Chromatography Forum

Hi all,

I have a GC7890A system, with a 5975C MSD. Both were serviced in late December and I have since started to notice an increase in the Nitrogen + O2 levels. Before service, levels were below 1% for N2 and ~0.3% for O2. I (stupidly) did not tune after the engineer's visit and did not use the MSD until a few weeks later, when I noticed an increase to 6% in N2 and ~1.5-2% in O2. In all cases, the water has remained below 0.6%. At this point, I also noticed a decrease in the relative abundance of m/z 502 (above 5% to below 3%).

Since my lack of tuning, I can't pinpoint the exact time at which the leak appeared but since the only maintenance on the MSD was the service, I assume it was at that point. I think that the levels of air with low water show that we might have a small leak (?) and if this was the case, I'd rather fix it before it gets worse. I have re-tightened the nut at the interface and did some inlet maintenance in case this was the cause. I am now thinking about checking the o-ring at the side door of the MSD.

1-Do you agree with a small leak being present?
2-If so, are there any other obvious places than the side door to check (we have limited leak-checking capabilities)?
3-Given that we only use our MSD in SIM mode (m/z 62), is the decrease in relative abundance at high mass an issue?

Thanks for help!

I don't think it's much of an issue.

To check if you have a leak at the MS side, use a can of dust remover spray (to remove dirt from your keyboard). Put the MS in scan between 10-100 m/z and spray on the interface nut while checking the mass spectrum. If a peak appears due to spraying, you can tighten it slightly and check again. Same trick for the MS door.

I don't think it's much of an issue.

To check if you have a leak at the MS side, use a can of dust remover spray (to remove dirt from your keyboard). Put the MS in scan between 10-100 m/z and spray on the interface nut while checking the mass spectrum. If a peak appears due to spraying, you can tighten it slightly and check again. Same trick for the MS door.

This is the best method to find a leak in MS. If you do not see a spike in the spectrum from the dusting spray( normally in the 55-85m/z range depending on the chemical in the spray) then it is most likely just a shift in the relative mass abundance across the scan range due to the maintenance. Since you noticed a lower 502 peak, it could be the 28 and 32 peaks are now a little higher than normal. Another tune may fix it if you don't find a leak.

Thanks to both of you for the replies. I will give the spray a go and see what I find.