Hplc validation for degradation products - low level linearity : non linear response

Hello to all,

We are currently validating an HPLC method for impurities of an Active Pharmaceutical Ingredient, and we have found a non-linear behavior between response and concentration of API at impurity levels.

According to ICH, the study was performed on an API reference solution (we have not tested placebo yet) and the range is from 0.2 (LOQ = 0.3) to 3.8% (120% of the specification): [0.2 – 1.1 – 2.0 – 2.9 – 3.8].
The response factor for the 0.2% concentration is much lower than the rest of the studied points and as a result, the recovery % with the 2.0% concentration is very low (%RSD is not higher than 10%).

We thought that maybe our product was not stable enough in water at low concentrations, because the values were stable from the 1% point.

We tried a lot of improvements, the main ones are:

  • Change of the diluents so as to stabilize the solution: we obtained good results with acidified methanol once, but we did not obtained the same results twice.
  • Range extension: 12 points with regular intervals. We observed an increase from 0.2 to 0.8% and then, the response factors were stable.

We can not figure out another way to stabilize our solution, and we have not tested the placebo solution, for which we are expecting even worse results!

Here are some other ideas, not tested yet:
Change the processing method which is currently adapted to the API regular dosage. But the areas measured for impurities are large enough, so we are not sure it will change a lot…?

Maybe find a mathematical artifact, for example increase the number of measurement at low concentrations and thus not respect a regular interval? I think I remember there is a mathematical model to analyze this kind of range…

What do you think of our situation? Has someone ever been in that case?
I am looking forward to other ideas and advices!
Thank you,


Your LOQ is 0.3% and you are testing from 0.2%. I think you can start your range from LOQ level and omit concentrations below LOQ. This may help a bit, but may not resolve all your problems.

Thanks for the answer,
We have already tested a low level concentration range, including 0.3%, and the results remained the same: we really see a stability from 1% approx. only. So our recovery results are still non-linear…

Dear SRD
Pl.change chromatographic condition like Injection volume ,increase test concentration and change wavelength and also check in PDA whetere any co-eluted peak is merge or seprate at lower level which is not merge or not seperate at higher level.

Some times uniformity of the batch can cause this kind of aberrency

Check pricision of impurity, if the test sample has it.

Other wise change the mobile phase and column for better results.

derivatization methods can be useful in such cases sometimes.

Best Regards,
Bujji Reddy Kanchi