How do I solve high oxygen and nitrogen levels after a gas cannister change?

After changing to a new gas cannister, I have been chasing high Oxygen and Nitrogen levels in my system. The gas filters for both instruments show they are still fresh (the right colour indicator), I've switched out for another new gas tank, purged the regulator, tightened fittings and wiped down the analyzer chamber seal. How can these levels still persist?

My triple quad and single quad systems are hooked up to the same He tank. The triple quad has O2 levels at around 7% and N2 levels at around 25%, the single quad has O2 levels at around 3% and N2 levels at around 15%. After each of the above troubleshooting attempts, the levels are the exact same, no real change. Since they are close to atmospheric levels, I thought maybe an air leak, however, the H2O levels are below 1%. 

Any help is appreciated. 


  • Hi  thanks for participating in the Community!

    I moved your question to the GC/MS Forum for better visibility.



  • Were the N2:O2 and Water levels about the same before?  Have you used an electronic leak detector on every fitting?  When you say you purged the regulator, did you turn it on at normal supply pressure and turn it off and let the pressure decrease all the way to atmospheric, then turn it on again to supply, off, let decay, and back on again at least eight or ten times?   New gas filters need this same procedure after they are replaced, too. 

    You may have a contaminated tank.  Could you put the original one back on or try a different one?

  • Hi Paul, 

    The N2:O2 levels were both below 1% before the tank switch. Unfortunately I don't have an electronic leak detector, but I have been trying the canned air trick. No signal has shown though when I spray on each of the fittings (but I'm also not entirely sure I'm doing it right). Also your description of the regulator purge is exactly right, I would repeat that procedure 8x. Gas filters are not new, but their colour indicators show no sign of saturation. I've tried a new tank of He with no change, but cannot confirm if the two tanks are the same lot. Unfortunately the old tank of He was picked up when I ordered the first tank. 

    I've also noticed that N2:O2 levels go up as I reduce the flow in the inlet. I've replaced the septum, liner, o-ring, and ferrule with no change. 

    I'm thinking of trying to replace some copper lines in case they got twisted out of shape. Also maybe the filter is saturated and the colour indicator isn't showing it?

    I appreciate the help though, thank you. 

  • I did a soap and water test on the regulator connection to the tank, and it showed some leakage (disconnected from the instruments of course). I retightened the fitting with some plumbers tape and the leak is sealed. It appears to have fixed the N2:O2 levels in the single quad. 

    For the triple quad, I noticed the levels of N2:O2 would decrease if I increased inlet flow. I replaced the consumables around the inlet, and also double checked my glass connector between my column and my guard column. The appeared a little loose, and so I tightened that and my O2 levels are now below 1%. I wonder if it came loose during the gas purge, or when I was playing with the column connection. 

  • Soap and water or Snoop type leak detection fluids are not recommended to be used on GC or GC/MS gas connections.  If gas is leaking out, gas is also leaking in - that's why a leak in the helium line will show as air in the background. That means that leak detection fluids get into the line at the leak and will contaminate the system. The recommended tool is an electronic leak detector like the Agilent G6699A: Electronic GC Leak Detector (Hydrogen, Nitrogen, Helium) | Agilent

    The regulator connection to the tank is not sealed at the threads. The seal is metal to metal at the tip so thread tape only helps smooth out tightening the large nut.  Sometimes just disconnecting and reconnecting the regulator at the tank fitting is sufficient. If the tip of the regulator gets mangled the only solution is a new regulator.

    Glad it's fixed.

Was this helpful?