[nimbus8]

Hey everyone ive been looking into dry chromatography as a safer and cheaper method to separate compounds than flash chromatography.
Here is the setup im currently looking into:


Does anyone have any information as to the differences in separation each one can achieve. I read on several websites that they get about the same degree of compound separation so im curious.

[orgopete]

I don't know whether there is a difference in separation efficiency of this vacuum filtration per amount of silica used. A colleague used to do separations this way with what I thought was poor results. This is what I perceive to be the weakness of this system. I'm doing this without doing any calculations, so I could be wrong on this.

When I've done regular flash or pseudo-flash chromatography, early fractions are contained in relatively small column volumes. Later fractions require larger and larger volumes of eluent unless the eluting power of the eluent is increased. If it is increased, the fractions will again be contained in relatively smaller volumes. Keeping these smaller fractions separate is much easier with regular flash chromoteaphy than with the types of apparatus shown. I'm not saying it cannot succeed, but with the rate of solvent collection that I have seen being used, it is not a surprise a poorer separation might result.

If you want speed, it is fast though. For very simple separations, it may give a better result per man hour.

[nimbus8]

Well with this type of apparatus you use much less eluent, dont quote me on this but when i was reading through how to do vacuum chromatography it seemed they would only put like 15ml(or whatever fraction size) of eluent and then they would suck the silica completely dry. And they seemed to do this for every fraction.
In reference to your friend what type of silica was he using, with this apparatus your supposed to use fine tlc grade silica with absolutely no bonding agents. If he was using regular flash silica that might be where his problem arose.

[orgopete]

Yup, regular flash silica and he used too large of fractions. We were generally purifying multigram samples. For me, 15 mL doesn't describe the fraction size accurately. This could be a small fraction of the column volume or a multiple of it, depending upon the amount of sample being purified and silica used. This does sound more rational for an effective separation, especially if a finer grade of silica is being used to increase the surface area. I'd try it.

[Corribus]

I think it depends on what you are doing.  We used to use this exact setup for purification of the products of porphyrin condensations and it worked great - but only as a first step. Basically to get rid of most of the polymers that were formed as side products. We always followed with a regular silica column to remove some minor impurities that couldn't be separated this way.