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MJLPHD

Dry column vacuum chromatography (DCVC)

25/10/2013

3 Comments

 
Picture
I had a compound that I was preparing on a 10-50 g scale and kept getting one spot with a consistently lower Rf which when present would stop my compound from recrystallizing. I heard about this method from a colleague who gave me a copy of Daniel Sejer Pedersen's 2001 Paper in Synthesis (Synthesis, 2001, (16), 2431-2434). Daniel has further detailed this technique on his blog.
I made the closest adaptation I could with our existing glassware and it worked great. In this photo 13 g of crude compound is loaded onto 40 g of silica. It took 300 mL to see any trace of the compound and after 500 mL I increased the solvent polarity from 70% hexanes : 30% EtOAc to 50% hexanes : 50% EtOAc to recover 12 g of compound that recrystallized easily. I have since managed to upscale to 25 g with exactly the same setup.
I used DCM to load my compound to the silica and care had to be taken when removing the DCM which bumps violently on the rotavap even when the vacuum is lowered with care, so you might want to use a splashguard with cotton in it. The dry loaded silica is then distributed evenly over the wide column of clean silica using a glass funnel. The Synthesis paper suggests placing a filter paper on top of the loaded silica when pouring solvent. I tried once with this and once without and found that in my case I got a more evenly spread pour with no paper but by using a wide mouthed beaker moved in a circular fashion as poured. It is true what it says in the paper that when a compound comes through a foaming is seen in the solvent. It would definitely be better to have a separatory funnel on the bottom with a vacuum inlet near the neck as this would more conveniently control the release of each fraction.

3 Comments
Jim
16/5/2014 09:52:00 am

Nice video. Have you been successfully using this in smaller scale (1 g, perhaps 3 cm diameter column) ? I tried the setup, including making the sep funnel, however the spots often overlap between the fraction.

One more question, why do you have to adsorb your compound on silica and not celite ?

It will be really great for educational purpose if you can make a video showing the process from the beginning to the end, showing successful separation of a mixture. I am looking forward to that.

Thanks,

Reply
mjlphd link
16/5/2014 04:04:48 pm

Jimmy,

Thanks for your thoughts. Yes, I have succeeded with this on smaller scale. A compound library that I prepared needed to be purified for characterization. I used this method with between 800-1200 mg of crude compound and as I wanted to obtain them quickly and in great purity I would often be happy to gather 400-500 mg of the pure compound. For this scale I had a column that was 4 cm in diameter, but it would have worked equally well with a 3 cm column. I was able to clean and re-use the silica this way without a problem. I find this method less effort than a flash column, however the disadvantage is that spots eluting very close together can't be separated as well this way as than can using flash chromatography.

Using celite instead of silica should also work as long as the spots are far apart. Celite doesn't have as strong resolving power as silica. If it's for a particular compound that you're going to be repeating, I'd try with both and compare, especially if you plan to upscale. If it's for a compound library, I'd stick with the silica.

Reply
Daniel
24/4/2015 03:53:26 pm

I think what Jimmy is getting at is that it was mentioned on the CurlyArrow blog that it is better to absorb your compound onto celite for dry-loading, rather than using silica gel. The column is still run using silica as the stationary phase.

When I worked in PR&D, this would be how we did chromatography on anything more than a few grams (and some people would use this method exclusively even for <1g). I've personally purified 150g using ca 1800g silica in a 4 L sinter funnel. If you're having co-eluting spots, you're either a) not using enough silica or b) you're increasing the polarity of your solvent too fast (sometimes its helpful to run these isocratically.

In regards to a), we'd use between 8 and 15 wts ("grams silica per gram crude") depending on how easy the separation is. Contrast this to regular flash, where it is advised that 50 to 100 wts silica is used (unless it is known less provides satisfactory separation).

The other thing to be mindful of is that you should be collecting appropriately sized fractions. Pouring solvent on the top until it just comes out the bottom is about right - in terms of numbers, this typically works out to be about 0.45 CV (column volumes), or ca 0.9 x mass of silica used.

You don't even have to "dry-load" (pre-adsorb) your compound - So long as it doesn't move far off the baseline in DCM (check by TLC), use a minimal volume to dissolve your compound. When you load the solution to the column, ensure that it is applied evenly and thinly. A thin application band is key to good separation!


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