. ;)
Worth trying, optimize the separation with HPLC first. I have purified lipids on reverse phase silica.
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I've heard of people running columns with some acetic acid in, but never TFA. I think we used to have a mix called CMAW in my old lab that was chloroform/MeOH/AcOH/water. I don't remember the proportions but I think it was something like 80/15/3/2.
You can get away with quite a lot in terms of polarity and silica. In many cases I think the dissolving silica issue is over exaggerated (though it will happen, especially if you're using a lot of MeOH).
A polar eluent I use occasionally is 45:5:1 EtOAc/IPA/water. This doesn't dissolve silica, so maybe spiking it with AcOH or TFA will work for you. Mix the IPA/water first, then add EtOAc otherwise you can have miscibility issues.
All this said, I think when you're considering putting water down a silica column then reverse phase is worth a look into.
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I've heard of people running columns with some acetic acid in, but never TFA. I think we used to have a mix called CMAW in my old lab that was chloroform/MeOH/AcOH/water. I don't remember the proportions but I think it was something like 80/15/3/2.
You can get away with quite a lot in terms of polarity and silica. In many cases I think the dissolving silica issue is over exaggerated (though it will happen, especially if you're using a lot of MeOH).
A polar eluent I use occasionally is 45:5:1 EtOAc/IPA/water. This doesn't dissolve silica, so maybe spiking it with AcOH or TFA will work for you. Mix the IPA/water first, then add EtOAc otherwise you can have miscibility issues.
All this said, I think when you're considering putting water down a silica column then reverse phase is worth a look into.
Discoderlimide: Which lipids did you purify and under what conditions? My guy has a 1,2-di-stearoyl-glycerol moiety (and 4 glutamic acid groups) attached and seems to stick pretty solidly to non-polar stationary phases. I know of certain columns that have both polar and nonpolar character (such as diol columns) that are suitable for this kind of things, but sadly I currently don't have access to those.
Dan: I've used CMAW in different ratios a great deal and it has been marvelous for most of my componds. But for this one, the AcOH simply doesn't cut it for dissolution - I can get great Rf's but as it wont dissolve the sample in a reasonable concentration it is quite useless. So my idea is to substitute the AcOH for TFA and mix it with the old friends CHCL3, MeOH and water.
I haven't been playing to much with isopropanol in this case, but it might be a nice idea. I know that it is quite used with amphiphilic compounds.
I am not looking for an especially polar eluant. Something like CHCL3:MeOH:TFA:water - (75:20:3:2). The water is needed in order to get well-defined spots.
The problem with reverse phase (and normal phase) is that this compound is quite the amphiphile and therefore nothing is really optimal.
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I've heard of people running columns with some acetic acid in, but never TFA. I think we used to have a mix called CMAW in my old lab that was chloroform/MeOH/AcOH/water. I don't remember the proportions but I think it was something like 80/15/3/2.
You can get away with quite a lot in terms of polarity and silica. In many cases I think the dissolving silica issue is over exaggerated (though it will happen, especially if you're using a lot of MeOH).
A polar eluent I use occasionally is 45:5:1 EtOAc/IPA/water. This doesn't dissolve silica, so maybe spiking it with AcOH or TFA will work for you. Mix the IPA/water first, then add EtOAc otherwise you can have miscibility issues.
All this said, I think when you're considering putting water down a silica column then reverse phase is worth a look into.
Discoderlimide: Which lipids did you purify and under what conditions? My guy has a 1,2-di-stearoyl-glycerol moiety (and 4 glutamic acid groups) attached and seems to stick pretty solidly to non-polar stationary phases. I know of certain columns that have both polar and nonpolar character (such as diol columns) that are suitable for this kind of things, but sadly I currently don't have access to those.
Dan: I've used CMAW in different ratios a great deal and it has been marvelous for most of my componds. But for this one, the AcOH simply doesn't cut it for dissolution - I can get great Rf's but as it wont dissolve the sample in a reasonable concentration it is quite useless. So my idea is to substitute the AcOH for TFA and mix it with the old friends CHCL3, MeOH and water.
I haven't been playing to much with isopropanol in this case, but it might be a nice idea. I know that it is quite used with amphiphilic compounds.
I am not looking for an especially polar eluant. Something like CHCL3:MeOH:TFA:water - (75:20:3:2). The water is needed in order to get well-defined spots.
The problem with reverse phase (and normal phase) is that this compound is quite the amphiphile and therefore nothing is really optimal.
I purified phosphinic acid analogues of phospholipids , phosphatidyl choline analogues etc. Unfortunately I cannot tell you the exact conditions, company secrets and so forth. The CMAW mentioned above is a good system. Try replacing the acetic acid with propionic acid