HP 6850 FID not lighting

[jenseeli]

Could I have some advice or suggestions, please?
I've been having trouble with our HP 6850 GC with FID. A few weeks ago, the FID didn't light as usual when my students tried to use the instrument. I increased the proportion of H2 a little in the detector gas controls and that worked for a few days. When the problem happened again, I tried turning up the H2 even more, but that didn't work.

We use ChemStation to turn the FID "on" (although I've tried the handheld remote control too). After the system brings the gases up to their set points, I do hear the pop noise that indicates an attempt to light the FID, but it doesn't seem to light. The gas flow and the FID signal fall back to zero. The system automatically tries again several times, but I don't hear any pops on subsequent attempts. The online plot shows kind of an "h" shaped signal for each lighting attempt, with a small flat shoulder after the initial large peak.

I've changed and reinstalled the column, hoping that it would help, but it did not. We normally use an H2 generator, but I've connected a compressed H2 cylinder and I get the same problem.
I am thinking of following Agilent's FID cleaning procedure next (or maybe rebuilding the FID), but since that requires significant time and cost, I wanted to ask you all first for advice.

[James_Ball]

How long since it was last cleaned?

If the jet that screws into the bottom of the detector becomes dirty it can cause the flow from the column and makeup to be turbulent and more difficult to light, also the stainless steel collector can become dirty and have a similar effect. Cleaning is simple and probably takes less time than cooling down and heating back up.

Another problem is the igniter can stop working, they have to be replaced every few years. Have you tried to hold a lit lighter (the long stem ones for lighting a grill work best) over the top of the FID for a few seconds and see if that helps it to stay lit?

There is also a setting in Chemstation that sets the minimum signal required for the detector to be considered as lit. If the signal drops below that it will try again but eventually shut down to prevent hydrogen from building up and creating an explosion hazzard. I can't remember the exact menu item but if you lower that it may fix the problem.

[Consumer Products Guy]

See above response. And on a couple of units we had to gently blow into the detector to light the flame.

[AndyA]

I've seen this before on our 7890s.
When you try to ignite the FID try gently blowing on it and this should help it to start. if this works then chances are you had the same issue as us and its the ignitor glow plug is starting to go. Its a simple job to replace and a new one is around £45 and only takes a few minutes to do.

Cleaning the FID won't do any harm either I've tried the Agilent cleaning kit and find it tends to scratch up the metal components. for the most part we just soak the metal components in methanol and sonicate them for a few minutes and leave them to dry in the air before re installing. taking the FID apart only should require an alan key and a nut driver (for removing the jet) most of the components are screwed together.

[Indiana2525]

See here: https://az621941.vo.msecnd.net/documents/d46e0989-82d4-4995-bd99-795864a409d4.pdf

[Peter Apps]

Check the actual flows for hydrogen, air and makeup using a flow meter, not the onboard EFC readouts.

Hold a cold, shiney object at the outlet of the FID - if you see condensation there is a flame in there somewhere, but it is not burning at the flame tip.

Peter

[Oatman]

Recently our GC/FID has been having troubles igniting its flame. So far I have checked the air gas line leading into the machine MyKFCExperience, trimmed the back column going into the detector, and cleaned the jet via the sonication procedure.

[Consumer Products Guy]

Recently our GC/FID has been having troubles igniting its flame. So far I have checked the air gas line leading into the machine, trimmed the back column going into the detector, and cleaned the jet via the sonication procedure.

State whether the ignitor glows while trying to light the flame (visual check); does it even pop ?

Sometimes just loosening the ignitor a tad then re-tightening it restores good electrical contact.

[Peter Apps]

Recently our GC/FID has been having troubles igniting its flame. So far I have checked the air gas line leading into the machine, trimmed the back column going into the detector, and cleaned the jet via the sonication procedure.

Review previous comments and try what they suggest.

Peter

[LALman]

Monitor if you are getting a signal by using the "Det Control" button. You may hear the pop of it lighting but if you see no signal (e.g. 10-20) it immediately cuts off the hydrogen flow. On my 6890 I had a brand new detector. After a few months it started refusing to light. It turned out to be the spring that touches the disk of the stack in the detector. That spring needs to make firm contact with the rim of the disk. Its OK to stretch it a bit so it reaches.

This is fresh in my mind because my FID just started getting noisy signals and after cleaning jet, and changing inlet septa and liner, all to no effect; I realized it might be the spring not making contact. I stretched it just a bit and that solved the problem.