Chromatography Forum

Hi,

Short question: Will I get retention of a zwitter-ionic compound on ion-exchange when I am using a pH where the molecule has both a positive and a negative charge (net charge = zero)?

Depends on how far apart the charged centers are. If they're on adjacent carbon atoms, cf. an amino acid, then probably not unless you operate at an extreme of pH where one of the two functional groups is uncharged while the other remains charged (as does the stationary phase at that extreme pH). If the charged centers are attached to carbon atoms with one or more carbon atoms between them, then you can get retention if the analyte can assume a well-defined orientation with one charged group (the one with the opposite charge from that of the stationary phase) facing the stationary phase and the other located more distantly from the stationary phase. My paper on the subject describes this in detail, cf. the following link: pubs.acs.org/doi/pdf/10.1021/ac100651k

If you want to discuss your application further, then please provide some detail about both the analyte and the column that you propose to use.

if your zwitter-ion has carboxylic acid and amine, and if stationary phase has sulfate, and if your pH is between 3 and 6, you will have some retention (so many ifs) You will have much longer retention if you have one of the groups in non-ionized state, which is easier to do with carboxylate at lower pH. Here is application which shows difference in retention for amino acids and basic compounds at two different pH (1.8 vs. 2.9):
http://www.sielc.com/Application-HPLC-S ... thway.html

Primesep 200 has pKa of 2, on the column with stronger acid (sulfate) you will have longer retention of zwitter-ions (all this is assuming that you acidic fragment is not sulfate or phosphate)

Sorry for the huge picture...

I have been asking about this compound before. It is present in a mix with natural flavour from oranges and the chromatogram is full of peaks. I would need a method where I only see this peak (preferably) or at least can reduce the interferences from the flavour.

My idea was to use the sulfate group for this, since it is negatively charged down to pH 0 (or something like that). Not many of the flavour peaks are likely to show the same behaviour. But at low pH the nitrogen in the pyridine ring will be positively charged. But maybe anion exchange would work anyhow? The positive charge should be repelled by the positive column surface?

Or if you have any other ideas than anion-exchange to clean this up, I am equally thankful

Mattias,

Pyridine pKa is about 5.25, so if you use anion-exchange column and mobile phase with pH above 5.5, your pyridine should be not ionized but your acid will be fully ionized. Alternatively you can try to use Primesep B2 mixed-mode anion-exchange column with higher organic (50%) and buffer. Higher organic will help you flash all other compounds and anion-exchange will help you to retain your target.

email me if you have questions.