Warnings - too high electron multiplier voltage

[intereco]

dear chrom and ms guru,

finally we was finnished recover our G1800 which equiped with SmartCard II and connected to PC via Chemstation B.02.01. MS was cleaned up, device start and works good, vacuum is ok, calibration vial full (but with old liquid). After 6 hours of pumping MS got following results:



Could you please comment this information - what is the condition of device and what to do to improve it. Really only replacememnt of electron multipier can help us?

thanks a lot!

[James_Ball]

Do you have good clean Helium for carrier gas? If so what flow are you using?

Definitely have a huge amount of air coming in somewhere. If I remember the GCD has the same analyzer setup as the 5971, which means you have the fairly wide rubber gasket between the top plate and the analyzer. If even a piece of lint gets on those you can get an air leak. If you have some Apeizon L grease, coat that gasket with it, then use a lint free Kemwipe to remove all but a very light film, then replace the gasket on the analyzer. I usually wipe the surface again with a Kemwipe before setting the top plate back on and when I place it on I move it slightly side to side to make sure it is sealing well. You may also want to do the same thing to the o-ring gasket on the vacuum interface.

I had a co-worker once who had a really bad leak in a 5971 and when we opened it back up we found a hair lying across the gasket, removed it and the leak was gone. It doesn't take much on those instruments to cause problems.

Once the leak is fixed, re-tune and if the multiplier is still 2000 volts or above, I would consider replacing it.

[intereco]

Do you have good clean Helium for carrier gas? If so what flow are you using?

Helium purity 99.995, flow 0.5 ml/min.

Definitely have a huge amount of air coming in somewhere. If I remember the GCD has the same analyzer setup as the 5971, which means you have the fairly wide rubber gasket between the top plate and the analyzer. If even a piece of lint gets on those you can get an air leak. If you have some Apeizon L grease, coat that gasket with it, then use a lint free Kemwipe to remove all but a very light film, then replace the gasket on the analyzer. I usually wipe the surface again with a Kemwipe before setting the top plate back on and when I place it on I move it slightly side to side to make sure it is sealing well. You may also want to do the same thing to the o-ring gasket on the vacuum interface.

I had a co-worker once who had a really bad leak in a 5971 and when we opened it back up we found a hair lying across the gasket, removed it and the leak was gone. It doesn't take much on those instruments to cause problems.

Seems I can shut up MS interface by usual needle and re-tune device in order to check whether it's leaks or dirty carrier gas?

Once the leak is fixed, re-tune and if the multiplier is still 2000 volts or above, I would consider replacing it.

Do you mean that excess air in detector can be a reason of so big voltage on electon multipier?

[Bigbear]

When you run a tune you should be in split mode, with your low flow you could be getting air in through the injection port.
In manual tune close the calibration valve and scan 16,28, and 44. If the rato 16/28 is about 1:4 look for an air leak somewhere. If you see very little 16, check the age of your gas filter ( if you use one).
For leak checking I use canned air and set the scan ions for the compound in the "air".

[James_Ball]

Do you have good clean Helium for carrier gas? If so what flow are you using?

Helium purity 99.995, flow 0.5 ml/min.

Definitely have a huge amount of air coming in somewhere. If I remember the GCD has the same analyzer setup as the 5971, which means you have the fairly wide rubber gasket between the top plate and the analyzer. If even a piece of lint gets on those you can get an air leak. If you have some Apeizon L grease, coat that gasket with it, then use a lint free Kemwipe to remove all but a very light film, then replace the gasket on the analyzer. I usually wipe the surface again with a Kemwipe before setting the top plate back on and when I place it on I move it slightly side to side to make sure it is sealing well. You may also want to do the same thing to the o-ring gasket on the vacuum interface.

I had a co-worker once who had a really bad leak in a 5971 and when we opened it back up we found a hair lying across the gasket, removed it and the leak was gone. It doesn't take much on those instruments to cause problems.

Seems I can shut up MS interface by usual needle and re-tune device in order to check whether it's leaks or dirty carrier gas?

Once the leak is fixed, re-tune and if the multiplier is still 2000 volts or above, I would consider replacing it.

Do you mean that excess air in detector can be a reason of so big voltage on electon multipier?

If excess air is causing high pressure in the vacuum chamber it will cause lower sensitivity which will cause the autotune procedure to increase the EM voltage.

Yes if you block off the inlet to the Detector, normally using a ferrule with no hole, you can eliminate the carrier as the problem.

Does the instrument have a high vacuum ion gauge so you can see what the vacuum in the analyzer is? It should be in the 5x10^-5 torr range with about 1ml/minute carrier going through. Lower is better. Higher will give reduced sensitivity. If it is too high then the oil in the diffusion pump may be low or contaminated. Also make sure there is fresh oil in the rough pump, preferably something like Inland 45 which is low vapor pressure.