Nitrogen at 1200% of m/z 69

Hi guys, I've got a 5975 that was vented for around 2 months due to the whole helium thing. The analyzer chamber was sitting at ambient for the duration, multiplier was still pretty new so I did some maintenance and pumped it down to try and save what was left of it. I tuned it 2 days later and I'm seeing a huge nitrogen peak. 69 is at ~620k counts and 28 is around 7.5 million, I don't think the turbo would even spool if it was this bad of a leak. I sprayed the door and mass spec inlet with duster and didn't see a spike on mass 51. Vacuum is reading 7.12e-6, does it just need more time to pump down since it was vented for so long or am I missing something here?

Thanks,

Parents Reply Children
  • The best way to get nitrogen out of gas filters is to do a number of full pressurize/depressurize to ambient cycles.
    Disconnect the gas line just before the GC plumbing.
    It's easiest if you can install a shutoff valve there, but a cap will do.
    Set the helium supply to 60 or 80 psi.
    Open the exit valve (or remove the cap), and let it blow out for at least three to five trap volumes or more, then close the tank supply valve and allow the trap to depressurize all the way to ambient pressure.
    Close the vent valve, open the supply valve, pressurizing the trap to 80psi.
    Close the supply valve, open the vent valve, allow the trap to depressurize all the way to ambient pressure.
    Repeat…how many times? Well I’d think eight or ten is probably a good start.
    You cannot get the same effect by just pushing your thumb over the exit and letting it build up pressure and then blowing out. Depressurizing all the way to ambient on every cycle is necessary.
    The last time make sure that the vent valve and supply valve are both opened and reconnect the gas supply to the GC.

  • Nitrogen is under 100% at this point. I'm going to chalk it up to the trap. Instrument hasn't ran an injection in over a year yet we still have it on a service contract for some reason, but that's a different can of worms.. Thanks guys.

Was this helpful?