Varian 3900 GC Split woes

Discussions about GC and other "gas phase" separation techniques.

2 posts Page 1 of 1
Hi All, this might be a long one so buckle in.

We have a Varian 3900 gas chromatography instrument and recently have been having trouble with using it in split mode. When we try to set the method to split mode starting at a ratio of 50:1 we found that the GC would never stabilize because the actual pressure would always be higher than the set pressure. Reducing the ratio to 40:1 would rectify the problem, but this is apparently just kicking the can down the road as we are now having the same issue at a 40:1 split. Running splitless has no issues other than it's running splitless.

Investigating here and other places on the internet I found out that the split lines are something that should be cleaned out with some regularity - apparently this had never been done here before!

So, I rinsed the split lines from the split trap with various solvents and collected an interesting cocktail of brown components. When I reinstalled the lines however the issue persisted, though at slightly lower pressure. I also rinsed and dried the split trap at this time too.

I'm stuck for what the issue could really be. We've talked to technicians from where we bought the instrument (it was second-hand) and they are recommending we replace the gold seal and split trap to ensure it's not something easy before we replace the EFC (EPC?).

My questions are:

Has anyone had similar issues? Did you solve them?

Does anyone have a good explanation on how the split vent valve works? Things are gradually pointing towards an issue with the EFC not controlling the valve correctly, I'm hoping to learn how it mechanically functions so I can figure out if that's the issue.

Are Gold seals standard in most machines? I've seen that agilent sells them but I don't know enough to say if they're the right size for our machine.
The easy bit first - there are no gold seals in a Varian.

First step is to measure actual gas flows at the split outlet and septum purge outlet with an independent flow meter. Then measure the inlet pressure with a gauge connected to a needle poked through the septum. That will tell you whether the EFC's readings are accurate. If not there is a procedure to recalibrate it - check the manuals.

If they are accurate, it sounds as if you have gunk in the plumbing downstream of the inlet that is restricting the gas flow. You might be able to clean it by flushing solvent through, but beware that it might affect seals along the way.
Peter Apps
2 posts Page 1 of 1

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