Help for chiral separation of a 2,3-dihydroxy-butyl ester.

[bluer]

Hi, everyone:
I have the enantiomers of Benzoic acid 2,3-dihydroxy-butyl ester to be separated by chiral columns. I have ChiraDex (Merck), ChiroBiotic T (Astec), and ES-Pepsin (Agilent) columns in my hands. I have tried many moblie phases, such as the mixtures of H2O/MeOH, H2O/acetonitrile or H2O/EtOH, and many additives, such as isopropanol, ethylene glycol and diethylene glycol. And I also tried many runs at different temperature and different PH value.
But I still can not separate the two enantiomers by those three chiral columns.
Could you please give me some advices?
Do I have to order another kind of chiral column for the enantiomers?

Thanks a lot!

[tom jupille]

Chiral separations differ from other analyses in that most of the selectivity resides in the stationary phase, with the mobile phase having only a minor effect (the antibiotic and protein columns you have already tried probably allow the greatest degree of control via the mobile phase).

If this were my problem, I would contact the major vendors and ask for their recommendations. Three sources that come to mind are:
- Astec (for cyclodextrin or antibiotic columns)
- Daicel / Chiral Technologies (for derivatized cellulose columns)
- Regis Technologies (for Pirkle columns)

You can find their web sites in the FAQ (drill down to the "where to obtain additional information" section).

[bluer]

Thanks for your help!
I also think I need to try other chiral columns.

[HW Mueller]

Not having done any chiral separations myself, I wonder wether there should not be a relatively narrow range of effective retention.
So, bluer,
did your mobile phase span a very wide range of retention or were all of them near the dead time, or all with very high retention?

[bluer]

Those three chiral columns I have tried are all reverse phase columns. I just use the mixtures of water and alcohols as mobile phases. The retention time will be from about 5 to 20 minutes for my samples. But none of those elute conditions can get separated peaks in chromatograms for the enantiomers.

[tom jupille]

Hans, your point is a good one. In one sense, chiral separations are no different from any other LC separations. The dominant retention mechanism is (usually) either normal- or reversed-phase, and retention is controlled in the usual manner. The enantioselectivity is a "secondary" effect that allows one enantiomer to interact more strongly with the stationary phase than the other.

The retention requirement is the same as for anything else: you want k' values between about 2 and 10.

[HW Mueller]

Which probably means that bluer might get a book to see wether there are some more ideas on this, and if he can´t get his columns to work get other ones or try derivatization (making diastereomers)?