Random sharp air peaks

[zegockie]

Hello,
I wonder if somebody could suggest the origin of random sharp air peaks in my chromatograms any other run and at various RT. Vacuum is OK, oxygen and nitrogen are low. I changed both ferrules, changed inlet to back, tried isoterm runs to eliminate change of temp influence - the same result. Cannot determine the origin. Please hint if you have ideas.

[DR]

Examine them very closely - if they actually look like peaks, then you have a leak. If they're straight up and down spikes, too narrow and squared off to be peaks, it's an electronic problem.

[zegockie]

They are peaks, just ion 32 (I have cut off at 28.5, so I don't see nitrogen). Ok, but what part of GC/MS or helium supply line? Helium supply itself is fine, there are no peaks on other instrument. They develop randomly, at various RTs, pretty much the same intensity. MS vacuum is very good.

[cleh]

Sounds like you have a GC/MS. Please post GC and MS models. What type of injector? Split/splitless or split injection? Do you have an autosampler and, if so, what type/model. Also would be helpful if you post a scan from mass 10 to 650 from mtun. Is there a He purifier in line to this GC that doesn't go to the other instrument you mentioned that is sharing the He cylinder? If so, what kind and how many cylinders have you used with it? Have you changed the septum?

[zegockie]

The system is Agilent 7890/5975C; Standard split/ splitless inlet; split ratio currently is 50:1; Just changed gas trap, liner, Merlin septum, column, switched inlets. He supply is common for several instruments - no indications of air peaks on others. Standard autosampler, nothing unusual. Each instrument has a dedicated gas trap. Autotune data is as usual.
I use self-tightening nuts - it looks like vacuum is good.
Those peaks appear randomly, around 500k intensity, looks like with minor not tailing, but I would say the right side is less steep than the left. One or two per run, sometimes there are no peaks at all.

[Peter Apps]

As a troubleshooting excercise try replacing the Merlin seal with an ordinary septum.

Peter

[zegockie]

I determined that MS part is a problem - likely one of gaskets is "burping". Side plate gasket looks OK.

[cleh]

I determined that MS part is a problem - likely one of gaskets is "burping". Side plate gasket looks OK.

I would still clean the side plate gasket, groove it sits in, and analyzer surface it sits against with MEOH. When you pump down, press your hand firmly against the side plate and turn on the power (don't insert the screws on the side plate). When the rough pump clears the air from the system, you can take your hand off and screw the front screw into the analyzer just tight enough to keep the door from opening if you should lose power.

[zegockie]

To clean a groove - do you suggest to remove rubber seal from the groove? I afraid to damage rubber while doing this.