Chromatography Forum

I'm trying to get a system that was in storage for the last few years back up and running. It is an Agilent 6890/5973 set up with a diffusion pump (DP) for the high vacuum. The foreline pump is a pfeiffer duo 2.5.

The issues i'm having is that i can't get the system down to any lower than 124 mTorr. I've tried duster on the side panel and inside the GC at the transfer sleeve and can't find a leak. When i cap off the MS it gets down to ~15 mTorr and if i add the column and cap the front it gets down to about the same.

But the moment I hook it up to the inlet it shoots up to 120-140 mTorr. So far I've tried replacing all the seals I could find, it was a Restek inlet and had 3, and then replaced the weldment assembly entirely, with one from Agilent, with absolutely no affect what-so-ever and now I'm out of ideas.

Does anyone have any suggestions?

Thanks in advance!

If that is the rough pump vacuum then it isn't terribly high. 15mTorr is good with no flow and when you add column flow it should go up to around 70mTorr with 1ml/min helium. If it has been sitting a long time then it will take a while to come down completely as the Diffusion Pump oil degases and the traces of moisture and air bake out of the source parts.

Do you have the ion gauge on the unit? If so what vacuum does it read?

The 140 mTorr is with the DP on hot, i can see the oil bubbling. the ion gauge is showing 5*10^-5, which is about what i expect at idle.

Typically what I'd expect from the pump down is ~70 mTorr, the other unit that is set up identically usually sits about 63 mTorr, with a similar reading from the ion gauge that i have now with the constant flow of 1.2 mL/min that we're running.

I hadn't thought of the DP oil off gassing. Prior to turning it on the first time i replaced all the fluids having no real idea how long they'd been sitting there. I had let it set while on overnight to come in with the same 140 mTorr.

If you can get the correct analyzer vacuum and not be seeing excessive air and water then you should be good to go. It could be the rough vacuum gauge is a little out of calibration or just dirty.