strange issue inlet-FID detector problem...

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

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Hi everyone,
I am working with a GC FID 6890 Agilent. The column installed is a 530 um column (BP21 SGE). Our method works with nitrogen as a carrier gas, at a column flow of 1.6 (16 cm/sec), and a split ratio of 1:15. All giving a pressure in the inlet of 1.4 psi and a total flow of aprox 27mL/min.
We have been working with this method for about 3 years.
Slowly, we started noticing problems to keep the pressure at 1.4 in the inlet, until the GC actually gave up. We can not keep a pressure lower than 2psi. This led as well into problems keeping the flame on in the detector. The flows of H2, air and make up N2 are well mantained, but it can not keep the flame on.

This is the isolation of the problem that I have been doing:
- Starting from the detector. The JEt and the FID have been cleaned (by a technitian), and we see that flame can be mantained when instead of the column there is an stopper nut attached to the FID. So we do not think it is the FID itself, but rather leaks upstream from the detector.
- We have also seen that the FID can keep the flame on when the column is installed in the FID, but not connected to the inlet. Instead, we cap the column with a septum in the inlet side. This tells me that the column itself is not damaged.
- When I install the column in the inlet, the detector fails to ingite. I have tried with several pressures in the inlet (2-8-10-25 psi), and the same thing happens. (I can not keep any lower than 2 psi)
This would tell me that the issue might be related to the connection between the column and the inlet, and might be the same reason why we can not keep pressures lower than 2 in the inlet.
- To see if we had porblems in the connection between the inlet and the column I left only a few cm of column in the inlet and capped it with a septum. The inlet could mantain without problems the pressure at 1.4psi. So this tells me that the issue is actually not in the connection between the inlet and the column. So now I do not know anymore.....

Any ideas??

Thanks in advance

Anna
Is there a problem with the carrier gas? Perhaps if moisture is present the FID cannot be lit. Are the moisture traps okay. If the carrier gas is not pure maybe the EPC cannot set the flows correctly? If the flow is too high maybe it is extinguishing the flame?

I found this note useful previously

https://az621941.vo.msecnd.net/document ... a409d4.pdf

Kind REgards
Steve
Dear Steve,
Many thanks for your answer. The file you provided was useful. I do not think it is most in the traps, because the FID can light perfectly only with the make-up flow of Nitrogen (without the flow from the column). I am suspecting now that it might be that the flow through the column is not what it is saying it is, but a lot higher...
That makes sense.
If the flow through the column is very high maybe that could extinguish the flame or prevent the flame from lighting? What is causing the flows to be too high; if you have checked all the common possibilities, maybe the EPC is faulty? If you have a flow meter you could measure the flow at the split vent. You could aslo measure the flows at the FID with and without the column connected.
If it's the EPC you will probably need an engineer to come and change it. I think the 6890 EPC is one module but if you have dual channel system you could swap the EPCs over to check or maybe more easily install a column on the other channel and see if the problem is still there.
Hello

Check Lit offset for FID detector. If it is 2.0 change it to 1.0 or 0.5
Sometimes that is the reason for flame going off.

Regards

Tomasz Kubowicz
A possibility on a 6890 is to check the inlet to see if there is a crack in the weldment. Put some helium as carrier gas to the system and use a leak detector to see if there is a break somewhere. I have a 6890 GC-FID and I am using hydrogen as my carrier with nitrogen makeup gas. First setting up, the tiniest leak wreaks havoc with flow control.

On another 6890 I had a break in the weldment that was invisible (until I wiggled it to see the crack that my leak detector said was there). Replacing it took care of the problem.
tkubowicz wrote:
Hello

Check Lit offset for FID detector. If it is 2.0 change it to 1.0 or 0.5
Sometimes that is the reason for flame going off.

Regards

Tomasz Kubowicz


This can definitely be a problem. What is the signal of the detector when it is first lit? If it is near 2.0, then it will turn off the gas if the signal drops below that threshold.

Also check the column flow at the end of the column, if there is a problem with the EPC as mentioned above, you could be getting a much higher flow than you expect. The pressure sensor can go bad and you may have 10 psi instead of 1.4 psi which would cause many problems.
The past is there to guide us into the future, not to dwell in.
Anna_Bur,

Problem with a leak will be very hard to track down with a normal leak detector when using nitrogen as the reference for most leak detectors is air.

Make sure the o-ring on the bottom of the inlet nut on the 6890 is on the correct side of the Gold seal, underneath on the oven side. this can be a source of leakage.

Are you already running nitrogen as make-up? FID's are still made to have to run with higher flow rates than you are providing via you column so you need to have make-up running to make up the difference. Also, make sure you are using the appropriate jet since you are using a capillary at low flow.

Finally, are you using puff-the-magic dragon across the top of the detector to get it to light?

Best regards,

AICMM
Hi Anna,

Never trust GC flow values in my opinion. I am using a revamped perkin elmer Clarus 500 (from 2004) and I prefer to use pressure set points for the packed column carrier gas. Check with a mass flow controller or bubble flow meter if necessary. Only use flow for the PPC detector modules (TCD and FID).

On a different matter, do you know how to calculate column retention times for Oxygen and Argon? I am using a Haysep DB packed column.

I am using Helium in my TCD internal standard and packed column carrier gas. 30 ml/min of Helium as TCD ref and 140 kPa (approx 30) of the packed column carrier.
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