1 Reply Latest reply on Nov 8, 2018 1:46 AM by gchaplain

    GC/MS leak while inlet pressure and column flow keeps fluctuating


      Hi everyone!

      First of all, I would like to thanks in advice any answer to the problem specified below. Also, I'd like to mention that I'm a GC beginner (my Ph.D. started 4 months ago) so I apologize in advance for any so-obvious question.

      At our lab, we've been experiencing some problems with what we assumed to be a gas leak. Under normal conditions, a He bottle lasts approx. 6 months; taking into account that during the summer season we didn't run any sample and that the He bottle was replaced in June, this doesn't seem to be a regular thing. Also, it can be observed how the needle of the pressure gauge slightly fluctuates while the device is on. In the end, %N2 reached a higher value than normal (7.5) when autotuning, which led us to an air leak.

      Once we started to replace and adjust any consumable that could affect this, we came to a point where inlet pressure and column-1 flow was fluctuating during so long (well, technically it never settled). We checked leaks but we didn't see anything. Anyway and according to what the manual says as well as I could read many threads in this website, I already replaced almost every consumable that could affect to that fluctuation:

      • Gold ring.
      • Liner + o-rings.
      • Septums.
      • Split vent cartridge.

      Once we replaced the last one, we finally observed that both pressure and column-1 flow stabilized. BUT, the needle from pressure gauge still fluctuates a bit.

      I read in some forums that it could be a welding problem, or even worse: one related to the EPC.

      What else I have left to do?


      GC Model: Agilent 6890N attached to an MS Agilent 5973.


      Thanks again for the help.


      Best regards,



        • Re: GC/MS leak while inlet pressure and column flow keeps fluctuating

          Good morning,


          7.5% is still a little high for my liking personally. Pressure inside the GC should be fine if its reading stable (what is your working pressure?), but the needle on the pressure guage is external, as long as it is around 100 PSI going into the GC it should be OK.

          I suspect you may have a leak external to the instrument, maybe on the manifold connected to the helium, or even on the copper line going into the GC's EPC. I would check there first as any leak here will put air directly into your helium stream and no matter what you do to the GC, if there is a leak there, it will keep reading high.