What are some things to consider when the Lachat method does not give a good calibration or QC checks fail?

Dokument-ID

Dokument-ID TE7966

Veröffentlichungsdatum

Veröffentlichungsdatum 04.03.2022
Frage
What are some things to consider when the Lachat method does not give a good calibration or QC checks fail?
Zusammenfassung
Lachat bad calibration and QC checks.
Antwort
Here is a check list of things to consider:
 
  • How do the peaks and baselines look?
    • Do the peaks have good shapes?
    • Is the baseline stable?
    • Are the peaks properly integrated?
  • What is the source of standards? Do multiple sources have the same behavior?
  • Are the reagents fresh? Are the source chemicals old?
  • Are the pump tubes the correct material and size? Have they been replaced since the issue began?
  • Are the pump tube cartridges in good shape and tensioned correctly? Cartridge tension levers should be set back one click after pushing lever all the way to the right (to interior of pump). Cartridges should not be worn after a few months, but they can possibly be worn after a few years.
  • Is the pump speed at 35?
  • Do the pump tubes go to the right reagents? Do they lead to the right place on the manifold?  Check and verify that everything matches the manifold diagram for the Lachat method being run.
  • Is the sample loop the correct length and diameter?
  • Has the valve timing been checked by running dye? Please see the following article: Is there a replacement for the Lachat Dye, Universal 250ml?
  • If the chemistry is heated, is the temperature set correctly and is the correct length of tubing on the heater being used? The standard heater block comes with both 175cm and 650cm lengths of tubing on it.
  • Is the correct wavelength filter installed in the detector?
  • Is there a back-pressure loop installed and does the method call for it?
  • Is the waste line free flowing? Consider removing the waste line and letting the waste drip into a beaker to confirm that it is flowing.
  • Has the flow of each reagent line been checked? The lines can be placed in water for this. Lift each line out of the water one at a time and watch the flow of air to verify the flow is predominately forward.
  • Are the reagent recipes and the range used the same as specified in the published Lachat method?
  • Do the parameters entered into the software match the Lachat method?
  • Is the method is appropriate for the sample matrix? Different sample matrixes include acid preserved samples, digested or distilled samples, extracts.
  • Consider running 10 replicates of a midrange standard to see if the results are consistent or if they fluctuate.

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