Using prep HPLC for ion exchange

[thiazole]

I noticed quite some time ago that if you load an amine as an HCl salt (for example) to a prep HPLC system and flush with pure water that it can and will actually free base your the amine as long as the compound doesn't rapidly come off the column. I've also seen examples where it will come off in two peaks, one that comes off very early that NMR shows as a salt and the second peak that comes off much later that the NMR shows as the free base. From these, I can conclude that ion exchange could also be performed via prep HPLC, ie, load an HCl salt and flush with TFA water and exchange to a TFA salt. What I'm interested in is a quaternary ammonium salt (4 alkyl groups on a + charged nitrogen) with a I- counter ion. If I flush this on the column with a KBr solution, it should exchange the I- to a Br-. My question to everyone out there is, have any of you done this or seen papers on this? I'm not a big fan of ion exchange resin and think this is a much better way to perform an ion exchange, but I'd like some literature to back it up (so I don't have to pay for elemental analysis to prove it works).

[DJ]

I don't believe you can separate neutral and ionized forms of the same amine

[thiazole]

That isn't what I meant (although when you use no modifier in the mobile phase, it can happen). I meant you load the column with 100% salt form and flush with pure water (ie, no modifiers) - assuming this molecule is polar enough to run quickly, you may see some compound come off as the salt (first) and some that comes off as the free base. If you load a greasier salt and flush with pure water long enough, the water will pull off all the salt and essentially free base your compound. It isn't surprising this happens - water has an affinity for HCl (for example) and even though the affinity is less than that of an amine, you are swamping it with 1000sX more water than amine and once the HCl comes off, it is moved through the column, ie, it is irreversible.

[DJ]

What ion exchange column are you using? What is the nature of your sample?

[thiazole]

Sorry, I guess I didn't really explain this. No ion exchange column. If you take a C18 reverse phase column and equilibrate it with unmodified water, then load an HCl salt (say diphenhydramine HCl) and flush it with sufficient water only, the water will slowly pull off the HCl and leave the free based diphenhyramine on the top of the column. Then you can flush with ACN, collect your compound, and take an NMR and it will verify the disappearance of the HCl salt. I guess this is news to everyone. I'm not an analytical chemist. I'm a synthetic organic chemist with 13 years of lab experience who recently started doing some QC analytical chemistry part time (why I'm on this forum). This is a trick I've been using for the past 8 years or so. The first time I learned this, I made an HCl salt of an amine and wanted to preserve the HCl but it needed some purification. Since purifying with TFA in as a modifier using reverse phase would exchange the salt to TFA, I used pure water. Once I isolated the product and took an NMR, it was a free base. After trying it a few more times, I got the same result each time. I figured this was common knowledge among analytical chemists.

[Vlad Orlovsky]

Your RP column will have really low capacity since pKa of silanols is around 5, and there is very small amount of them on a modern RP columns. Ion-exchange and mixed-mode columns have much much higher capacity. The best way for you is probably get some IE resins and this in the flask.