Chromatography Forum

Well this is frustrating. We've had the system up and running fine for 13+ years continuously without issue. The MS is a Thermo DSQ II, purchased new 2008. It was working fine, up until a recent power failure. After that, we can't get it to retain vacuum. We've removed the column and capped the transfer line. It still won't maintain a usable vacuum. Even with the column removed and the transfer line capped, it will not pump down to <300 mtorr in 5 min (as described in the manual for acceptable vacuum performance). After a few a hours it will get below 100 mtorr. When it's that low, we spray tech duster around all the common leak points (transfer line ferrules, transfer line connect to MS, top glass seal, calibrant thimble, etc.) and do not see any signs of a leak (m/z 69, 83 in the mass spectrum). If we leave overnight, it will loss vacuum, rising to 3000+ mtorr.

- Roughing pump appears to be working fine. It was completely rebuilt in 2017 due to oil leaks. Oil was recently changed (it looked good) and there are no oil leaks. We confirmed ballast is off, and mode is set to maximum vacuum.

- The foreline at the roughing pump had hardened considerably, so we cut off about a foot to where it became more flexible. No improvement. In fact, it seems like anything we do with the foreline makes matters worse. If it were easier to do, I'd probably just replace the foreline entirely, but I don't see how to get to it at the turbo pump end.

Again, it doesn't appear to be a leak at any of the common leak point; I've dealt with those in the past, but this doesn't seem like any of those.

Any ideas/suggestions what else to check?

Thanks





John

I've had better luck with a cylinder of argon than tech duster while attempting to locate a leak. Maybe give that a shot at the same points you've already checked?

I've never used that exact system, but we had our MSD experience a leak out of nowhere and I eventually found it to be the interface transfer line itself had a small crack.

Good luck!

I had a similar problem with an ISQ once. The problem was where the hose from the rough pump connects to the turbo pump. There is a steel fitting clamped to the turbo pump. On its backside it has a threaded opening. This opening has a T-shaped brass piece connecting to the cal gas system to flush it for tuning. There an O ring was degraded. This leak cant be found by argon checking, since its behind the mass analyzer. I pulled it out, replaced the o ring and sealed it with high vac grease.

Thanks you two. Good suggestions, especially ddoerder's. That's exactly along the lines of what we were thinking, especially since anything we do with the foreline seems to exacerbate the problem. Hopefully it's as simple as that.