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jlawrence
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[*] posted on 3-9-2011 at 21:46
dry vacuum column chromatography


Hello,

Has anyone here tried dry vacuum column chromatography ?(especially if you are in the US). Can you share any info about the catalog no of Silica that you use, and how do you seal the frit with silica after washing it with MeOH, EtOAc and hexane ? The paper mentioned parafilm and suba seal rubber, but I doubted how it will work, especially parafilm..
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drago57
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[*] posted on 3-9-2011 at 23:25


Quote: Originally posted by jlawrence  
Hello,

Has anyone here tried dry vacuum column chromatography ?(especially if you are in the US).


Haven't actually tried it to completely resolve mixtures, but have used it quite a bit to simplify mixtures before recrystallising.

Quote: Originally posted by jlawrence  

Can you share any info about the catalog no of Silica that you use, and how do you seal the frit with silica after washing it with MeOH, EtOAc and hexane ?


I just use the same silica I'd use for a normal column (can't remember the details of the top of my head and I'm nowhere near my lab to check atm, sorry). I use a fritted funnel to hold the silica so no need to seal it. I do tend to put a piece of filter paper between the frit and the silica to stop small particles blocking up the frit though. Not sure if this answers your question though.

Quote: Originally posted by jlawrence  

The paper mentioned parafilm and suba seal rubber, but I doubted how it will work, especially parafilm..


Can you give the reference for that paper? Parafilm does tend to dissolve in a lot of organic solvents. I use PTFE tape (same stuff plumbers use to seal pipe joints) where solvent contact is likely, costs a couple of £ a roll from any hardware store.
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smuv
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[*] posted on 4-9-2011 at 10:14


use it all the time; use the same silica you use for a column; I don't understand what you are saying about sealing the frit. All you need is a fritted glass funnel, silica and a vacuum flask. Fill the funnel with the amount of silica you will use, then add the solvent/mixture you will be eluting with to moisten the silica. Stir wet/solvent ab/adsorbed silica around with a spatula and allow it to settle while you are pulling a vacuum/solvent through the funnel, to give you a nice flat bed of silica. Load your compound onto the silica using the least polar solvent system that will dissolve your compound. At this point you can cap with sand if you are anal and trying a real hard separation (probably should just run a column) or you can just go ahead and elute your compound.

Also...no one calls it Dry vacuum column chromatography, even in literature. People just say filtered through silica, and in lab people just say, I ran a plug. I can't believe a guy got a paper for this, people have been doing this for so long. It is not a spectacular purification technique, it is simply more convenient than running a column for easy separations.





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jlawrence
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[*] posted on 4-9-2011 at 11:59


drago57, smuv>
Interesting, so regular silica works ? is using flash silica going to make it better ?

I am guessing that this is a little bit different than running a regular plug, but I might be wrong. I posted the link. The guy emphasized the importance of really pressing the silica to pack it tight and use gradient eluent.

The sealing was described in the link as well.

http://curlyarrow.blogspot.com/2006/10/dry-column-vacuum-chr...
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chemrox
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[*] posted on 4-9-2011 at 18:30


@smuv what size sample vs what thickness silica has worked well for you? I ask because the last two times I tried it I ended up recovering everything and recrystallizing a few times instead.



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[*] posted on 4-9-2011 at 19:33


@smuv - The technique as described in the paper actually varies from a simple plug, and I've tried it a couple of times with success on larger amounts of material than I'd comfortably run down a column.

There's two key points. First, use the finer TLC grade silica (I can't remember the diameter, but it's far smaller than the 43-60 um silica commonly used for columns). This allows for better resolution - finer particle size = more even packing = better separation. Second, best results are definitely achieved using a gradient system and collecting fractions. And the advantage is you can easily control the gradient, without having to stop your column, open it up, add solvent, run that through, repeat... you just pour the new stuff in the top!

I've run it in a 4 cm fritted funnel, with a rubber stopper on the B14 joint at the bottom. Attach the vacuum to your funnel, then push a test tube onto the stopper so it seals in place (I wouldn't, however, recommend just letting it hang when you add solvent - hold onto the tube in case it drops!). Add solvent, allow it to elute, change tubes, add the next batch of solvent... easy. I just add one fraction worth of solvent at a time (50 mL - the volume my tubes could comfortably hold) and allow it to fully elute before switching tubes. Also, a layer of sand topped with a piece of filter paper protects the surface of the column really well, so you can just pour the solvent in without disturbing the top of the silica (for a simple plug, the filter paper alone works excellently).

One other point of note - the method isn't great for low boiling solvents like DCM or Et2O. They evaporate too readily, which can be an issue.
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smuv
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[*] posted on 4-9-2011 at 20:28


@ziqquratu if you ran that seperation in a column of the same diamater as your frit funnel, with the same amount of silica, and eluted at the same rate, you would have gotten the same separation

The only practical difference betwen a plug and a column is the shape of the stationary phase. Columns are generally long and skinny as opposed to plugs which are short and fat (this simply stems from the shape of a filter funnel vs. a column). Therefore plugs sacrifice total number of theoretical plates for material throughput. On the other hand the the column has a much longer sationary phase, with a greater number of theoretical plates; the problem is when you load a lot of material onto a column, due to the small diamater of the stationary phase it starts "choaking" (for lack of a better/more technical word). To make an analogy to distillation a plug is equivalent to a 50mm wide x 200 mm tall fractionating column while column chromatography would be like a 25mm wide 800mm tall column; the taller skinnier column 10's of times more theoretical plates despite the same volume, but is easy to flood at high take off rates.

