Iron/EDTA Ion Pair method

[Noser222]

I'm running a reverse phase method for EDTA by which it is complexed with Iron and then form the ion pair with tetrabutylammonium bromide. The mobile phase is a sodium acetate buffer (pH ~4) with tetrabutylammonium bromide added. I've experienced a massive buildup in column pressure and wondering if it could be due to the Iron concetration in my samples, which is about 0.2 mM. Has anyone run a method like this and run into problems?

[Einar Ponten]

A long time ago I was also in for a similar approach on a pharmaceutical formulation and the determination of EDTA. I never found it sufficiently robust.

I think you must assure proper buffering at even slightly lower pH to make sure that iron precipitation is avoided. Do you really need that high iron concentration? Some millimolar should be enough?

You also need a column oven and continous mixing in the mobile phase bottle. Both the complexing and ion-pairing reactions are temperature dependent (as well as column kinetics).

Are there really not any alternative methods today?

[Noser222]

I don't have Iron in the mobile phase, just in my samples, at 0.2 mM. The buffer in the mobile phase is acetic acid/sodium acetate and we have verified that it is acidic.
What kind of method did you end up using? I've read of another method using Nickel to complex with MS detection bud don't know of any others.

[Chris Pohl]

Noser222,

Your pressure problem could be related to the specifics of your sample preparation or the stoichiometry of your sample. Since you say that your sample contains both EDTA and Fe, what is the oxidation state of your iron and what is the mole ratio of the ingredients? Is the Fe concentration constant and the EDTA concentration variable in your samples? If you have more Fe in your sample than EDTA on a stoichiometric basis (both Fe II and Fe III form one-to-one complexes with EDTA), you can expect problems with colloidal iron precipitate being formed at the pH you mention. This could be the cause of your pressure buildup problem. In this case, you might benefit from adding a second chelating agent to tie up the remaining ion (such as NTA).