FID occasionally extinguishing

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

17 posts Page 1 of 2
What are we missing here?

We're having an issue with our Agilent 7890B's FID occasionally extinguishing. We bought this new several years ago, and it currently has 62,900 injections. It's single channel (on back channel), MMI, and of course FID. Hydrogen carrier and fuel provided by a Parker H2-1300 hydrogen generator, with a drier in line as an added precaution. This generator supplies hydrogen to two additional GCs. Air is provided by an oil-less compressor through a Parker HPZA-7000 Zero air generator which feeds a total of six GCs. Each GC has a Restek moisture/hydrocarbon trap (yes, overkill) for the air line feeding it. Nitrogen is used as make-up and provided from a tank. So a total of 6 Agilent GCs in the lab, with all but one functioning as usual without issue. Until recently, we haven't had any issues with the lab configuration or the specific GC. A few weeks ago, at the beginning of a weekend run, the FID flame started going out during runs. It would immediately automatically re-ignite without issue. At first it appeared to be happening at the beginning of runs, but then it became clear it was random. Here's what we've done so far on the specific GC, basically trying any- and everything regarding the detector:

1) Remove air moisture/hydrocarbon trap on problem GC.
2) Replace jet
3) Cut/reinstall column.

This looked like it helped, but about 3 days later, same problem

4) Replace entire collector, carefully inspected all detector components

We went about a week before the issue returned.

5) Now we're feeding that specific GC only with a zero air line directly from a tank.

No issue yet, but it's only been through the weekend. Again, no problems with the other GCs, which share the gas sources, run the same samples, etc.

When it first started happening, I could have sworn I saw something along the lines of "detector temp. not stable," but I can't find that in any of the logs. I thought there could be an intermittent problem with the detector's temp. sensor, but I don't know anything about how that works.

Thanks for reading.
Jake wrote:
What are we missing here?

We're having an issue with our Agilent 7890B's FID occasionally extinguishing. We bought this new several years ago, and it currently has 62,900 injections. It's single channel (on back channel), MMI, and of course FID. Hydrogen carrier and fuel provided by a Parker H2-1300 hydrogen generator, with a drier in line as an added precaution. This generator supplies hydrogen to two additional GCs. Air is provided by an oil-less compressor through a Parker HPZA-7000 Zero air generator which feeds a total of six GCs. Each GC has a Restek moisture/hydrocarbon trap (yes, overkill) for the air line feeding it. Nitrogen is used as make-up and provided from a tank. So a total of 6 Agilent GCs in the lab, with all but one functioning as usual without issue. Until recently, we haven't had any issues with the lab configuration or the specific GC. A few weeks ago, at the beginning of a weekend run, the FID flame started going out during runs. It would immediately automatically re-ignite without issue. At first it appeared to be happening at the beginning of runs, but then it became clear it was random. Here's what we've done so far on the specific GC, basically trying any- and everything regarding the detector:

1) Remove air moisture/hydrocarbon trap on problem GC.
2) Replace jet
3) Cut/reinstall column.

This looked like it helped, but about 3 days later, same problem

4) Replace entire collector, carefully inspected all detector components

We went about a week before the issue returned.

5) Now we're feeding that specific GC only with a zero air line directly from a tank.

No issue yet, but it's only been through the weekend. Again, no problems with the other GCs, which share the gas sources, run the same samples, etc.

When it first started happening, I could have sworn I saw something along the lines of "detector temp. not stable," but I can't find that in any of the logs. I thought there could be an intermittent problem with the detector's temp. sensor, but I don't know anything about how that works.

Thanks for reading.


The temp sensor/heater element can go bad and that would be the cause of the temp not stable error. Unless it gets very cold I can't see that causing a loss of flame though. If the tank you are using for zero air now has a slightly higher pressure setting then maybe it could be a problem with the EPC on the air for the FID not being stable but works good enough with the tank as the source. Try putting a flow meter on the FID and turning on only the air and see how stable the flow is. If it seems unstable could be the EPC.
The past is there to guide us into the future, not to dwell in.
Thanks James. You're of course correct regarding the temp. of the detector. I was thinking more along the lines that it was the temperature sensor reading a temp. below the ignition temp. set point, so the controller was turning of the gas flows. But writing this, I doubt it's that sophisticated.

