Chromatography Forum

I am running a Bruker/Agilent/Varian GCMS with Helium mobile phase. I'm scanning down to 14 m/z
and was really puzzled by the MS being dominated by m/z =20. Then I realized it must be Neon?
I also see Neon 21 and 22 and water at 18 and N2 and O2.

My second highest peak though is m/z = 19. What in the world can that be?

Hello

First of all if you see air and water ions you should bake out MSD to remove water and make sure there is no air leak. (Check carrier gas trap - it can be saturated)
If you're not sure where leak comes from just plug MSD transfer line with blank nut (to isolate MSD from GC) and tune MS.

Regards

Tomasz Kubowicz

I have never had a GC/MS system that I didn't see some air and water and other gasses present. Normally I get 44 CO2 and 40 Ar along with 32 O2, 28 N2 and 18 H20. http://www.sisweb.com/referenc/source/exactmas.htm looks like 19 could be F or possibly H2O(17) but that would be unlikely since O(17) has such low abundance.

The way to tell if you have a leak big enough to worry about is to look at the abundance of those ions versus m/z 69 from the PFTBA. If they are greater the 5% you could have a leak, greater than 10% and you will begin to see problems from oxidization in the source over time. If they are down around 1% they you have a seal on the system that is nearly perfect, at least as far as I have ever experienced.