GC/MS contamination peak

Recently did full maintenance on GC/MS; cleaned the source, replaced the gold seal, inlet liner, septa as well as eventually installed a new column with a new guard column (we were starting to observe lots of ghost peaks and bleed since it had been in use for now a while). I'm observing a contamination peak with 129 and 147 main ions, which seems to correspond to Bis(2-ethylhexyl) adipate - https://imgur.com/us01aBd - both in split and splitless mode. Pretty confident it's from the GC side since if I do a run while keeping the temperatures of the inlet and oven low (50-60), I do not observe this peak (plus the peak is consistent in its retention time).

Interestingly enough, when running a method with a higher starting oven temperature, the abundance of the peak decreases until it pretty much disappears if the starting temperature is above ~175-200 degC. The peak also seems to be absent if I run while heating the oven, but keeping the inlet temperature off. So, thinking this might be an inlet issue, I then replaced the split vent trap, cleaned the inlet itself, as well as the vent line, and replaced both septa and liner again - also baked out the column some before re-installing. I am still seeing this contamination peak!  Note that all runs were conducted without actual injections.

At this point, I feel I have replaced everything what am I missing? Anyone know what this peak could be characteristic of?


  • Hi

    I assume you have done everything.

    Now change helium cylinder with new lot.

    If any moisture/nitrogen trap filter is connected with helium line then change it...

  • The cylinder wasn't changed during the whole process (been in use for about 6 months without issue), so the contamination should not be coming from there.

  • As you are saying that from 6 month, you are using cylinder so there is chances that it's already consumed in good quantity.So there is chances of impurities from cylinder.

    Are you using any moisture/nitrogen trap filter...

  • Please check above pdf file...

  • Can you share autotune report?

  • Do you get that peak when you do a no-inject run, just pressing start or removing the ALS syringe?

    Do you get that peak when you do a solvent-only run with no peaks of interest?

    When is the last time you replaced the ALS wash and waste vials and the cone cap inserts?  The ALS syringe?

    Here's one procedure to try:  You do not need to setup for acquisition in MassHunter as you really don't want the filament lit.  Remove the autosampler. Oven at the final oven temperature. Inlet hot, transferline hot.  Set the column flow to 0.2 ml/min - two psi inlet pressure or so, the inlet to SPLIT mode and about oh, 400ml/min or more with GasSaver turned off. Using a new syringe and new, never touched plastic solvent, inject five microliters slowly - insert the syringe and push the plunger down over a few seconds.  Do this some few times, four or five or so.  Then get a coffee and wait for a few minutes to allow anything to come off the column.

    Then reload your normal run method, which resets all the parameters, and try a solvent-only run.   Peak or no peak?

    I've seen weird peaks from plastic pipettes, solvent in squeeze bottles, vial cap septa, and many other odd places. If the peak has shape and a relatively consistent retention time, it's before the column or at the very head end of the column.  Make sure that the GCMS is leak free first, of course.

  • No; all runs are done without injections (manual start run on GC after trigerring in software), so it cannot come from sample/solvent/syringe/etc.

  • Unable to open it

    U can take photo and share it..

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