Drunkguy
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Flash Chromatography
Nobody got anything to say about this?
I am not really interested in HPLC or GC currently.
It is my opinion that these are analytical methods that must come after preparative flash chromatography (FC).
Especially given that FC employs many of the same principles as in the above mentioned techniques; Namely it can be automated with a fraction
collector, and it is possible to hook it up to a UV detector.
I'm surprised not more people are interested in FC, but then again it is specific to organic chemists which slashes the number of potential users, and
secondly it is more common amongst professional chemists of which only a small fraction of whom account for the traffic generated in these forums.
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Sandmeyer
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| Quote: | Originally posted by Drunkguy
Nobody got anything to say about this? |
What can be said about it, it is (IMHO) the most boring part of the organic chemist's work...
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Klute
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I prefer coluum chromatography to washing up  Lots of TLC to do if you haven't
got a detector or other means, so I never have time to get bored 
The most boring part of it is distilling the fractionse when you don't haev a rotovap....
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The_Davster
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I would rather(and always do) try recrystalization with every solvent in the cabinet at work, and then if none work, I use a collumn.
Of course a collumn is preferable to GC and LC if you want a usefull ammount of product.
I tend to collect many many small (~3mL or so) fractions when I run collumns. I use tape to stick as many vials as possible onto a meter stick, and
just slide it when I want to change the fraction. But we don't have fancy equipment like automated ones with fraction collectors.
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Ozone
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I'm with Dav on this one. Unfortunately, it seems, more often than not (in research rather than preestablished, optimized syntheses) I get some nasty
brown oil. TLC indicates that my product is in there, but so are a hundred other things...
So far as it goes, flash really isn't too bad. It's fast, easy to set up and provides some g's of product. Unfortunately, the number of plates is
relatively small (which is improved, somewhat, by the rapid elution rates used in flash), so I usually prefer to spot each fraction on a TLC plate to
rapidly evaluate the separation.
The other down-side is that the amount of solvent used is rather large. A rotavap or Zymark turbovap (my preference for flash fractions which tend to
be just-too-small to be handy with the rotavap) is very handy. The turbovap is especially nice b/c it uses N2 as a sweep gas which protects your
precious fraction from air (unfortunately, it is not equipped, stock, with solvent recovery).
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Nicodem
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Columns are a nightmare, especially when after so much work you get to compounds traveling more or less together contaminating each other. Better use
radial chromatography whenever possible - so much less work, less solvents, you can watch your compounds moving in real time, and reusable plates if
you wash them with MeOH. The only and very serious drawback is that the loading amount is limited to <1g even for the thicker 4mm plates.
Of course, for those lucky bastards having an automated flash with premade cartridges… that's another story. 
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Drunkguy
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http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&...
^^^Only one person bid on it. First of all he listed it for £500 but nobody was interested in it at that price. This time around, he only got a bid a
few hours before closing, so it's hardly a popular item in high demand.
I think they have their marketing strategy a bit wrong. The reason why there is so many slots is not to save the busy organic research team TIME. The
*real* reason why they are there is because there is the SPACE to accomodate them. It's almost like the "making a mountain out of a molehill" kind of
analogy. Behind those heavy steel doors is alot of sophisticated electronic gadgetry, and it is this that constitutes the bulk of the product.
I only have ONE 27mm "plunger" so when I get some ISOLUTE cartridges, I will just attempt a separation several times to get some practice under my
belt.
Obviously, buying a case load of different sized plungers and cartridges is ofcourse DESIRABLE, but that would clock up close to £1000 and is
therefore not especially feasible.
I also bought a cheap Pharmacia "Fraction Collector" which cost me next to nothing [Note: this has the same initials as "flash chromatography" so the
use of the short-hand notation "FC" is ambigious].
I think to start with im going to be using my FlashMaster Personal to test out the ISOLUE brand of disposable cartridges. It will be interesting to
see how this compares to the glass columns they use to teach students the principles of chromatography.
Interestingly, I think an important contrast is that the compound is applied to the BOTTOM of the column and the solvent elutes from the TOP of the
cartridge.
Although I am sure of my "long-range plan" I still will need a super freezer (-85C) before I can do this and some other chemicals etc which will still
take me about 5 years before my lab is sophiticated enough to cope with all the demands that will be placed on doing this project.
In the mean-time, im not to sure what projects I can do to test chromatography out.
I think those cartridges are re-usable. I know a guy who used to use them all the time at work (spicy brown, RIP).
He didnt think they were reusable because the silica is hard like concrete, but if u leave it to soak in water, then its only silica, so it should
come loose. Obviously, when u add some chromatogaphy grade silica the next time it will be loose (not hard), but theres no reason why this cant work.
I think reusing it is more of an issue for really large columns, not the 20g ones. Say for example, if using between 100-200g columns to separate
10-20g's crude product.
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