Chromatography Forum

Hi there,

I'm using a quaternary ammonium anion exchange column. The manual dictates NaOH (aq) should be used as a mobile phase. NaOH is not favourable for my experiments.

I want to separate phosphates and was wondering what other mobile phases I can use. I know I need a counter ion, can it simply be replaced by NaCl ?

Thanks

No, because Cl- is much more strongly bound to the anion exchanger than is OH-

That doesn't mean you can't use it, but it's not a case of "simply" replacing.

We routinely use bicarbonate, benzoic acid, phthalic acid

Hi DK,

Tom is right Cl- as a counter ion is a stronger binder; if you are using it in conjunction with suppressed conductivity its weak conjugate acid in any suppressor is HCl which gives much higher background than the suppression product for NaOH which is water. Phosphate is also associated in solution as a suppressed ion and as such has issues as it is the least conductive ion to analyze by conductivity. It also binds strongly depending on eluant pH as you use more basic buffers it tends to be trivalent. If you are looking at polyphosphates the problem of polyvalencey increases hence the need for gradients to enhance selectivity and get these polyvalent species off the column.

Regards

Mark Albertson
Thermo Fisher Scientific
Sales Mgr ANZ.
IC, Sample Prep

Thank you for your responses.

Ok so (Na+Cl-) is an option. I am not using conductivity detection so that is not an issue. The fact that it is a stronger binder is one to take into account. Is it a matter of simply using lower concentrations of the salt?

I read sodium bicarbonate is also frequently used?

D

Yes sodium bicarbonate can be used, but it will affect your buffer (by the way, what pH were you plsnning on?).

The pH I did not think of yet to be honest. For good retention I guess the polyphosphate needs to be ionized?

Yes, and the degree of ionization will affect how strongly they will be retained.

What you might want to do is to pick a buffer at a pH to ensure ionization (ad compatible with your detector) and a reasonably low concentration (something on the orfer of 25 mM or so) and then run a gradient from 0 to 1 N NaCl. That will give you an idea about a reasonable ionic strength for your samples.