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separation of two isomers
Posted: Tue May 12, 2009 2:03 pm
by mueller
Can anyone of you guys help me out?
I have the ester of a carbonic acid and I'm not able to separate it from another isomer (with LC/MS)
I tested a lot of more or less "normal" silica C18 columns. I use MeOH and Water because they are the best.
I need a straight tip for a good column, because I guess that is the key for my problem - any suggestions?
Thanks
Posted: Tue May 12, 2009 2:26 pm
by Uwe Neue
Please describe you LC method first, and why you prefer MeOH over MeCN. It could for example be that your gradient is too steep...
Posted: Tue May 12, 2009 2:42 pm
by mueller
the problem is, that I couldn´t generate a good ion (fragment) with Acetonitrile/ water - with MEOH/ water the intensity is much (!) greater. I also tried it with MeCn, intensity was poor and no separation could become obvious (same result with MeOH)
Method: Doesn´t matter if gradient or isocratic flow - the eluation time of the two isomers is more or less always the same.
Posted: Tue May 12, 2009 3:55 pm
by tom jupille
The word "isomers" encompasses a lot of things. I've argued (while being too lazy to dig up proof
) that bonded-phase columns inherently give up selectivity for isomers compared to adsorbents because the bonded phase is "squishy" (how's that for a technical term?) and can to a certain extent deform to accommodate different analyte geometries, whereas an adsorbent surface is "rigid".
That argument suggests something like porous graphite: you can use it with MeOH water, it provides reversed-phase retention behavior, and as a rigid surface it has been shown to provide unique isomer selectivity (most pronounced for analytes that can interact with pi-electrons). Another possibility is to move to a shorter-chain bonded phase column (less squishy, proportionally more silanol accessibility).
Posted: Tue May 12, 2009 4:47 pm
by Uwe Neue
In the absense of good information...
Try a phenyl column or a column with an embedded polar group (XBridge Phenyl, XBridge RP18).
Also, significant (!!!) differences in ligand density can influence the selectivity of closely related analytes. Try Atlantis T3 with less than half the ligand density of a standard C18.
Posted: Tue May 12, 2009 4:54 pm
by bhuvfe
Posted: Wed May 13, 2009 9:34 am
by Alex Buske
What kind of isomers? If the carbonic acids have different chain lengths, try a C8.
Alex
Posted: Wed May 13, 2009 11:27 am
by mueller
Thanks for all your advices:
Well, I have two constitutional isomers - the only difference is -CH2-CH2- and -CHCH3- .
Waters xBridge didn´t help. C8 looks better, but a real separation couldn´t be observed.
Has anybody of you experience with THF as MP or will cyclodextrine help???
Other advises are welcome too...
Posted: Wed May 13, 2009 12:43 pm
by Uwe Neue
I had suggested XBridge RP18, which is the packing with the embedded polar group, not just an XBridge packing.
To go from one random C18 to another one, or from a C18 to a C8 gives the LOWEST amount of selectivity change, no matter what the problem is. The only exception specifically for your problem at hand is to change the ligand density drastically.
I can send you a few publications on selectivity of different packings, if you want.
Posted: Wed May 13, 2009 1:51 pm
by Peter Apps
Who wants to take a bet that the mobile phase is so strong that everything is coming out at the dead time
It wouldn't be the first time.
Peter
Posted: Wed May 13, 2009 2:02 pm
by HW Mueller
Branched chain not separable from straight chain? Is this part of a huge substituent or did Peter hit on the answer?
Posted: Fri May 22, 2009 2:29 am
by Bryan Evans
What is the value of k' (or rough estimate)?
Does the molecule have any ionizable functional groups?
Large aromatic substituents? Is peak shape ok?