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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: OrganicDan96 on July 02, 2019, 03:58:42 PM

Title: effects of gradient rate on separation with column chromatography
Post by: OrganicDan96 on July 02, 2019, 03:58:42 PM

I have been trying to separate compounds from a reaction crude using column chromatography. I have successfully separated my desired compound from the mixture but there are two other compounds (one is starting material) that did not separate. I am using a gradient 0-->10% MeOH in DCM using a biotage chromatography system. would separation be improved with a slower gradient?

i have a second question. my compound is an amine and streaks quite a lot i have tried additives such as NEt₃ and methanolic MeOH but these reduce the separation. is there some way to narrow the band? This does not seem to be a big deal on a small scale as its the last thing off the column but it may be problematic when do it on a bigger scale.

Title: Re: effects of gradient rate on separation with column chromatography
Post by: rolnor on July 03, 2019, 06:43:04 AM

I have used TEA/Hexane/EtOAc 1:9:9 for amines, maybe your compound is more hydrophilic then mine? Also TEA/EtOAc 1:20 can do fine.

Title: Re: effects of gradient rate on separation with column chromatography
Post by: OrganicDan96 on July 03, 2019, 07:21:22 AM

I was using 2% TEA 10%MeOH in DCM which looked like it was giving good separation and very narrow bands by TLC but it was hiding the fact that some bands were overlapping and several attempts at the column couldn't replicate what the TLC was showing. It appears to be a very polar compound. I can live with the fact that it streaks a bit as long as it all comes off the column

Title: Re: effects of gradient rate on separation with column chromatography
Post by: Babcock_Hall on July 03, 2019, 10:46:26 AM

Let the buyer beware, it is not always wise to extrapolate from one kind of chromatography to another, and I have no direct experience with your situation.  But for some kinds of chromatography, making the gradient shallower improves resolution.  By shallower I mean reducing the percentage increase of the stronger eluant per mL of the gradient.  I would be interested in hearing what did or did not work.

Title: Re: effects of gradient rate on separation with column chromatography
Post by: rolnor on July 03, 2019, 11:08:47 AM

You should pack the column without TEA and MeOH if you want to replicate TLC-results.

Title: Re: effects of gradient rate on separation with column chromatography
Post by: OrganicDan96 on July 03, 2019, 01:15:04 PM

Quote from: rolnor on July 03, 2019, 11:08:47 AM

You should pack the column without TEA and MeOH if you want to replicate TLC-results.

thanks i think the TLC with TEA was misleading me anyway so i have dropped it.

i was able to separate the bands and identify them using analytical HPLC/MS using a machine that can run prep as well.

Title: Re: effects of gradient rate on separation with column chromatography
Post by: hypervalent_iodine on July 03, 2019, 08:29:02 PM

Is basic alumina a possibility?

Title: Re: effects of gradient rate on separation with column chromatography
Post by: OrganicDan96 on July 04, 2019, 06:41:08 AM

Quote from: hypervalent_iodine on July 03, 2019, 08:29:02 PM

Is basic alumina a possibility?

that's a good idea I had not thought of that, thanks