High N2 but not oxygen

[MSCHemist]

Hi all

I switched cylinders the other day and now I have N2 up 6-7% but Oxygen is at 1.3% I checked for leaks and couldn't find any in the lines nor GC connections and the ratio doesn not indicate air. Could it be a bad cylinder? The He is highest grade built in purifier.

I am also seeing random spikes averaging every 0.2 min of m/Z 40.1 argon in the TIC.

[Steve Reimer]

We have had problems with batches of He with high nitrogen. If you have another GC/MS on that tank it will show up ther too.

[cleh]

I've received bad He lately so it could be that. What grade is it? I would make sure there are no leaks in the injection port. What GC and MS do you have? Is it direct inject or do you have a purge and trap or headspace attached?

[MSCHemist]

It is Airgas's built in purifier high grade. The spikes went away late today and it went down to 4%. I'd still like it to be lower. It is just a plain old Split splitless 6890-5973 with a quickswap/aux 5. The only other GC I have is my 5890-FID. It has been running a bit elevated with nitrogen for a while ~2.5 to 3.5% but the O2 ratio never seems to indicate a leak and I've checked all the lines and connections thuroughly. The traps are old I'm not sure if that could do it.

[James_Ball]

Could be a little air contamination before the traps with the O2 being removed but the N2 passing through.

[aldehyde]

Yeah if the tank isn't itself contaminated with N2 I would suspect a leak at the regulator and your oxygen filter is scrubbing out the oxygen but allowing the N2 to pass through. Check for leaks (reseat the regulator, if you have old crappy teflon tape remove and replace it) and then check your spectrum again after 10 minutes or so.

[MSCHemist]

It ended up just going away on its own. Normally I run 2%N2 and 1%O2.

[dliebert]

As said previously, high N2 + low O2 = leak before the filter/trap. Keep your eye open for it next time you change cylinders.

[MSCHemist]

Now I have the opposite which is usual for the instrument 2.X%N2 and 1.X%O2 at a 2:1 ration. I am wondering if there is perhaps a minor leak in the gasket between the turbo pump and analyzer chamber. Also I just bought a diff pump 5973N system and the vacuum on that is 2E-5 torr and <<1% of each N2 and O2 where as my turbo (a 5 year old Pfieffer) runs a 4E-5 Torr and I can't really do pressure pulses as sometimes the vacuum gets into the high -4's and even close to the -3's. However I capped it off once and got E-6 once.

[James_Ball]

Is that a standard turbo or the high performance turbo? If high performance, that vacuum is pretty bad if the flows are the same between it and the diff pump. On my older one I get 7 E-6 torr when running 0.6ml/min helium(performance turbo) and about 2 E-5torr when running the same on my diff pump model.

[MSCHemist]

It's a standard turbo (an old Pfieffer) and the flow is always at 1.3ml/min He because of the quickswap. It is set at 4psig, 280 deg transferline, and has a 17cm 100 um restrictor.