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 levelsautomateandvalidate
Now check the instrument calibration,
Try inject three concentrations of caffeine each one injection and verify the linearity. Once it is OK
then come to conclusion that there is no problem with HPLC System
Normally in HPLC, Concentrations that yeild above 1AU absorbance can be non linear due the limitation of beers law.
Diluent selection in HPLC makes sometimes critical role for this kind of non linear issues.
Check the system suitability for every validation study.
Bujji Reddy K