Reoccurring peak (possibly noise or air bubbles) in HPLC analysis.

My company has an Agilent 1100 HPLC with a Post Column Reactor (PCR) for the analysis of carbamates (EPA 531.1). Recently I have been seeing reoccurring peaks that are not analytes showing up in the chromatogram as if there is an air bubble passing through the detector.

I have attached screenshots of some chromatograms. The first is our laboratory performance check (to check PGF, S/N, and resolution factor), a 0.50 ppb and a 10.0 ppb ICAL (a 5 point curve 0.5 - 10 ppb), a method blank, and a 2 ppb CCV. (Note: BDMC is our internal standard).

Each analytical run is about 55 minutes per injection. There is a 24 hour period between the LPC and the 2 ppb CCV. As you can see, the reoccurring peaks increases over time. I am not sure what is causing these peaks. There are no visible signs of leaking and the seals and reaction cells in the PCR have been replaced and conditioned.

Our most recent working theory is possible HPLC pump issues and may replace these piston seals to see if we have any improvement. If anyone has seen this type of issue I would appreciate if you shared your experience and how you solved it.

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