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Author: Subject: Column chromatography problem

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[*] posted on 1-7-2013 at 17:59
Column chromatography problem

Hi everyone,

I have some difficulties when I try to purify my product with a column chromatography using a MeOH/DCM (10:90) eluent... sometimes the silica cracks and many bubbles appears when the eluent runs down the column.

Someone know why?
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[*] posted on 1-7-2013 at 18:25

Not an expert in column chromatography but in my experience a column cracks only if it goes dry or was prepared incorrectly. Make sure you are doing the following:

Mix your silica gel with lots of your eluent. Make a thin slurry. Transfer this to your column and shake the column to help it to settle. All of the silica gel should go to the bottom and there should be plenty of liquid above. Drain out the liquid as necessary until all of your silica gel is in the column. Do not let the liquid level drop below the solid level.

Once you have your column packed, drain out enough liquid so that your liquid level is about even with your solid level. Add your compound that you are eluding, preferably dissolved concentrated in solvent if it is not a liquid. Drain the column until your new liquid level is even with the silica gel.

Now start adding eluent. Add some, let it pass through, keep adding and passing never letting your liquid level drop below the silica gel.

If you need to take a break just seal up the column, they can sit for some time (days in my experience) without the bands moving. Great for if you're waiting on yield.

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[*] posted on 1-7-2013 at 19:40

<strong>Carabistouilles</strong>, how dry is your methanol? What type of silica are you using?

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[*] posted on 2-7-2013 at 01:24

You should also pack your whole column in one addition. If you add only a little slurry, wait for it to settle and then add more increments later there will be inhomogenities in between the increments. If necessarry, extend the column temporarily with another tube or funnel that you can remove later when it is packed.

Also avoid large temperature variations. You can get gas bubbles in the column otherwise (from gas dissolved in you mobile phase), especially going from a cold to a warm environment.

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[*] posted on 2-7-2013 at 05:19

Methanol releases a lot of heat when it hits silica. This is probably the issue, so just prepare a slurry - wet load the column, don't dry load. Or if you're running a gradient, your gradient may simply be too steep.

That said, some advice coming from a lot of experience running columns: DCM/MeOH is a shitty eluent and if you can run your sample in anything else (such as hexanes/EtOAc/TEA, or hexanes/EtOAc/AcOH), do so. Things always look much better on the TLC than after you run the column. Diffusion can be a complete bastard with this solvent system. Also, use tongs when handling the test tubes, or you'll get splashed with DCM a thousand times over and not only does it wreck nitrile gloves in a hurry, it's carcinogenic and passes through them with little difficulty.

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[*] posted on 2-7-2013 at 07:28

I agree with madscientist, if your gradient is to steep, you will see those "bubbles" in your silica. Pain in the ass to see those and always frustrating.

Also, when loading the column, make sure you shake it up thoroughly to get the bubbles out. And after you pour your silica into the column, tap the sides with a cork ring, or something not hard, to loosen up any other bubbles trapped inside as you let it all settle and pack.

Guess what I'm doing right now? Separating diastereomers on a huge column. See ya in an hour or two. :cool:

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