Bracketing standard failure in long run

Hi,
I am facing bracketing standard failure while running the long sequence. we are including the B.Std at every 12 injections, where, the recovery will keep increasing until end of the sequence the difference would be more than 6% with the initial. the method is gradient, post injection needle was is 30 sec. % RSD and weight agreement are fine. 
One more observation, if we keep new filled vial, then B.Std will be fine. For this reason we verified the vial cap, septa and vial integrity everything is fine. But if we continue the same vial for repeated B.Std injection, then the results will keep increasing. 

Please let us know the possible cause and solution.

Thank you.
  

Answers

  • Most likely, the cause is evaporation of your standard's solvent.
    Assuming you do not have a chilled sample compartment, nor better cap liners available, the most likely solution is to use each standard vial for consecutive injections only, then not return to that vial once samples are injected after using a given standard vial. If your run time is long, even this may not work as well as making only 1 injection per standard vial.
  • Thank you, Dan.

    Thanks for your suggestion

    We are using the solvent which comprises more composition of water (Around 80%). Hence solvent evaporation may not be the cause. Sample compartment is provisioned with chiller. We are investigating with vial firmness (Septa and cap liners).


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