TQD internal standard area counts decrease significantly with each run

We operate Acquity TQD.  Finding in recent days that Internal Standard area counts for standards and first several samples are ~10,000 but gradually the counts decrease to less than 7000 by the end of our run of about 80 samples.  Controls throughout the run are acceptable, but we have some concern about the apparent loss of detector sensitivity.  Any insights from the community?

Comments

  • What matrix are you analyzing? This seems to be normal behaviour, at least for urine samples (10x diluted). In my case the sensitivity can be easily decreased by 50% after running 80-100 injections. It's due to the burned matrix impurities which sit on the sample cone. I clean it after every plate.
  • Serum protein crash with acetonitrile and zinc sulfate.  However, in some runs we see significant decrease in area counts even in our first few standards (surrogate matrix BSA).  We could try cleaning the cone more frequently.  Just not sure why this is occurring now, as we have used this method for 5 years.  Thank you for your input.
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