Air peaks with periodicity in GC/MS system

Would appreciate any advice.  I am experiencing periodic air peaks occuring during runs on my 7890B/5975 system (see TIC image below).  I have done a complete inlet maintenance, removed and re-installed the column, and checked the autotune and I still am experiencing this issue.  The images below are from a MeOH injection using a DB-Wax column and normal temp ramp.  All of the peaks have identical MS spectra as shown below.  I'm currently at a loss on how to troubleshoot this as the MS itself looks perfectly normal with the autotube and air/water check.

  • In case it helps, the most recent air/water check was 0.35% water, 5.5% nitrogen, 1.8% oxygen, 0.4% carbon dioxide, and 1560% N/water

  • Beautiful.  Does the spacing change with a different column flow rate?  If this is 1.2 ml/min, for example, is it different with only 1.0 ml/min?   If it changes and you did the inlet and column maintenance properly, the issue is before those.  If it does not change it is probably in the MS.

    If it's the MS, vent.  Carefully remove the analyzer door O ring from its groove. Wipe the groove out with a solvent-dipped cotton swab.  Wipe the O ring off with a solvent-dipped lint-free cloth. Reinstall the O ring.   Or just replace the O ring with a new one. The part number is 0905-1442.  They do not last forever...

    Side plate O ring

    While it is vented, also replace the O ring in the vent valve. You can use a typical injection port liner O ring.

    Vent Valve O ring

    Pump it down and try again.  

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