Seeking general advice for resolving air leak

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

8 posts Page 1 of 1
Hi, new to the forum, and seeking advice on resolving a gas leak in a dated instrument that I am just starting work on (GC-MSD 6890 5973). This instrument has been off-line for a few years and is being used for analysis of THMs and VOCs, and has a purge and trap attached.

Planning on beginning with replacement of the interface column nut and ferrule. If that doesn't work, then moving on to the inlet septum and liner. The MS appears to be holding vacuum and temperature.

Please let me know if anyone has any other suggested starting points, or general advice. Also does anyone have any recommendations for the easiest way to accurately determine the status of the leak? (I have been using manual tune, and just trying to monitor the counts of 28 and 18 to see if they are dropping).

Thanks for any help!
GC Tech under development...
Welcome to the forum.

You need to get yourself an electronic leak seeker - where air is leaking in, carrier gas will be leaking out.

Peter
Peter Apps
GrantWeink wrote:
..."Please let me know if anyone has any other suggested starting points, or general advice. "...


Good starting point is to find out whether a leak is in MS or GC.
Remove capillary column from transfer line and cap it off with no-hole ferrule.
Check if a leak still persist.

BTW - what are the symptoms of the leak you mentioned ?
Another way is to get some canned air and spray around suspected spots while scanning. Those spots are, the MS door (there is a little machined out spot to flood the entire door at the lower edge near the cal valve), the connection between the MS box and transfer assembly (get the spray nozzle into the insulation shroud between the GC and MS) . If the MS has CI, then there is a welded pipe on the transfer line that can fail.
Not as common, but more difficult to troubleshoot is a leak in the interface itself. It is made of two different size pipes welded together and it can leak at the internal weld. With the interface cool (AUX turned off) spray some Dustoff into the insulation behind the interface nut. If you spot a leak then it could be inside the interface itself.

As someone asked, what is your criteria for a leak? Are mass 28 and 18 more than 10% of mass 69 when scanning in manual tune? If so then a leak is possible. If it is 20% and falling, then it could just be air and water baking out of the ceramics in the source if it has been at atmospheric pressure while it was sitting idle.
The past is there to guide us into the future, not to dwell in.
You can use your MS as a leak detector as noted. I like to set it in prof mode and set one peak to mass 40 (Argon). Then bleed argon at all the possible leak points and see which ones cause Argon to spike up.
Thanks all, I used the air canister trick and in spraying areas (i.e. the inlet and the GC/MS interface) found increases in peaks 51 and 65. Going to re-vent the system and try to re-seal these areas (as the leak(s) were indicated at all points).

Is there also an o-ring in between the GC/MS interface which can leak and be a problem?.

Originally the air leak was assumed due to very high levels of N2 (28) and h2o(18), in the system of 60% and 6%, relative abundance.... still working on it.
Yes there is an O-ring inside the interface. To get to it you need to vent, remove the column from the MS. Pull the MS away from the GC, and under all the shielding and insulation there is the interface fitting screwed into the vacuum chamber. there is an O ring sealing this connection. I would think this is a low probability leak site.
Highest probability site is the large door O ring. Check that the O ring is inserted completely in the groove. Every time I vent I put some MeOH on a kim wipe and go over the O- ring, and the door's sealing surface prior to closing the door. When pumping down push on the "feet" on the electric board while turning on the MS. If you use the screw to hold the door closed, do not tighten it until the turbo pump has come up to speed.
Easy to replace, but if you don't have one, you can remove it and grease it with a very small dab of hi vac grease. Completely distribute the grease with your fingers THEN wipe off as much as you can with kim wipes.
There is also an O ring sealing the pressure sensor at the rear of the vacuum chamber. You might want too check if the mount is hand tight.
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