This is not to say I don't use plugs for easy large scale separations, I just can't believe someone got a JACS paper (or whatever it was) from reporting what is essentially a column.

@chemrox, if you can give me the Rfs or just tell me the compounds you are separating I can give you a hand. But it is case by case. Generally, unless the separation is very easy, you need to use a gradient solvent system. In this case, run some TLCs and find a solvent system where one spot pretty much does not move, and the other spot is reasonably mobile in; then find another system where the second spot is reasonably mobile but any other trash does not reasonably move in. Then run the plug with that gradient (really I never actually do a "gradient", once the first compound comes off I just switch to the next solvent system).




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[*] posted on 5-9-2011 at 04:50


Smuv, you're missing the selling points of DCVC - it looks alot faster to run than a column (even compared to flash) and gradient elution is tres facile. Add very cheap and easily assembled apparatus (how much would it cost a glassblower to make you a column 4cm wide!) and you're beginning to see the advantages. The author claims that it gives excellent separation and hasn't run a regular column in years - on the few occasions it failed he went straight to HPLC. The capacity to deal with large product quantities with good resolution and quick run time make DCVC ideal.

on a separate note, I wonder if colloidal silica from somewhere like Inoxia could be used for this type of chromatography - its particle size is just the right region for what is claimed to be essential to good results. Its just the pore size that I'm worried about...
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[*] posted on 5-9-2011 at 08:19


Sounds as if the linked article plus a paper it references, Dry Column Vacuum Chromatography, D.S. Pedersen and C. Rosenbohm, Synthesis, 2001, pp. 2431-2434, give good instructions - maybe worth someone grabbing and attaching that paper here.
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[*] posted on 5-9-2011 at 09:18


To me it looks like a technique for selective desorption by step-wise change in solvent elution power. Not really comparable to a simple filtration trough silicagel where all you do is selectively absorb the most polar crap. It looks more similar to flash chromatography, except that you flush the column dry in between fractions and make a step-wise gradient on the eluent. I must try it out one day, though on second thought maybe no, given we have one of those automatic flash chromatography machines at the job. It must have been more than a year since I did my last classical flash column.
Quote: Originally posted by smuv  
This is not to say I don't use plugs for easy large scale separations, I just can't believe someone got a JACS paper (or whatever it was) from reporting what is essentially a column.

It was Synthesis, but I don't see why an article on a lab technique would not be a worthy publication. We are too used to the lab techniques being spread via person to person communication or via Vogel's and similar manuals. Just consider that the JOC article introducing flash chromatography to the masses (DOI:10.1021/jo00408a041) is now one of the most cited papers in chemistry (7905 citations up to this day according to WoS!). For years almost everyone who used this now trivial technique cited that paper. Pedersen's article got 108 citations in these 10 years, which is excellent for these times when people tend to cite only people who cite them. In this regard, it appears that a chromatographic separation technique called "Vacuum Dry Column Chromatography" was first described in 1965 already and is described in detail in a JOC paper from 1982 (DOI: 10.1021/jo00144a042, attached).

PS: Currently I do not have access to Synthesis 2001 from home, but someone else will fetch that.


Attachment: Vacuum Dry Column Chromatography_JOC.pdf (444kB)
This file has been downloaded 766 times





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jlawrence
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[*] posted on 5-9-2011 at 09:52


The VDCC paper describes something that looks more like prep TLC to me. In my subjective opinion, although theoretically it is pretty nice (similar to TLC), it is not practical to setup nylon tubings with sep funnel + making holes to sample and cutting them. Imagine the hazards associated with it.

Now what I am not sure is what if you have a lot of polar junks in the mixture, maybe you need to run a plug first and then use this column perhaps, or maybe we can just scrape off the first 5 mm of Si layer and add a new one if necessary ?

I am interested in knowing more from someone here who uses this technique actively.
Ziqquratu, I have a similar idea using regular frit funnel + vacuum adapter+scintillation vials, that's why I am excited about it. What I hate sometimes about flash chro is the solvent addition+occasional drippings+spills and I cannot run TLC in peace, especially for new compound (I am not gifted with multitasking)

In the spirit of sharing, here is the pdf of DCVC (...ah acronyms).


Attachment: Synthesis-Stuttgart 2001 Pedersen.pdf (180kB)
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smuv
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[*] posted on 5-9-2011 at 11:29


Quote:
It must have been more than a year since I did my last classical flash column.


Now you are just bragging nicodem :P

...ugh...back to my column...


Edit: btw 108 citations according to WoS

[Edited on 9-5-2011 by smuv]




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[*] posted on 6-10-2011 at 12:31


It's a bit of a two sided story for dry vac chromatography.
I've used it and it can work well. Not recommended in warmer
climates where air humidity in the lab is an issue and with
solvents such as toluene. I have colleagues that swear by it.
I more often swear at it. I may be a bit old school but a regular
column works fine for me. Or go reverse phase.
No big issues just big dollars.




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stoichiometric_steve
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[*] posted on 15-10-2011 at 11:44


Quote: Originally posted by SmashGlass  
Not recommended in warmer
climates where air humidity in the lab is an issue and with
solvents such as toluene.


Care to elaborate?




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[*] posted on 17-10-2011 at 12:33


I've ha a bad experience once when toluene was used in a dry
vac setup and a very humid day. The air moisture was introduced
(sucked) onto the column and started to mix with the next fraction
of toluene. By the end the silica had started to come through with
the now water/toluene mix. It doesn't happen often in well
equipped labs, but with poor ventilation and or very humid
conditions this could happen.

I hope this explains my previous statement a little more.




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