GC-MS, high Nitrogen, low Oxygen leak???? Help!!!

[josh]

GCMS 5973/HP6890

Friday: Changed Helium gas cylinder as was running low.
Put long run on over weekend, During run %RSD deteriorated and became erratic. Performed Tune before and after run;

AutoTune prior to run AutoTune after run
Repeller 30.12 30.63
Peak widths 0.60/0.61 0.60/0.61
69 ABS: 467573 388924
EM Volt: 1576 1576
Water: 6.77% 3.95%
N2: 2.67% 24.33%
O2: 0.46% 0.18%
CO2: 0.39% 0.08%

This would appear like a contaminated Helium Cylinder. Changed to a different cylinder and no improvement. Changed back to original low cylinder and still high N2, low O2.

Can anyone explain what’s happening? It’s not a typical leak as the N2:O2 ratio is not 4:1. It now seems it’s not a contaminated Helium cylinder. What other possibilities are there?

Any advise, ideas or suggestions would be much appreciated.

Josh

[Fat Jonny]

deja-vu....

http://www.sepsci.com/chromforum/viewtopic.php?t=7699

Maybe the second cylinder is from the same batch and is also contaminated? Can you go back to the empty one you changed from? Hopefully there's enough He left to confirm the tank is the issue.

Changing cylinders and suddenly finding a N28 leak is too coincedental.

What happens if you turn off the He flow? Does the 28 peak disappear? If it does that would suggest it's coming with the gas and the problem is before the MS.

[AICMM]

Put a septum on your column at the inlet and look to see if the 28 goes away.
\
Best regards.

[josh]

Thanks for the link and the advice. I did try out the old nearly empty cylinder back but the problem did not diappear. This is a worry. I hope to have 99.9999% Helium arriving today.

As a precaution I will check MS and GC again for leaks but the fact that the low level of oxygen versus nitrogen would suggest its not a typical air leak. But good advise is never assume anything.

I will let you know how it works out!

Josh

[Peter Apps]

We had a similar problem, and I recall it coming up on the forum before also. The 28 was not nitrogen, we leak seeked and changed cylinders etc etc, all to no avail. In the end it was coming from column bleed or inlet junk.

The coincidence with the cyliber change might be just that - a coincidence.

Peter

[LCbob]

Hi Josh

Some things to take into account.

1. it can take some time for the He from the new cylinder to pass through the tubing and get to the MS, especially on large sites where the cylinders are located outside away from the building. There is often a 'lag' between changing a cylinder and the problem occurring. This may be the cause of the old cylinder also giving high N2.

2.do you have an oxygen filter fitted. If you have a leak before the oxygen filetr the oxygen is removed and only the N2 left.

Hope this helps

[mtorrey]

You can have a leak at the transfer line. Try tightening the transfer line nut a little. Do you have a O2 trap? I've seen them trap N2 and it takes forever for it to go away.

Mike

GCMS 5973/HP6890

Friday: Changed Helium gas cylinder as was running low.
Put long run on over weekend, During run %RSD deteriorated and became erratic. Performed Tune before and after run;

AutoTune prior to run AutoTune after run
Repeller 30.12 30.63
Peak widths 0.60/0.61 0.60/0.61
69 ABS: 467573 388924
EM Volt: 1576 1576
Water: 6.77% 3.95%
N2: 2.67% 24.33%
O2: 0.46% 0.18%
CO2: 0.39% 0.08%

This would appear like a contaminated Helium Cylinder. Changed to a different cylinder and no improvement. Changed back to original low cylinder and still high N2, low O2.

Can anyone explain what’s happening? It’s not a typical leak as the N2:O2 ratio is not 4:1. It now seems it’s not a contaminated Helium cylinder. What other possibilities are there?

Any advise, ideas or suggestions would be much appreciated.

Josh

[josh]

Thanks

As previouly said the original nearly empty Cylinder of Helium was put back while waiting for new higher grade 99.9999% helium to arrive, didn't seem to help. Tried the septum on the inlet of the column and pumped down, checked after 30 mins of pumping down and still quite alot of N2 but O2 at 4:1 ratio. Checked for leaks with 'dust-off', found none at the mass spec door or at the column to ms nut, unsure where I went wrong??

New Helium arrived, connected it and 1 hour later the N2 has dropped from the previously seen 20% to 7%, O2 is still ~0.4%??? Hopefully this will continue to fall overnight. Maybe it does take a long time for the lines to purge of the contaminated Helium, if contaminated helium was the problem. I reckon we have 50m of line.

A suggestion maybe would be to take the 3-in-1 trap off line and if a leak occurs somewhere in the line, the 4:1 ratio should return. And if N2 is trapped in the filter, this would help to find it out.

I'll follow with an update
Josh

[jdlh199]

We have a similar long-line setup with one tank connected to 4 GCs. When I had the problem linked above, I had to drag the tank down to one machine, otherwise I would have to wait hours to see an effect when switching between tanks.

Another option is to create a T in the gas line right at the connection to the instrument and stick a switch in there. Change gas tanks, then flip the switch to vent the line from the tank to the instrument with the new gas! Voila, new gas in a few minutes!

[josh]

Just to finish with this problem. It was indeed a contaminated helium cylinder that caused the problem and the contaminate took quiet a while to purge from the long line from cylinder to instrument. I will only use N6 grade helium from now on, 99.9999%, Autotunes have improved since the switch.

Thanks everyone for your replies
Josh

[JI2002]

Just to finish with this problem. It was indeed a contaminated helium cylinder that caused the problem and the contaminate took quiet a while to purge from the long line from cylinder to instrument. I will only use N6 grade helium from now on, 99.9999%, Autotunes have improved since the switch.

Thanks everyone for your replies
Josh

contaminated helium cylinder doesn't explain why you still had the leak after capping off the column at the inlet end.