EDTA in mobile phase

[Mattias]

Hi,

I am currently using an old LC-method that is used for the determination of impurities of an antioxidant.

The samples are diluted in mobile phase, and they are very unstable. The analysis must take place within a very short period of time after dilution. This is of course not very practical if you want to go home to your familiy in the evening

I tested to add a very small amount of EDTA to the mobile phase, and now the samples are stable for days. It is best to have the EDTA in both the samples and the mobile phase (to reduce the front peak)

Just to be sure, is there a theoretical risk that EDTA could cause problems with the separation? Or some other aspects that I have missed? How should I validate this (in my opinion) minor change.

LC-method: isocratic, C18, mobile phase: MeOH/SDS in buffer, UV detection.

[gbalock]

Mattias,
I think it would be safe to say that you have stopped some metal catalyzed degradatioin by addition of the EDTA. To answer your question, I would want to do some validation on the analysis, but not necessarily a full validation. Specificty and peak purity would be my top picks.

[prashant.pqa]

I think adding EDTA will not make any difference for anlysis . It id good to add analysis for speacially for antioxident. But check for peak purity.

[Mattias]

Thanks for your replies,

Does anyone know a good way to remove the metal ions (divalent) from a solution. It was not popular to propose a change in the separation conditions of a pharmacopeial method.

My results show that the metals come from the SDS chemical. I have tried the purest SDS I could find but I still see degradation. If possible I could prepare a stock solution of SDS that I can purify before I mix the mobile phase.

many thanks!

[rhaefe]

Mathias,

if you are sure where the metal ions are coming from you can try to remove them by using a chelating cation exchange resin based on immobilized IDA (iminodiacetic acid). Rohm and Haas for example has one (Amberlite 748) but there are others out there as well. If you want to get fancy you might be able to pack a column and install it in-line to constantly remove metal ions from the mobile phase. I have done this in the past and it did work quite well with the cartridge installed prior to the injector.

cheers,

[Mattias]

Rhaefe, thanks for your suggestion!

The degradation is occuring in the LC-vials, since I dilute the samples with mobile phase. So I need the purified mobile phase in a larger quantity (approx 100 ml / analysis).

The ideal solution would be to purify the part of the mobile phase that will be used for sample dilution. The reaction is not so fast that I will have problem with on-column degradation.

The SDS contains only 5 ppm of Fe according to the certificate, but it still seems to have a great impact on my sample stability.

[rhaefe]

Mattias,

the chelating resins should work. I know that you decided not to add EDTA to your mobile phase. How about you prepare some mobile phase with EDTA and use that to dilute the samples. Don't add it to the actual MP you use to run the samples. Would this be considered "cheating"...?

cheers,

[Mattias]

The problem I have with EDTA only in the sample is that I get a disturbing front peak. One of the most important "true" degradation products will elute in tail of that front peak.

I could try to reduce the EDTA concentration to a minimum. Since my metal contamination is very low I will probably not need much.

It is also a question of all the paper work involved if I do a change that will require a "change to be approved" by the authorities. I guess we have to file about 20 of these... A "filtration" of the mobile phase for dilution is a simpler modification.

[rhaefe]

The regulatory impact has to be considered. In that case I really would try treatment with a chelating resin and filter the MP.

http://www.rohmhaas.com/ionexchange/IP/ ... IRC748.pdf

It is available through Aldrich. And I am neither affiliated with Aldrich or Rohm and Haas...

[Mattias]

Just to tell you the end of the story:

I found a prepacked column with immobilized IDA (iminodiacetic acid) material that I have tried successfully for this application.

It is manufactured by Amersham (GE Healthcare) and it is called "HiTrap chelating". Thanks for leading me to the IDA track!