Newbie problem with ion exchange

[Random Guy]

Forgive me for bothering you with this problem, but I'm a molecular biologist who has to do a small chemical step using ion exchange simply to remove salts from a number of small samples. I have some Dowex -50 that has been in the lab for a few years but the little info I've been able to find about how to do this is contradictory. If my samples are essentially 100-500 microlitres of saline, how much resin would I need to desalt it? One text said 4-5 meq per g but the manufacturer's page said 1.7 meq per litre. Also, the methods I am following are a bit vague and confusing. They suggest a 0.5 ml resin volume stored inside a pipette tip. That works fine. But they also suggest washing each with 100 ml of water. Washing with 1 ml took quite some time... I can't imagine that the text is correct.

I figure that this is very simple, old-time stuff and a few words of advice here would save me a lot of time digging through the chem library. Many thanks in advance,

Random Guy

[tom jupille]

First off, the ion exchange capacity of resins is usually quoted in milliequivalents per milliliter of packed bed volume. It will probably be in the vicinity of 2 - 4 mEq/mL for a Dowex 50WX?? resin. The exact value will depend on the ionic form and on what comes after the X in the specification. That refers to the amount of crosslinker (divinylbenzene) in the resin. 50WX4 would be 4%; 50WX8 would be 8%, and so on. The lower the crosslinking, the more the resin will shrink or swell when you change the ionic form (hence the vagueness about the capacity).

Next, Dowex 50W- resins are cation exchangers; that means that they must be in the H+ form to be used for removing Na+. You need to check that the resin is in the proper ionic form. It's been a while since I looked at it, but try BioRad's web site for information on that procedure.

More: when you replace the Na+ by H+, you are effectively converting your NaCl to HCl (i.e, the pH will drop). To completely desalt you need a mixed-bed resin that has both anion and cation exchangers.

Finally, I would say that ten times the bed volume is the minimum you would want to rinse with (in your case, about 5 mL). 100 mL sounds high to me; that would be required only if you were trying to regenerate say, a Na+ form resin back to (more weakly bound) H+ form.

[Random Guy]

Thank you very much Tom. The resin is in the H form and your explanation cleared up my questions as well as a few other things that I hadn't mentioned. I'll email the company to point out what I assume is the spelling error in their online description of the ion exchange capacity of their resin.

Best wishes,

Random Guy (who is too embarassed at his ignorance of this matter to include his name).

[tom jupille]

There's nothing to be embarassed about. Ignorance is curable (stupidity,on the other hand, is not). All I did was pass along information that people like Roy Wood and Larry Cummings at BioRad taught me back when I was ignorant.

[Jade.Barker]

Random guy,
Do NOT be embarrased! This is a good question, best of all you caught it with a temperature check of logic... That means you are paying attention - Thats good!

A couple months ago my old lab had to recalculate some data because the Manager gave us a calculation that added up to more than 100%... It took the tech a couple of weeks to catch the Logic poblem with that... It can happen to the best of us.

Glad to have another new face. Together we are smarter than anyone of us is alone. Welcome!