FID Loss of Sensitivity

[itspip]

Hello all, I'm having some issues with my Agilent 7890A with FID. We recently had an issue where we lost sensitivity. This was not gradual and seems to have happened overnight. Our solvent peak had a height of ~30000pA and now the solvent peak has a height near 5pA.

I took out the jet and cleaned it with solvent and a cleaning wire. There don't appear to be any deposits on the jet. When the flame is on and the instrument is idle the signal is ~1.8pA.

I checked for leaks in the lines and could not find any. The gas cylinders have plenty of pressure left in them and the column nuts are snug.

I have recently replaced the septum and liner thinking that the issue may be there. Replacing the liner and septum yielded no change in the sensitivity.

Here is where I have some possible side questions. The primary method calls for a 100:1 split, but the liners are splitless. When I changed the method to reflect a splitless injection the peakheight of the solvent remained unchanged, ~5pA, but the peak did broaden.

The injection port is at 250˚C and the oven starts at 100˚C and ramps to 300˚C at 15˚/min.

I'm not as familiar with GC as I am LC, so my questions; what is my next step with the loss of sensitivity? Should I replace the liner with a split/splitless one?

[uzman]

An easy check for FID electronics : touch the FID chimney with a syringe needle , you must see a very high signal .

[la_donz27]

Hi,

First thing I would do is re-install the column with a new ferrule into the injector. It may have loosened, slipped down, or something.

Also, you can pressure test the inlet by putting a no-hole ferrule in the nut and pressurize the inlet. Turn off the pressure then watch to see if it drops over 5 minutes. Agilent says it shouldn't drop more than 0.1 psi over 5 min.

I have also tested for column fractures by putting a septum on the column end (pull it out of the FID) and pressurizing the inlet similar to above. Pressure shouldn't drop much at all if the column is fine.

Second, if you were getting your 30000 pA with that splitless liner, then that's a side issue and will have nothing to do with the sensitivity loss. But I would suggest using a split liner once you solve your sensitivity issue.

Hope this helps, good luck!

[itspip]

Thanks for the responses.

I tried your trick uzman and got responses. So I guess that means the electronics are functioning as they should.

la_donz27, I did reseat the column into the injection port with a new ferrule. We don't have any no-hole ferrules to check the injection port to see if it is holding pressure.

The higher ups don't seem as concerned about fixing the issue as I am so our samples are going to be run on our GC/MS.

I'll keep you updated if I manage to get the problem resolved. Thanks for your help everyone.

[la_donz27]

Hi, again;

If you don't have a no-hole ferrule, you can easily make one if you have graphite ferrules on hand. Just install the ferrule and column nut without the column, and it will squish down to solid as you tighten it.

Give that a try if you want to troubleshoot further.

Good luck!

[itspip]

I'll see what tomorrow has in store for me and see if I have time to try that. Thanks for the help.

[Peter Apps]

Your signal has gone from 30000 to 3 - a factor of 10000 that might correspond to a setting change on the instrument or in the software.

Peter

[Consumer Products Guy]

This was the "fun" type of problem that I enjoyed working on (back in my working days, before I retired), so much more than confirming the level of glycerin added to a consumer product, or determining "why" a hand sanitizer product or clear gel antiperspirant "became cloudy" after prolonged storage where there was considerable weight loss. And there was always something to learn, too.

By the way: when I suggested decades ago that products get weighed (and the initial weight written on the container) before storage, so that weight loss could be determined first thing upon retrieval, the response was "that's too much extra work".....

I did become discouraged that Product Development. Processing, and Manufacturing seemed to feel that ALL responsibility for solving problems had to come from Analytical Chemistry....guess it was "easier" for them to submit samples and clear themselves of responsibility...

[itspip]

I had some more time to devote to the issue here and this is what I found/did.

I reset the injection liner and column in the injection port. I also trimmed a few inches off the front of the column. This had little to no effect.

I replaced the syringe, this solved the sensitivity issue.

The FID also started getting peaks from relighting every minute. I tightened the nut into the FID and reset the column into the FID.

I had been watching the GC make the injections. I am unsure why the syringe was the culprit. Does anyone else have some clues for that?

[Peter Apps]

Thanks for the progress report. For a syringe to malfunction by four orders of magnitude implies that plunger not moving at all, which needs it to be both stuck in the barrel and loose from the plunger driver, or the needle being completely blocked, which is entirely possible.

Peter