Baphomet
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Column Chromatography
Has anyone here used column or flash chromatography to separate different fractions? I'm particularly though not exclusively interested in organic
mixtures.
The mixture I have is composed of a target compound whose molecules are larger and has a higher boiling point than the other compounds in the mixture.
So I could use distillation but I just wanted to know if anyone has pointers on FC using silica gel in a homemade apparatus.
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nightflight
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I´ve ased this before on another board and got no feedback(so maybe it works too good*lol*)
but seriously, there is no point in using this IMHO, when destillation is a possible option.
First, you need to choose the perfect carrier and eluant that dissolves the product and has just the right polarity, to adhere to the medium, then
there´re many things to consider in the set-up, like no bubbles in the silica, carefully adding the eluant-product, and finally there´s no way in
telling by sight, when your product will be coming out, except it has a different colour, so you´ll have to do a TLC and develop it, plus using many
beakers, not to speak of the pressure to be generated in flashc.
.
But that´s just my uneducated and solely theoretical opinion.
Of course, I´d be courius to do it, but that´s a bit too much hassle for me, for having so little advanteges over other methods.
So maybe ther´re matrials and products, where it´s superieur over other methods, it´s just that I wouldn´t know which these´d be.
[Edited on 21-12-2006 by nightflight]
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Tsjerk
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Column Chromatography does work great if you want product of high purity. But the downsides are that you will need a LOT of solvent and you must be
able to do TLC. The times I used this technique (not at home) I already knew which solvents to use and in which concentrations. But if you have a lot
of cheap solvents and you know which ones to use it's a technique, I think, is well usable at home.
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Nick F
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Yeah, it's not the size of the molecules that is important, as in gel electrophoresis, but the polarity. You need to balance your compound's affinity
for the solid phase (your silica gel) and the solvent. As has been said, you will need a lot of solvent (well, that depends on how much you are trying
to seperate) and you would really need to be able to do TLC. Normally it is actually quite easy to find a good solvent mixture using TLC - I normally
start off with DCM, and if that doesn't work just add EtOAc to the DCM, 10% at a time, until you find a ratio that is suitable. I find that that
approach works unless your product is very similar to your waste products.
To set up for FCC, poke a bit of cotton wool down to the bottom of your column, so that it gets wedged just above the tap, then add about 1cm of fine
sand on top of that, and wet it with whatever solvent mixture you've selected. Then put some of your silica gel into a large conical flask (wear a
face mask, you don't want to breathe in the dust!), and make a thin slurry of it with your solvent mixture. The thinner the slurry, the easier it is
to make sure that there are no bubbles in your column. Pour some of this slurry into your column, tap it to free any trapped air bubbles, open the
tap, and pressurise the column to force some of the solvent out. Stop when there is a cm or so of solvent above your silica. This packs the gel down.
Repeat this until you have the desired height of silica. If the Rf values of the different fractions (as found by TLC) are similar, then you will need
quite a long column to get a good seperation. Then put about 5mm of fine sand in, open the tap (without pressure) and let the solvent drip out until
the solvent level is just at the top of the sand. Then add your compound (neat if liquid, or dissolved in a bit of your solvent) and again let solvent
drip out until the liquid is just at the top of the sand, or slightly below. Then very carefully and slowly (with a pipette) add more solvent on top,
taking care not to disturb this sand layer. Once you get an inch or so above the sand, then you can start adding it more quickly. Fill the column to
the top with solvent, and you're ready to go! There are other variations on this method, it doesn't really matter which you use.
It is a bit more hassle than distillation, but sometimes it's the only way to go. Last term at university I wasted a week of work on my fourth year
research project because I tried a bulb-to-bulb distillation of my product, but it turned out that even at 0.2mm Hg, the decomposition point was below
the boiling point and my product was turned into a black sticky mess! 
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Baphomet
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Thanks guys for all the info! The subject appears to be covered quite comprehensively here. Distillation will be the way to go from now on, but I'll
keep this in mind in case I get a black sticky mess some day
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