Chromatography Forum

We have a Agilent 7820A GC with two symptoms. 1. The air pressure actual is much higher than the setpoint (~500 vs. 400). 2. The FID will not stay lit.

Just so you know everything that has been done:

-The GC has been leak tested numerous times and I have found no leaks.

-I have changed out everything in the inlet imaginable just to be safe (liner, septa, gold seal and washer, O-rings). Cleaned out the septa nut and casing unit and checked all nut fittings and tightened them.

-I changed out the split vent cartride and O-ring.

-Changed out the jet in the detector. Cleaned the chimney assembly entirely. Re-installed and even tried to get some friction but unscrewing/screwing the screws to try to get it to ignite. Performed jet test by removing column from detector and setting H2 to 75mL/min, with makeup turned off. 0 flow indication in makeup so no pressure backing up into makeup channel. So jet test passed.

-Tried adjusting the H2:Air ratio to optimal range by using the acutal air pressure output of 500 instead of the set point of 400. This allowed the FID to stay lit for approx 2 min. Then went out again, this is also deviating from the method. The glow plug is fully functional.

Anyone have any ideas for me to try? I probally need to manually check the air flow, can someone tell me how to do this? Also could this be an issue with the EPC? I tried restoring all pressure sensors back to 0 but no luck. I am not familiar with trying to resolve any sort of pressure issues or the EPC at all, and we are missing our senior technician.

Any help would be great. TYVM
-Summer

Are you sure that the indication of 500 is for pressure and not flow ? And if it is pressure what are the units ? How are you setting the pressure / flow to 400 ?

You next step needs to be to measure the actual flows of each gas through the detector using a flow meter that is independent of the GC's onboard gas controls.

Peter

Hi

Out of interest when you did the jet test did you have the column installed or blanking ferrule?
As Pete points out you need to manually check the flow ( and yes I believe it is a flow @ml/min ). My guess is your EPC is faulty if it is reading incorrectly and that air flow could be just about anything!!

do you have moisture filters/traps on your system gases? trace water can cause the FID flame to die.

My experience has been that in this situation, you want to run higher than normal on the hydrogen not the air. More air just dilutes out the fuel. Try to feed less air and see if you can get it to stay lit.

It should work at Air:H2 350:35 (what I run on my 6890's). For my old 5890's I usually run 300:30.

We have a Agilent ... GC

I probally need to manually check the air flow, can someone tell me how to do this? -Summer

There is a flow adapter from Agilent (likely got one in your start up kit) that press-fits into the FID. So turn off the H2, the make up gas, and the carrier gas to measure the air flow with a bubble meter or flow meter. Similar to measure the H2.

We have a Agilent ... GC

2. The FID will not stay lit.

Anyone have any ideas for me to try?
-Summer

Summer: check the software, under GC parameters, Detector, to see what the "Lit offset" is set for. Typically, these default to 2.0 pA, which can cause detector signal to drop below 2.0 and extinguish flame, so tries to re-ignite periodically, like you describe. So we set our Lit offset to 0.5 pA and don't have this issue any more. And that value gets saved with the Method.