Chromatography Forum

Hi,

I have googled this simple question, but no useful answer appeared. So I hope you have an answer.

I would like to add some EDTA to my samples to avoid oxidation in my LC-vials. My problem is that the drug is most soluble in acidic pH, and I believe that EDTA is not working at low pH. Does anyone know at what pH-values EDTA is useful as a metal ion scavenger?

Second (independant) question: is is normal that my UV-detector noise is higher (as µAu) at higher wavelengths? You would think that the noise would get lower. I run with water/TFA/ACN, and my noise at 300 nm is much higher than at 220 nm.

Hi,

I have googled this simple question, but no useful answer appeared. So I hope you have an answer.

I would like to add some EDTA to my samples to avoid oxidation in my LC-vials. My problem is that the drug is most soluble in acidic pH, and I believe that EDTA is not working at low pH. Does anyone know at what pH-values EDTA is useful as a metal ion scavenger?

As you say, it is pH dependent but also dependent of which ions you want to protect your sample against. Did not find a good table on the internet so here are some approximate minimum pHs:
Mg2+ 10
Ca2+ 8
Fe2+/Mn2+ 5
Co/Cu/Zn/Al 4

Fe 3+ 1

So below pH 5 EDTA fast loses effect, still Mg and Ca ions can cause troubles.

Acid washed vials (for example HCL for AAS use.) could be an alternative, works well in non trace AAS to remove Cu/Fe etc. But inconvinient if many samples.

Just wonder what sort of organic compound would be easily oxidized in acid medium?

Normally you have more noise at lower wavelength because of strong background absorption. If you have a stronger background at longer wavelength you may have the stronger noise there. Also, could be that your light source is dwindling at the longer wavelength.

Thanks!

I don't know what ions that catalyse my oxidation, and I don't know where they come from. But I do know that EDTA (tripotassium edetate) make my chromatograms look much cleaner.

I am testing now at pH 2.5, and it looks very good (at least after one hour in solution). I assume that it is Fe3+ then that causes most of my problem. I guess I was lucky...

HW Mueller> I just missed your reply...

I am not sure if it is oxidation, but I see e.g. dimers forming in the solution where EDTA was not added. The molecule is 5-ASA. If dissolved in just water, the chromatograms are really ugly - with lots of unknown peaks.

I am using an old 1100 DAD detector, and I think it might be the optics that is getting bad. I know that it just passed the noise test during the last PQ - with a brand new lamp.

Seems like the instability of 5-ASA is known since before.


http://www.ingentaconnect.com/content/e ... .alexandra

Also, EDTA is not water-soluble at low pH.

In spite of krickos´ link I still don´t know what ASA is, but since it´s primarily a pH effect it is not nearly as interesting (to me, might look the compund up later).

(Incidentally, metal ions are all around us and catalyze all kinds of things at extremely low levels).

Hans,

5-ASA is 5-aminosalicylic acid which forms dimer called olsalazine:

http://en.wikipedia.org/wiki/Olsalazine

Consumer products guy> The potassium salt seems to dissolve easily, but the acid is insoluble (which I learned the hard way).

OK, judging from the formular of the dimer there is certainly oxidation involved in going there. But if low pH prevents it, it could be another example, of many, on pH control of oxidation.
Mattias, if you have time to play a bit: Another way to prevent oxidation at the higher pH´s is to put in an antioxidant, if that stops the reaction you almost have proved that oxidation is the culprit.