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,

  • What is the O2 level you're seeing?  If it's an air leak, the N2/O2 ratio will be approx. 4.  If the O2 level is really low, then it's either a big leak before a gas filter (filter traps O2 but lets N2 pass) or a gas supply problem.  I have seen a customer accidentally connect a N2 cylinder instead of He - both use a CGA 580 fitting so it's an easy mistake to make.

  • Running off an 8 tank helium bank, instrument further down this T off the main line looks fine. I've been up and down most of the connections with a leak detector including split vent line and I'm not seeing anything. Main bank hasn't indicated a leak since I opened this line. Oxygen is negligible, it looks better this morning but still not ready to run injections.

      

    I'm kind of starving it right now with a total flow of 5.4 mL/min and I've seen water climb when you do that. I loaded a run method for a couple hours yesterday but it didn't do anything for my nitrogen count.

  • Is it lower with higher GC oven temperature?  If so it is likely in the GC. I always suspect the GC as there are more connections. Kevin

  • Have you changed a gas filter recently?  Some of them are filled with N2 during manufacturing and will take some time to purge when they're installed.

  • Nitrogen to oxygen ratios don’t match for an air leak downstream of the gas filter. I have seen baselines climb on my ECDs when I cool them down, those giant ferrules will shrink when you aren’t at 300 degrees 

  • It has a new helium specific trap on it. As far as I know the only difference is what gas they fill it with vs a nitrogen trap. This was the last of our current supply of big boy helium traps on hand, we do have 7 of the small Agilent visual traps hanging around, I’d just need to cut some new gas lines.

  • 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.

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