Jack, Here are few alternative thoughts:
Your problem is common for analysis of cough and cold compositions. My guess is that right now you have 20-30 minutes gap between brompheniramine and other two compounds. Cough and cold compositions usually contain mixture of hydrophilic neutral (acetaminophen, guaifenesin, guaiacol, etc.), hydrophylic basic (phenylephrine, epinephrine, ephedrine, etc.) and hydrophobic basic compounds (pheniramines, dextromethorphan, doxylamine, pyrilamine). It is not easy to analyze them and elute all of them in a reasonable time. In most of the cases you end up with fast elution of compounds from first two groups and much longer time with hydrophobic basic compounds.
Ion-pairing reagents will help you to retain hydrophilic basic compounds but at the same time increase retention time of basic hydrophobic compounds. IP will have no effect on neutral components (which you don’t have). This becomes even more serious with hydrophobic compounds which have more than one basic group as ion-pairing reagent will interact with both groups creating even longer retention (pheniramines, pyrilamine, triprolidine). You can use IP reagent with shorter chain as Tom suggested but this will decrease retention time of you hydrophilic basic compounds (norephedrine and phenylephrine) and you might lose resolution between these two. Another approach is to use sharp gradient of ACN from 0% to 70% with no IP reagents-a lot of modern C18 columns will handle 0% organic.
You can use
our mixed mode approach and elute compounds of your mixture within 10 (even shorter) minutes with good resolution-no ion-pairing reagent needed and you can use ELSD/LC/MS compatible conditions.
In the example below you have a mixture of compounds which mimic your situation (hydrophilic basic and hydrophobic basic). You can also see what a regular C18 column will give you under similar isocratic conditions with no IP reagent:
http://www.sielc.com/application_131.html
If your requirements can tolerate gradient you can do double gradient (ACN and buffer) you can do even better. In this case you will have extremely sharp peaks and achieve very high efficiency- 500K-2M plates/meter due to focusing phenomena.
Although some people will say that this is not correct plate count and I can agree with this argument, but at the end you should not care how to call or calculate it-you will have good separation:
http://www.sielc.com/application_122.html
You can also check other methods related to cough and cold compositions:
http://www.sielc.com/compound_167.html
We will set up a method for your tomorrow (I don’t think that we have brompheniramine but can use chrlopheniramine instead) and I will update you when we have results.
Kind regards,
Vlad