We do have a flow meter, so we'll measure the flow as you describe when the instrument's not in use. We'll also install in-line regulators for the H2 and air lines leading into the problem GC. It isn't the GC with the most number of injections in our lab (we have several others in the 100K+ range), but it has been our main workhorse for years, so it could be time for an EPC failure.
We had this issue on one of our GCs; I'm retired now, so can't take a look at the software to look up the correct terms.

Our issue was similar of flame going out then trying/reigniting. We had to change the threshold setting on the detector from its default to a lower value; think that the default value was like 2.0 and we made it smaller, saved our Chemstation Methods with that, and issues were gone.
ConsumProdGuy above is talking about the Lit Offset. Default value is 2.0. If your FID signal goes below this, the GC thinks the flame is out and will shut down. As an example, your FID signal may be starting out around 2.2 and gradually decreasing to <2.0.

Dave
dlbenach wrote:
ConsumProdGuy above is talking about the Lit Offset. Default value is 2.0. If your FID signal goes below this, the GC thinks the flame is out and will shut down. As an example, your FID signal may be starting out around 2.2 and gradually decreasing to <2.0.
Dave


Yeah, Dave's right ! What I was thinkin' !!!
Consumer Products Guy wrote:
dlbenach wrote:
ConsumProdGuy above is talking about the Lit Offset. Default value is 2.0. If your FID signal goes below this, the GC thinks the flame is out and will shut down. As an example, your FID signal may be starting out around 2.2 and gradually decreasing to <2.0.
Dave


Yeah, Dave's right ! What I was thinkin' !!!


I agree also, I forgot about that setting on the 7890, I am used to the 6890 and 5890, but that is definitely a problem sometimes.
The past is there to guide us into the future, not to dwell in.
Ah! Excellent. Thanks so much all. This really sounds like it could be the problem. Service engineers have commented on how remarkably low our system background was, especially after so many samples. We really try to keep our systems clean and well maintained. I've never really looked into what the lit offset is and just used the default value. Will update in the future.

Thanks again.
James_Ball wrote:
I agree also, I forgot about that setting on the 7890, I am used to the 6890 and 5890, but that is definitely a problem sometimes.


We didn't have 7890 !!! But we formerly had 5830 and 5840 !!!
Consumer Products Guy wrote:
James_Ball wrote:
I agree also, I forgot about that setting on the 7890, I am used to the 6890 and 5890, but that is definitely a problem sometimes.


We didn't have 7890 !!! But we formerly had 5830 and 5840 !!!


Well now I actually feel young :)
The past is there to guide us into the future, not to dwell in.
And we also had Agilent/HP's first HPLC 1084B, built on their 5840 GC chassis.
Consumer Products Guy wrote:
And we also had Agilent/HP's first HPLC 1084B, built on their 5840 GC chassis.


The oldest I worked with was the 5995 GC/MS all in one.
The past is there to guide us into the future, not to dwell in.
Lordy I hated the 1084. What a complete POS. Of course HP recognized it and the reward was the 1090. Still one of the better HPLCs I've worked with, and if you got bored you could always play the ski game!

The Lit Offset is how Agilent rewards you for doing your job and keeping your gas supplies clean; they just randomly shut you down! Political correctness and helicopters moms joined forces to design the 7890 and thus assumed you were an abject moron and a complete slob.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
Hi, have you got the method running as constant pressure or constant flow. I had a similair issue with a 6890. When I switched from constant pressure to constant flow it resolved the issue.
So another Friday, and it happened again, this time on a dedicated air line. The system's been in use all week, doing fine. We didn't stop to do any maintenance or anything. My memory was correct: checking the system logs this time there is a message: "back detector temp. too low at 0.21" (that's retention time), followed by "back detector gas flow 0.23" (no other info, but I guess that means there's a problem with the gas flow at that time). The run then proceeds without issue after an immediate re-ignite. The detector temp is at 270 C and always reads stable, at least while we're watching it.

We just did set the lit offset from 0.2 to 0.5 per the suggestions here, but the detector temp. message suggests a different problem. Haven't had a chance to measure gas flows and add the inline regulators. We'll do that when we switch back to our Zero Air generator, since the air supply doesn't appear to be the issue.

@ Chemiger: We are using constant flow. Thanks for the suggestion.
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