Chromatography Forum

I have a 7890A/5975C GC/MS (equipped with a turbopump) that stopped working. I first noticed the foreline pump wasn't operating and the MS temperatures were low. I tried power-cycling the MS and pumping down a few times, and each time the turbopump was able to get up to about 10% of speed before it (and the foreline pump) would shut off. It appears that it's able to pull some vacuum, as air rushes in when I vent the system.

Is there a way to tell whether it is the foreline pump or the turbopump (or something else) that is having problems? It seems that it's the turbopump, since I'm only able to reach 10% speed, but I want to be certain before forking over the money for a new one and attempting to install it myself.

I've also seen some posts about electrical communication issues causing this problem, but I'm not sure whether that applies to my particular setup.

Thanks for your help!

Does the foreline pump sound like it is still pulling air? If you can plug it in directly, not into the MS, and block off the inlet it should quiet down very quickly.

A bigger question, is the door not sealing or do you have a large leak somewhere else?

The door is sealing and it doesn't sound like it's pulling in air

It may be that the system doesn't read the vacuum correctly. What does the MS display for vacuum when pumping down? Does it get to a meaningful number?

Thanks for your reply, Steve! I do recall seeing a very high number for the pressure (9.9e9 or something like that). The Agilent technician suggested that I should expect the gauge to not work when we're not able to pull a high vacuum (when the turbo pump fails); has that not been the case in your experience?

You might get better responses on the GC/MS forum.

It sounds like either the system is leaky, the foreline pump is not working well, or the turbo is dying. Are there any aberrant noises from the turbo ie bearings bad?

Otherwise the most common culprit is the side plate o-ring. It isn't stated very prominant in the manual but it is recommended to put a very small ammount of Apiezon L vacuum grease on the side plate and vent vale o-rings. Just wipe them off with a methanol soaked kimwipe, smear some grease on a kimwipe and wipe that over the o-ring, then wipe off any excess with a clean kim wipe. Again a very small ammount of grease. If the o-ring is in bad shape replace it. On the 73 the vent o-ring is a standard gc inlet o-ring and sits in a groove on the analyzer, on the 75 the o-ring is on the vent knob and I can't confirm if it is a standard inlet/liner o-ring.

You can do a quick check on the foreline pump by disconnecting it from the MS and checking if it pulls a good vacuum on startup. I've had a Pfeiffer fail to pump at all but still sound OK.
I have a 5973 that I have to hold the door closed on start up because it has a slight warp and won't seal without help. That one also had the seal fail at the high voltage line (looks like a car ignition wire.)

If the vacuum is good but the MS doesn't read it correctly, it's time for a service call.

BerkeleyPhD wrote:
The Agilent technician suggested that I should expect the gauge to not work when we're not able to pull a high vacuum (when the turbo pump fails); has that not been the case in your experience?

My gauge on my 5975 works fine with just the foreline pump on - it reads something like 1.5e-2 torr.

The foreline needs to get the vacuum down pretty low for the turbopump to stand a chance.

On my 73 I had a stupid problem where the MSD would report the foreline pressure in mtorr and Chemstation would think it was in torr and thus 75mTorr was read by Chemstation as 75 torr and too high so it would not start the high vac pump. But that was a 73 and diff pump. The solution to that was to pump it down from the instrument console and start Chemstation after pumpdown was complete.

It may be that your ion gauge is not working. Disconnect the ion gauge from the mass spec and restart the system.

If you have some Dustoff and can open the chamber and blow a little on the fan of the turbo and it spins freely and continues to spin then it is ok, if the turbine stops within a few seconds, the bearings are failing.