Hi, there,

I am doing a HPLC method validaton (gradient, UV detection). I am using 2 same types of instruments for the validation. One instrument consistently gives perfect linearity for response vs standard concentrations, but another instrument behaves inconsistently when checked with the same standard solutions: sometimes the linearity is ok, sometimes very bad. I am wondering what's the possible cause of the problem.

The causes could be wide.
Change elution solvents (degas), clean solvent filters, check the pump flow (and maybe change seals), check also the injection part (seals too and injection volume precision).
If not resolved then time to find out the old troubleshouting guide, remove the dust and check all the possibilities :-)

Agree with Laurent that the possibilities could be numerous. Check your user manual for possible causes, or call the manufacturer. Anyway, at first glance, your problematic LC's faulty part(s) could be isolated in the injector area (if any of these apply to your instrument) i.e. injector seal (leaky), faulty injector metering motor, plugged injector, switching valve, or the old plunger. I hope this help.

maybe the lamp of your second detector is at the end of its lifetime