[zhangh]

I am currently running column chromatography to isolate a mixture of 4 component, my desired product run very close to another component, Rf value difference is about 0.06 using PE:EA = 3:1,

how to run column chromatography to isolate them efficiently? By the way,  I have no flash chromatograph in hand.  Thanks.

Please share your tips and tricks about running chromatography!

[russellm72]

What do you mean you have no flash chromatograhy in hand?

Are you running a column with positive pressure (bellows) or negative pressure e.g using a water pump pulling the solvent through a sinter type assembly?

A bit more info would help. Also please run loads of different TLC systems of your mixture. You'll be amazed of the difference using different solvent combinations. e.g if you can get your compound to run at about Rf 0.3 or even higher leaving a few spots on the baseline a quick silica plug will help your cause extremly and of course lower the loading for your next column.

A useful book is Advanced Practical Organic Chemistry by J. Leonard, B. Lygo and G. Proctor which you might find useful.

R.

[chiralic]

Hello zhangh:

1st Question:
How do you know that you don't have Flash Chromatography in hand?

As said russellm, please give us more information about mix, p.e. polarity, what kind of compound you
are separating....

Regards,

Chiralic

[DavidW]

One nice way to run chromatography with compounds that are very close to each other is to do a "gradient".  In your case, you said that PE:EA (3:1) gave you a very small separation on TLC.  So instead of eluding your column with 3:1 PE:EA, start running your column with 50:1 PE:EA.  Then gradually increase the polarity of your solvent: 40:1, 30:1, 20:1, etc....  Yes you will be collecting a lot of blank fractions, and your compound may also be coming out over a larger number of fractions, but this gradient method is very effective at separating hard-to-separate mixtures.  I've used it to successfully separate two spots that have no separation on TLC at all.

Good luck! 

[nottingham]

One nice way to run chromatography with compounds that are very close to each other is to do a "gradient".  In your case, you said that PE:EA (3:1) gave you a very small separation on TLC.  So instead of eluding your column with 3:1 PE:EA, start running your column with 50:1 PE:EA.  Then gradually increase the polarity of your solvent: 40:1, 30:1, 20:1, etc....  Yes you will be collecting a lot of blank fractions, and your compound may also be coming out over a larger number of fractions, but this gradient method is very effective at separating hard-to-separate mixtures.  I've used it to successfully separate two spots that have no separation on TLC at all.

Good luck! 

I strongly agree with what you said. Also just like the guy said try different solvent system would be helpful.

[mir]

A useful book is Advanced Practical Organic Chemistry by J. Leonard, B. Lygo and G. Proctor which you might find useful.

R.

Thanks for the book reference, I ordered it from the library :-)

I find flash chromatography a bit stressful, and I wish I had ten arms and 8 eyes each time I do it. How can you run TLC, watch so the column dont ran dry, elute fractions at the same time? And also change the eluent, as one proposed? Mye tutor say the eluent flow have to run in the speed of 2.5 cm/30 s. A bit fast, dont you think?

[DavidW]

.

I find flash chromatography a bit stressful, and I wish I had ten arms and 8 eyes each time I do it. How can you run TLC, watch so the column dont ran dry, elute fractions at the same time? And also change the eluent, as one proposed? Mye tutor say the eluent flow have to run in the speed of 2.5 cm/30 s. A bit fast, dont you think?

You certainly don't need to be running TLCs the same time you column is running.    What I sometimes do is, after I elute for awhile, I would stop the air flow so the column won't run dry, and run a TLC or two.  When the TLC's are done, I would restart the air flow and continue eluting.  I've run so many columns that I am able to manage takiing TLC's during eluting now, but it takes some practice   

2.5 cm/30s sounds okay, not too fast.  But it's not an absolute number that you can use for all situations.  It really depends on your separation and/or your method of elution (e.g. single solvent system or the gradient method I suggested above)

[mir]

On the wall on my lab, we have a instruction (table) from an very famous article about flash.
I am following it very close. Well I just try it anyway... I guess I have to run some columns more before I got the hang of it 

[Yogi]

You could take a look at the Stille paper on flash. 

Ref: Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-2925.

It's a good place to start.  Then the use of gradient is probably the most appropriate.

[firassh2000]

u should change your solvent mixture to get a good seperation at the TLC paper then you try to seperate them, and u can use HPLC technich