We are working on the separation of acetaminophen, pseudoephedrine, dextromethorphan and benzoic acid. APAP elutes with the solvent front using a C8 or C18 column. We switched to CN column and APAP was slightly retained. To get the spacing we desired among the other compounds we incorporated ion-pairing reagents (hexyl, heptyl and octyl sulfonates). However, the amines were broad and tailed extremely bad with increasing chain length and concentration compared to not using any ion-paring reagent at all. Any suggestions on why this is so.

I could do some hand-waving about the IP reagent covering the column surface and decreasing its loading capacity, but my $0.02 worth would be that if you're going to have to use an IP reagent anyway, you might as well start over with a C8 or C18 column. The longer chain columns are typically longer-lived anyway, and they are a much more common choice for IP work.

Anybody got any direct experience with this kind of problem?

-- Tom Jupille / LC Resources

Have had a lot of luck using ion-pairing agents for cough and cold analysis. Use a C8 column and 0.1% octane sulfonate. Current method is 50:40:10 20mM Potassium phosphate pH 3: Methanol: ACN with 0.1% OSA in each bottle on a 150x4.6mm C8. ALternatively, can pre-mix all three into one bottle and then add 0.1% OSA. Using longer-chain or shorter chain IP reagents (hex, SDS) will affect retention of basic components. Modifying conc of IP will also modify retention of bases.

Sounds good. Just out of curiosity, have you had a chance to look at what happens if you change the ratio of MeOH to ACN? (In other words, is there something special about that 50:40 ratio?)

-- Tom Jupille