Ion-pair Chromatography: Revalidation issue

[zokitano]

Dear colleagues,

I shall greatly appreciate if you could share your opinions and experience in this particular subject in IPC (ion-pair chromatography).

What is the problem?
It is clear that use of ion-pair agents can enhance peak shape and retention time when common approaches, such as modifying eluent ratios or changing stationary phase, fail to improve resolution of polar and ionized analytes, especially when they are in large numbers, e.g. impurities of active pharmaceutical ingredient. Changing one ion-pairing agent with another also changes the chromatographic separation. This change may lead to different resolution, inversion of retention and fail in achieving the system suitability.
So, when using ion-pairing agent that slightly differs in structure with the prescribed ion-pairing agent in the method, questions arise whether the changed method should or should not be revalidated.
The chain lengths of the ion-pair reagents enable selective separation of the analytes. The longer is the chain, the more hydrophobic is the counter-ion, and therefore, the greater is the retention. Retention may increase by a factor of almost 20 when going from pentyl to dodecyl alkyl chain. In either case, increase in the alkyl chain length of the counter-ion increases retention in reverse-phase ion pair chromatography by up to 2.5 times per added - CH2- group in the counter-ion.

My question is: if you change the prescribed ion-pairing reagent with similar (e.g. heptane sulfonic with hexane sulfonic) and if you check the critical chromatography parameters of the changed method (resolution, k', peak symmetry) is it sufficient after performing the system suitability, checking the selectivity and method robustness to accept the changed method (after partial revalidation)?

Your help is greatly appreciated

Sincerely,
Zoran

[Dan]

Zoran,

This is a tough issue without an easy answer. It may not even be covered in your SOP. That usually means that your QA or Regulatory Compliance group will say do a complete validation.

However, it may be possible to do a partial revalidation as you suggest. The important point is to see what, if anything, has changed.

You mention for the revalidation to check critical chromatography parameters, system suitability, selectivity and robustness. I would agree with that but I would add a precision experiment to compare sample results between methods (i.e. before the change and after the change).

If your ion pair reagent is in your sample/standard solvent, then there would be a good argument for re-doing the solution stability experiment. But I don't think that would be necessary.

You may find that you are re-doing or nearly re-doing an entire validation. So your QA or RC group would again have a good argument for doing a new, entire method validation.

Good luck convincing your QA/RC group.

One question: performing even a partial re-validation is a lot of work, so is it worth the effort? In other words, is there a compelling reason to change the ion pair reagent?

Regards,
Dan

[zokitano]

Dan, thank you for your reply.

I suppose that changing the ion-pair reagent will cost too much work, time, and money during revalidation. The only reason I posted this revalidation question is the debate at work that I am running with my colleagues.
I wanted to be sure about certain things, so I needed your opinions.

Thanks,
Zoran

[JM]

In regulated environment, some HPLC method parameters can be tweaked, within certain limits like flow rate, column length , MP ratio etc.( ref.USP) but changing to different buffer will call for full validation and if you are changing already approved method you have to do the method equivalency study as well.

JM