Peak drift of 1/3 peaks

[GDowsett19]

Hi all,

Here is an issue I'm experiencing, I'm wondering if I can get people's ideas/opinions?

Background: I've been running active content/ quantitative HPLC runs (using Thermo Ultimate 3000 systems, MWD detector) with established, validated analytical and HPLC methods. Column: Phenomenex Kinetex C18, 250 x 4.6mm, 5u, 100A. Wavelength: 318nm (bandwith: 10nm, no ref). Mobile phases are A - DI-Water and B - MeCN (100%). It's just a simple gradient (10% B-70% B over 12 minutes) with a re-equilibration step.

The issue: I see 3 peaks, very well resolved at 5 minutes, 7 minutes and 9 minutes. Recently I've been analysing some new samples and I see perfect chromatography at first, but then the middle peak (7 mins) starts to negatively drift and ends up at about 6 mins. This is true for samples, but the standards drift only about 20 seconds (similar peak sizes).

So I guess I'm asking is there any reason a) for only 1 peak to drift and b) if there is any reason the sample peak drifts more than the standard? (please note that I've ruled out the possibility of this drifted sample peak being a deg product/impurity)

Thanks

[Consumer Products Guy]

My first investigation would be to add some pH control to the aqueous portion, like a little acetic or phosphoric acid.

[Multidimensional]

I agree with above comment. Your method may not be rugged enough and need further development. Using just pure DI water and ACN will not account for any changes in ionization. This may lead to drift, poor RSD or related problems with one or more peaks (samples).