Chromatography Forum

Hello again,
I have problems of high nitrogen content in air/water check. Wait, no need to say "search". Helium filters were changed few days ago and we though that N2 is coming from filters. When purged line was connected to GC, N2 counts decreased drastically and happy smile on face apeared, but ... After one hour Air/water check showed N2 increase (from 7% to 18%). Than there was idea of leaking line before traps. He flow was turned off on GC and closed line to GC before traps. As MSD was still working, vacuum on GC apeared and N2 increased from 18% to 100% and more. Reconected line of helium gave decrease of N2 as it was after line purge. So what a hell? There is JAS Unis 500 inlet and I think maybe it is leaking, but I dont know how to dissamble it till linear.

P.S Septum (<350 C) of this inlet was baked to surface.

You have a leak; the trick is to locate it. Check everything with a leak seeker.

Peter

Leaks in GCMS can be very frustrating & tricky to locate. Personally, i find joy in the process of narrowing down the location and fixing it .

I could type it out but there are guides online. Good luck!

But where to look: before traps or after traps? It is possible that Unis 500 inlet is leaking even only N2 is appearing? It is hard to imagine, that air could enter to line where helium pressure is 6 bar.

Air (and any gas) can diffuse upstream against a flow of gas out through a leak - especially in the boundary layer of the hole where the gas in contact with the wall is hardly moving. This forms the basis of one way of finding leaks - puff a gas or vapour with a distinctive ion in its spectrum around any suspect areas and look for the ion to appear on the MS. Silarly helium will diffuse out where there is a flow of air into a leak into the vaccuum of the MS.

Peter

only N2 is appearing?

I just want to rule out the following, under the assumption that you see nitrogen but no oxygen... Which kind of MS do you have?

Because if it's a triple quad for instance, you will most likely have a small flow of nitrogen gas naturally in your MS - which you should turn off to do an air/water check. Even then, it takes a minute or 2 for the nitrogen to disappear.

An MS source employing chemical ionization instead of standard electron impact could also explain presence of nitrogen.

(Correct me if i'm wrong, but with air diffusing into your system, both nitrogen and oxygen should show up in roughly a 4:1 ratio right?)