Chromatography Forum

My group has left our Agilent 6890 GC/MSDs down for a little over two years during the pandemic. They were working prior to our shutting them down. However, recently we started some new analysis requiring GC/MS analysis. Consequently, we decided to bring the instrument back up after a two year shutdown.

We changed the oil in the roughing pump and started the instrument. We are able to pump down, but when we tune we see a large air leak (28 is 50% of 69 and N2 to O2 is around 3:1), not to mention a poor vacuum 9.0 x 10-5 torr.

We decided to isolate the leak and eliminated the GC side by capping off our MSD inlet. However, we still have the leak. We have changed out the large O-ring with the same result and we have even switched analyzer side plates from different instruments (we have three) with the same result of a large leak.

There is something fundamentally wrong with our approach and I am looking for additional suggestions. Note the best pressure we have achieved during this is around 6.5 x 10-5 and the lowest N2 is around 20%.

We sprayed the backside of the side plate and we see a large leak. Does someone have a suggestion on seeding the o-rings.We have seemed to have lost our touch. We have never had this problem before.

We sprayed the backside of the side plate and we see a large leak. Does someone have a suggestion on seeding the o-rings.We have seemed to have lost our touch. We have never had this problem before.

"Seating" or sealing of the O-rings. I would get brand-new O-rings or seals, and congratulations on finding the big leak.

I think the problem has been our roughing pumps. We put a large scroll pump scavenged from another instrument and connected that to our MS (lots of hosing adjustments) and we were able to pump down with no leaks.

Our roughing pumps are old (more than 10 years). We changed the oil in one pump but it failed shortly after we turned it on in the initial start up so we went to pump two (changing oil again before use) and I assumed it was good since we were able to get our turbo pump to turn on.

I also have to admit we "reseated" the large O-Ring and perhaps it was due to our initial "reseeding" that our MS did not work. I can not rule out this possibility as well.

I usually wipe out the grove with a kimwipe dampened with P&T methanol and then gently wipe the o-ring the same way to get it clean. Sometimes the o-ring stretches a bit doing that and it does not want to seat uniformly all around. So I run a dry kimwipe around the seated ring back and forth to smooth it into place.

I've been advised in the past to put APIEZON L grease on a kimwipe and rub it together so its absorbed into the kimwipe then lightly grease the large o-ring. Followed by wiping off as much as possible with a kimwipe before reseating it. The APIEZON L grease is supposed to be a good "getter" for residual offgassing in the vacuum chamber.
Other operators say never never do this.

I usually wipe out the grove with a kimwipe dampened with P&T methanol and then gently wipe the o-ring the same way to get it clean. Sometimes the o-ring stretches a bit doing that and it does not want to seat uniformly all around. So I run a dry kimwipe around the seated ring back and forth to smooth it into place.

I've been advised in the past to put APIEZON L grease on a kimwipe and rub it together so its absorbed into the kimwipe then lightly grease the large o-ring. Followed by wiping off as much as possible with a kimwipe before reseating it. The APIEZON L grease is supposed to be a good "getter" for residual offgassing in the vacuum chamber.
Other operators say never never do this.

I have always used Apeizon L, as far back at the 5970 days. I still have a tube given to me by our Agilent service engineer more than 20 years ago. Never had a problem with it. I usually but just a bit on my finger tips and rub them together, then slide the o-ring through my fingers, this way I can feel if there are any nicks, scratches, or contaminates like lint on the surface. Then I pull it through while holding it in a kemwipe to remove any excess before putting it back in.

Our roughing pumps are old (more than 10 years). We changed the oil in one pump but it failed shortly after we turned it on in the initial start up so we went to pump two (changing oil again before use) and I assumed it was good since we were able to get our turbo pump to turn on.

Are they the small E2M1.5 or equivalent pumps that are small enough to fit up under the vacuum chamber? I have never had one of those last very long and replaced all of mine with Edwards E2M2 pumps scavenged from our old 5970s and 5971/72s. Many of them are 30 years old and have run non stop the whole time with only an annual oil change, no other maintenance. Maybe that is why they made them obsolete because they were too well built.