Chromatography Forum

Hello everyone,

The source pressure of my MS (1200L MS/MS coupled with Varian CP-3800 GC) is too high (~500 mTorr at 80°C and ~700 mTorr at 200°C, and it should be below 150 mTorr). It's been too high since the switch of CI ion volume back to EI. Before the switch, when I turned the methane off, the pressure went down to ~30 mTorr. Then the MS was vented, ion volume changed and since then, there has been a problem.

High source pressure is likely caused by a leak. Although no changes were done to GC, I checked the fittings. They all seem to be fine. Also all the screws, seals of the vacuum interlock were cleaned/changed and it didn't help.

What is more, when the scan is run for air, there are no peaks visible, only noise. It's the same with calibration gas (no peaks are detected even with a high voltage of the electron multiplier). The intensity of the noise is lower than usual too.

I will be very grateful for any suggestions about what else can be done, checked etc.

Thank you,

Kamila

Did you try capping off the CI line and toggling the CI valve on/off? Methane shouldn't have put liquid in the line, but this will help clear it out. You may have to slowly open the CI metering valve to speed things up a bit. The turbo will probably shutdown around 10 torr, so go easy.

Is the column too long/short and no longer in the right position relative to the source volume? Its possible when you switched the ion volume that the column got hung up or even broke if it was installed into the source volume.

Longshot: run an electronics diagnostic test to see if the valves are operating properly?

I've got two of these systems baking out this weekend. One had been disconnected from CID and CI gases for a couple years and it took a while to get all the moisture out of the lines. When I opened the CID gas to 4 mTorr it nearly shutdown the turbo. I got the vacuum down to normal, but it took some adjusting of the CID and CI metering valves with the cylinders closed off. I really wish these had heated quads.

Thank you tdgcuser. We've solved the problem. It appeared to be at the detector end of the column. Probably the ferrule had been put in a wrong way round. And I'm confused here. According to the manual, the tapered end of the ferrule goes into the nut. Our technician claims that she have always put the ferrule in the opposite direction (the tapered end towards the transfer line) and there never have been any problems... Interesting...

If you use Varian MS Data Review, please have a look at my new topic LOD & LOQ wonder and Varian MS data review. Maybe you'll be able to comment on it.

Thanks,

K.