chemrox
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flash chromatography column
I want to start using preparative flash chromatography with amounts of product in the 5-20 gram range. I have a couple of columns that could be
adapted to this use. I'm asking for recommendations. I plan to have a 24/40 joint put on the top of one of the comuns I have. One columns I could
adapt is about 30 mm diameter, 300 mm length and has a nice reservoir of about 500 ml. This one has a nice frit and stopcock built in. I would
simply have a joint added the top. I have a good gas adapter with a metering screw on a 24/40 male joint. Does this column seem like an appropriate
vessel? Any other ideas?
Thanks for giving this throught and attention.
CRX
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Klute
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5-20 g is already a large amount of column chromatography  With that 30x300, i
wouldn't do any more than a gram of product, depending on what it actually is. Otherwise you will have no seperation at all, part of it will only
start eluting when your first fraction is complete, especilyy with somewhat apolar eluants, alot of compounds oil out if present in a too large
quantity..
I would start by mastering traditional column chromatography, and then start applying argon/N2 pressure to speed things up.
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Swany
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For a 30mm diameter column, I wouldn't load on more than 1-2g of product, as Klute suggested. As he also mentioned, it does depend on how apolar your
product is. I have had troubles chromatographing apolar oils, they tend to self-elute. You need to sufficiently dilute them, but to do that you also
need to have a long enough column with enough volume to load it on in a coherent band. How complicated are you expecting your separations to be? That
is also a consideration. Again, traditional column chromatography will give better separation as well.
Your technical setup sounds good, it just sounds like something for a 1-2g (at most, probably overloaded). So, it is what it is. Can you give more
details on what sorts of columns you are expecting to run? Are you using silica as your medium?
Column and flash chromatography have always worked well for me, though UV sensitive TLC plates are just as valuable (if your compounds are good
chromophores). I've done flash and column chromotography on everything from thiophene derivatives to charged Ru-polypyridyl complexes. The UV lamp and
TLC plates really were helpful.
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chemrox
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Thanks. @Swany: evrything you're telling me I've read too except for the statement that regular column chromatography will give better separations.
I've read pretty much the opposite. http://www.saiadsorbents.com/flash.htm, http://siggy.chem.ucla.edu/voh/136/Flash_Chromatography.pdf. I was planning on using 60-70 mesh silica gel and/or same coarseness alumina gel
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Nicodem
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I agree that for 30 mm diameter column you will unlikely get a separation (or at least one without overlappings) for anything more than 1 g. However
if you post a TLC you obtained using the optimum separation mobile phase then maybe we could suggest you the maximum loading to use. Mind that you
need such a mobile phase that will keep your target compound bellow the 1/2 TLC height but above 1/4 height. Depending on how the TLC develops, you
could actually load up to 2 to 2.5 g on your column, particularly if you use gradient elution (but the feasibility of this depends on how the TLC
looks like). Also, if you are trying to separate diastereoisomers, you can forget about such high loads. Even if you manage to get a separation of
your diastereomers on TLC, you will unlikely obtain a separation without overlapings on a column, unless perhaps if you load it less than 0.5 g.
Edit: There are cases where you can load a 30 mm column with up to 5 g and get a separation. However, that is not really column chromatography any
more. For example, if your target travels really fast on TLC, like above 1/2 height while all the impurities stay nearly immobile (about bellow 1/8),
you can elute your product with petroleum ether or petroleum ether / ethyl acetate (8:1 or more). I call this flash elution rather than flash
chromatography.
[Edited on 7/12/2008 by Nicodem]
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stoichiometric_steve
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If you want to separate such large amounts, check out the excellent papers in the Rhodium archive on "DCVC = Dry Column Vaccuum Chromatography". It
uses slightly more expensive Silica Gel.
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chemrox
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@Nicodem: single or two dimensional plate workups?
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Swany
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If the compound you are trying to separate isn't overly soluble, it will tend to streak, this is worsened with flash chromatography. In cases like
this, running a column without pressure gave much better separation. If you are running a plug (you have impurities that stick to the top and your
product flushes out cleanly, easily), flash is obviously the way to go. If the products are readily soluble, bands can diffuse over time. Then flash
is good, however if the product is less soluble, I have only had trouble with flash. There is a good chance I am doing something wrongly, so take my
comments with a grain of salt, but they are my experience.
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grind
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In the chemical literature there is a tendency to chromatographe nearly all products. In many cases traditional purification methods (distillation,
crystallization) would be satisfactory, but are not tried.
Of course - chromatography works fine and is convenient, but on the other side very expensive and time-consuming. For hobby chemists purposes
chromatography is the last resort, if all other methods fail.
In this context - has anyone ever tried to recycle used silicagel? I imagine washing with methanol removes all remaining impurities (except for
special cases) and after drying the silicagel is reusable. Any comments or ideas?
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Swany
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I believe methanol will deactivate the gel to a point, as will letting it stick in the air. After washing it with methanol you may need to roast it
for a bit to remove the leftover ethanol or reactivate it. I have re-loaded the same column when running plugs.
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Klute
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Yes, I re-use clean silica gel a few times at home (except when I purifying a delicate/expensive compound), and I generally wash it with methanol,
then acetone, dry it up slightly (either on a hotplate or under vacuum), and repack a column. No problems up to now, and considering the price of the
silica, i think it's really worth it.
I always throw out the first cm or two though, as it's not worth the several MeOH/AcH washes to removes all the tar/deposit.
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grind
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Good to know!
What do you think is an ideal drying temperature for the silica gel? 100°C at 0,1 mm Hg for 1 hour?
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Klute
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I'm sure that will be more than enough, I usually left it at 80°C under 25torr for a few hours, or on the hotplate (>100°C) over night.
I wouldn't use such silica for delicate seperations, or products needing to be really pur, but for most intermediates where one just wants to remove
unreacted starting material etc before continuing another transfromation, I have found this to be very convienient.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
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