Inlet not reaching set point pressure/split vent line

[kelint]

Hi,
The split vent line has recently been cleaned on our GC (7890B model) and I believe since then the inlet pressure has been unable to reach set point (pressure is below 1psi – a major leakage!). I was wondering if an improperly installed split vent line can cause such problems?

We use split mode in our GC operations.

I tried re-tightening the fittings on the split vent line and in the inlet area and still no change. I will do a thorough leak check next time, but I am interested in anyone’s opinions and experiences in the meantime.

Many thanks.

[benhutcherson]

One of the 7820s(stripped down/more basic 7890) I look after likes the septum nut snugged down just a bit past finger tight-I doubt that it's more than 1/16 of a turn or so, but none the less it can throw this error if you don't put a wrench on it and snug it just a bit.

I looked at another one the other day(I don't ever use that particular one, but get asked sometimes to do maintenance/repairs on it) that was shutting down the inlet because it couldn't reach the pressure setpoint. I checked the counters and found 500+ injections on the septum, and when I pulled it the hole in it was big enough that I could see light.

Check the easy/obvious stuff(septum issues) first, but with that said there's actually a fair bit of plumbing between the injector and the split valve in the standard split/splitless injector-it runs from the injector to the trap, then from the trap to the valve before venting out to atmosphere. I'd take a careful look at all the fitting that were in any way disturbed when the line was replaced.

[Peter Apps]

A leak anywhere in the inlet plumbing, including at the septum, can cause low inlet pressure. Check with a leak seeker, not by dripping liquids on the connections.

Peter

[Bigbear]

I've never had good luck cleaning the split vent line. I just make a new one with 1/8" copper. I only has to be approximately as long as the one you are using. Length not that critical. Also check that the vent trap is not a problem.
Over tightening the septa can cause rapid coring. It only needs to be finger tight, in a SS injector all that's needed is to turn the nit until the C clip stops moving with the nut.

[James_Ball]

I've never had good luck cleaning the split vent line. I just make a new one with 1/8" copper. I only has to be approximately as long as the one you are using. Length not that critical. Also check that the vent trap is not a problem.
Over tightening the septa can cause rapid coring. It only needs to be finger tight, in a SS injector all that's needed is to turn the nit until the C clip stops moving with the nut.

Tightening until the C clip was loose or maybe a fingernail gap between it and the nut was always my gauge before, but I have a 7890B now that will not seal unless I go much farther down with the nut. I think maybe the new ones have a slightly different design, it is one with the yellow slide for locking the weldment to the port instead of the large green or gray nut that you need the special wrench for. I switched that one to a Merlin Microseal and haven't had trouble since.

[kelint]

Thanks very much for all your responses.

It turns out that the nut on the split vent line wasn’t screwed in tight enough. My colleague had to bend the copper wire to make nut fit into place.

[dblux_]

A leak anywhere in the inlet plumbing, including at the septum, can cause low inlet pressure. Check with a leak seeker, not by dripping liquids on the connections.

Peter

What would you suggest if not helium but nitrogen is a carrier gas ? (apart from this particular MSD case)

[James_Ball]

A leak anywhere in the inlet plumbing, including at the septum, can cause low inlet pressure. Check with a leak seeker, not by dripping liquids on the connections.

Peter

What would you suggest if not helium but nitrogen is a carrier gas ? (apart from this particular MSD case)

The electronic leak detector from Restek will detect Nitrogen leaks but with a little less sensitivity than hydrogen or helium. Considering air is about 80% nitrogen, when it increases to near 100% at the detector cell it still causes a detectable signal, since they are just a simplified TCD.

[dblux_]

The electronic leak detector from Restek will detect Nitrogen leaks but with a little less sensitivity than hydrogen or helium. Considering air is about 80% nitrogen, when it increases to near 100% at the detector cell it still causes a detectable signal, since they are just a simplified TCD.

James do you really think that 22655 from Restek is A LITTLE LESS sensitive for nitrogen than for helium ???
Man, it's 140 times less sensitive ! Really no use in GC with nitrogen carrier.
But if you hipothetically fill up your GC room with helium and conduct zeroing 22655 in helium atmosphere than yes, detecting small nitrogen leaks would be possible

[James_Ball]

The electronic leak detector from Restek will detect Nitrogen leaks but with a little less sensitivity than hydrogen or helium. Considering air is about 80% nitrogen, when it increases to near 100% at the detector cell it still causes a detectable signal, since they are just a simplified TCD.

James do you really think that 22655 from Restek is A LITTLE LESS sensitive for nitrogen than for helium ???
Man, it's 140 times less sensitive ! Really no use in GC with nitrogen carrier.
But if you hipothetically fill up your GC room with helium and conduct zeroing 22655 in helium atmosphere than yes, detecting small nitrogen leaks would be possible

I have detected small nitrogen leaks around ECD fitting with it before. It may not be as sensitive for nitrogen as it is for helium, but it is as sensitive or maybe more sensitive than soap solutions.

[LALman]

My older 5890 GC-FID would not seal well at the inlet until it had reached temperature. I always faked it by setting inlet pressure very low until the inlet temperature had been reached. It seems to work best to adjust tightness of the septum when the inlet is at temperature.

I thought I had a split vent problem when I first started trying to revive a used 6890 plus. The test block narrowed it down to the EPC needing to be reworked.
The optional split vent assembly (Restek catalog #23031) make a more reliable seal than the original split vent assembly. I don't know if these are usable for your more recent model and that goes for the fittings below. For which I only know 6890 equivalents.

You can buy the little EPC Test Kit (Restek catalog #24323) for your EPC to help isolate problems. o-rings for the weldment EPC interface are Restek catalog# 22064 (O-rings, 25 pack).

[dblux_]

I have detected small nitrogen leaks around ECD fitting with it before. It may not be as sensitive for nitrogen as it is for helium, but it is as sensitive or maybe more sensitive than soap solutions.

The term "small leak" is not defined anywhere. But it had to be at least 1.4 µL/sec to be detected by Restek 22655.
Bubble formed in Snoop growing 1.4 mm3 every second is easily visible and this is the measure of Snoop "sensitivity".