Chromatography Forum

Hello,

I am working on the separation of some epimers that carry through a series of synthetic steps. At many of the steps I have adequate separation using just a C18 column. I have not be so fortunate at a few key steps however. I would like to avoid going to chiral columns due to cost. Has anyone ever used a chiral guard column in series with a C18 RP column to improve separation of epimers?

Thanks!

Ok, additonal question...even if you have not tried something like this before...do you think there is any validity in trying it?

Do you feel lucky?.

Most chiral columns separate best in their normal phase, polar organic, or polar ionic modes, and not in their reversed phase mode.

How many columns are going to provide an improved separation with the same mobile phase that works for RP?.

If you are lucky you might find some that perform your separation, but most will not. I personally would not bother even trying, but if you have time....

Bruce Hamilton

As Bruce implied, do not waste your time. Just makes no sense. For one thing chiral columns are generally terrible unless you can place a large amount of the stationary phase in contact with the sample (relatively speaking. Of course you must first identify the correct/best CHIRAL column to begin with...). IMHO short chiral columns and/or chiral guard columns are just another way to give a column manufacturer "money for nothing" (can you hear the song reference in your head?)

Now, if memory serves correctly, epimers are diastereomers, which are NOT optical antipodes. That´s why there was separation on the C-18 column. It doesn´t make sense to use a chiral column or precolumn to separate diastereomers.

... It doesn´t make sense to use a chiral column or precolumn to separate diastereomers.

The original poster said that C18 column was not performing the separation, hence they need to try something. I would not bother - as there are easier options that I'd try first, but I also would not say that it's senseless.

There is a reasonable amount of literature out there about using chiral columns ( such as the macrocyclic glycopeptides), alone or with achiral columns, for non-chiral applications - especially for polar ionic molecules that are difficult to separate using conventional reverse phase columns and phases.

Chiral columns tend to be less robust than some reversed phase columns, but can use mobile phases ( eg >99% MeOH ), that may be more friendly to post-separation operations and detection.

Please keep having fun,

Bruce Hamilton

Instead of adding a chiral precolumn, why don't you try using a column with a selectivity that's different from that of a C18 column? Something like a graphite column, for example.

Depending of the molecular structure, you can used equivalent than C18 for chiral separation. This column is cholester columns which improve reversed phase interaction for chiral separation. And is less expensive than chiral phases. I had lot of success with this column.

Charles

So what is the sense here? Buy an expensive touchy "chiral" column after a C-18 couldn´t be made to work well?
I am well aware of cyclodextrine columns being used for "achiral" work, because of their chelating properties. Just think it doesn´t make much sense to jump to a chiral column, just because potentially optically active substances are involved.
How many people use "chiral" columns when they separate protein digests (amino acid analysis)?
There is nothing wrong with using a "chiral" column for "achiral" work, I just don´t see too much sense for me to suggest such a column when there is no separation of enantiomers (optical antipodes) involved.

It is possible to separate epimers on a regular column. Epimers are indeed diaastereomers. You might need to screen few columns and see for yourself. If your compound is ionizable then you can try mixed-mode columns which allow you to use small difference in properties (ionic and RP, ionic and HILIC) to separate compounds.
Here is separation of diastereomers in pure HILIC (alpha and beta-lactose):
http://www.sielc.com/application_175.html
and in reversed-phased mixed-mode:
http://www.sielc.com/compound_162.html (quinine and quinidine)

Also few examples of separation of positional isomers:
http://www.sielc.com/compound_215.html (toluidines)
http://www.sielc.com/compound_217.html (aminobenzoic acids)
http://www.sielc.com/compound_186.html (aminobiphenyls)
http://www.sielc.com/application_060.html (aminobutyric acids)

Any evidence that a cholesterol column will be good for this analysis, or is this just an advertisement?

Any evidence that a cholesterol column will be good for this analysis, ....

Probably not, but I like the idea of wandering around the laboratory muttering " my cholesterol isn't working today ".

Please keep having fun,

Bruce Hamilton

And I am spending my days fighting the cholesterol... Down, down, down you must go, you bastard...

I am usually winning...

Depending on the chemical structure of the epimeres I would also consider Phenyl, CN or Amino-type columns. Anyway, I don't think that simply using a chiral column guard would solve your problem.

EPIMERS ARE DIASTEREOMERS SO THERE IS NO NEED TO WASTE TIME AND MONEY ON CHIRAL COLUMNS THEY CAN BE EASILY SEPARATED USING C18 COLUMNS...I WORKED ON MANY METHODS IN WHICH WE CONVERT ENATIOMERS TO DIASTEREOMERS AND SEPARATE THEM EFFECTIVELY USING C18 COLUMNS.