Hey wasps
Hoping some of you have experience with flash chromatography in the ghetto. Because this happens in the ghetto where lab grade chromatography silica is not affordable, attempts to grind up kitty litter have been looked into (it works beautifully for TLC!!)

Here is the problem though, when using a compressed air added to the top of the column, so much pressure builds up that it blows the gas inlet off the top of the column, even if it is held in place by hand the the elution is painfully slow. So instead the column was connected to a filtration flask and vacuum is applied from beneath, the rate of elution goes way up BUT you can see air bubbles traveling upward through the stop cock and eventually it sucks the solvent out of the bottom of the column leaving gaps between the silica gel particles, but the solvent level above the silica doesn't really move. So it has to be aborted empties and started over.

It seems that the problem is very fine particles of silica form an upper layer (they are the last to settle out of the silica slurry) that under vacuum or pressure form a blockage in the column. (repeatedly) Attempts to remove the fine particles from the grinded up silica gel by exposing them to wind or a blast of air doesn't seem to do it.

Any ideas how to solve this problem? One thought is to get XX mesh screens and run the grinded silica through it to get uniform particle sizes, but I have no idea where I would find the right size mesh screens, especially locally. Thought?

A better solution would be to find some affordable source of silica gel already the right particle size, but where. The cheapest lab grade stuff I can find is like $150/kg (Yikes)

I think the issue with bubbles is that you are not packing the column properly(it will be hard with non-uniform media!). For flash chromatography I use a very fine mesh silica that is like flour, and most of the time I pull fractions with a vacuum. A pad of celite may help flow continue if you have a superfine layer clogging but the problem with bubbles just sounds like improper packing... Especially if your solvent is not moving but you have gaps in the column itself if this is how I understand it. The silica I am using is so light and fine I can hardly imagine a similar/smaller particle size being produced by grinding up litter so I find it hard to think it is a true clogging issue.
Of course you have no idea the mesh size and you should buy some mesh screens online, since you probably cannot find the correct ones OTC. Make sure there is not some additive on your silica that is a binder or anything like that.

Celite/diatomaceous earth does not work as media for chromatography... maybe TLC in some instances but definitely not a column, I tried a few times because I thought it may work even though I read it cannot be used. Well, it cannot, everything passes through.

150 bucks a kilo of silica is not a bad price BTW.

how many micron sieve.. like a bubble bag set for hash..   220,190,160,120,90,73,45,25 microns. think you can buy them separately. most grow shops sell em, or will order them.
usual size is 40-60 microns for columns?

a real cheap ass sieve alternative might be tights

Thanks b6badddawg, I'll look into that. I had no idea 

@ Bluebottle - you mean pump a liquid through? The level of the eluent is already a few inches above the top of the silica and it is under pressure, but doesn't percollate.

no probs, the downside might be the price. couple hundred bucks for a set.

have a look on ebay, them aquarium screen sieves 10-15 bucks each used for skimming plankton - shrimp- fish eggs might be another cheaper alternative if you can find a suitable size.
just type in 'micron sieve'
they look maybe better than the bubble hash bags, if you can get the right sizes

Well I managed to get a set of sieves from the local pottery house  Look out column!

@salat - I've triied using the silica sand that is available through pottery stores (assume that is what you are referring to) but it isn't very retentive, everything washes right through with little or no seperation. Using it for TLC everything had an rf of 0.9. I'm using silica gel from kitty litter run through a blender, which works great for TLC and will soon work for column I hope

silica from pottery supply stores is iirc fumed silica and has a very small surface area compared to proper chromatography grade silica. It may still work for separations, but I doubt it's efficacy and uniform nature(important factors for good resolution in flash chrom. I thought?). I fell into this idea some time ago myself and on researching gave up on it. Fumed sillica doesn't have the same silanols and surface chemistry as activated sillica does(unless I'm having another false memory). E-mailed some small producers/suppliers and did not hear back from them.

I did happen upon a kg or so of column chrom. grade eventually but it took some luck. Costed a good chunk of change but as long as you aren't using anything that will bind irreversibly to the surface, it can be recycled and reactivated IIRC. Not certain if flash mesh/activation is any different.

Anyhow I wish you luck! How are you applying the pressure by the way? Does there need to be a laminar flow?

edit - Not sure if this will help you, but I have heard of chemists using a vacuum filtering flasks an doing a sort of 'LPLC' with standard chromatography columns/burettes. It's still high-pressure atleast juxtaposed to the low pressure at the bottom of the column, and I hear the columns run quickly .