Inlet leak test

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

5 posts Page 1 of 1
Hi everyone,
we have been running PAH on our 7820/5977 system for a few years with pretty reliable sensitivity (2-250 ppb).
The past few months have been another story, with lower area counts, some PAH standards dropping out entirely at the lower concentration.
The mass spec side seems ok after cleaning the source, replacing the EM, and now getting good tune reports.
Still no joy.
So the GC has undergone many maintenance procedures - new column, liner, ferrule, septa clean the inlet body with solvent, replace split vent cartridge. No improvement.
About all I can think of to try on the GC is replace the inlet G1544-80570, in case ours is too far gone with active sites.

While waiting for that part I started leak checking again, and saw that the service mode Inlet Leak Check showed a small leak.
At 15 psi, the total flow is 3.7 and the column flow is 1.5. An Agilent engineer I spoke to said this is close to spec and would not cause a loss of area counts in some PAH, rather it would affect all of them if there were a large leak.
Is this right? I can't find the allowable limits for this test anywhere and without a leak tester it is hard to find small leaks.
If there's any thoughts on what direction to go here, I would greatly appreciate it.

thanks!
PAH are fairly stable analytes, normally broken down only by UV light. I have before seen tailing when the source was dirty or if the inlet was contaminated by some high boiling oils or other type of carbon contamination. Are you losing Naphthalene or the higher boiling Pyrenes, Anthracenes?

If the inlet temperature is too cold or hot them maybe that would be causing problems if it is not vaporizing them fast enough.

Also have you cleaned or changed out the copper line from the inlet to the split vent trap? When those get dirty it can cause a lot of problems. I like to just put in a piece of fresh copper tubing instead of trying to clean it. Also that port it connects to is difficult to clean, the orifice is small running into the area of the liner. I have used the aluminum oxide used for source cleaning mixed with citranox and the stick of a cotton swap to scrub it out before. You have to remove it from the GC in order to do that.
The past is there to guide us into the future, not to dwell in.
Thanks for your reply, James
We are losing the following compounds from the 16 EPA list: ACE, ANT, PYR, BAA,BAP
The rest are present, but have lower area counts than usual, approximately 5 to 10 x
The inlet and transfer line are at 315C, source 280, quad 175. These temps are the same as when we were seeing 5 ppb easily.

I cleaned out the inlet body and the copper line with qtip and solvent.

The leak seems minimal, and since we’re not seeing oxygen and nitrogen in the tune, I don’t suspect that’s the main issue

I don’t see why certain PAHs would drop out entirely while others would not.
I am also second-guessing whether or not our tune profile is satisfactory. We aren’t getting very high counts for 502 anymore but the EM volts aren’t increasing to try and improve that.
Here’s the most recent tune

Image
I've worked on this for weeks with no improvement, it's time to change tactics.
We have 7890 with a nearly new inlet. I'm going to hook that up to the mass spec to eliminate the GC side of the problem. If things don't drastically improve then it is probably the MS that needs more work.
Only slight improvement with the newer 7890. :shock:

That, and the fact that PHE has 100x more signal than ANT makes me suspect the standards now.
Is there any instrument issue that would cause ANT to drop out preferentially like that?
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