Chromatography Forum

Hello,

I have been working on a 5980 GC for the past few months, adding and removing parts and pieces to turn it into a dual FID instrument. I believe I have everything put back together correctly, but the signal output on both detectors seems really low. When the flame is lit the background signal fluctuates between 0.1 and 0.6, and the solvent peak doesn't get much higher than 12.

This seems really low, but I don't have a good idea what I should be seeing in this instrument. Does anyone have any advice on how to get more signal? Thanks!

Extra info: I'm using helium as a carrier gas, the column is a Restek Rtx-225 15 m column (we want to look at FAME's), the computer has Windows 95 and MS Chemstation G1701AA software (the computer is also connected to a GC/MS). Let me know if more info is needed!

My background is usual about 20 and solvent peaks about 10^7 or so. I always operate with a 10:1 split since it doesn
t have a gas saver.

My background signal on my 5890A and my 5890 Series II at 40 °C with a DB-5-type phase is more toward 3.0 pA.

Are you sure the flame is lit? I get as much as 0.1-0.3 change on the GC readout when I merely turn on the FID support gases.

Do you see column bleed if you perform a GC run with no injection?

I can see condensation form when I hold a wrench over the chimney, so the flame is lit.

Check your split ratio if you do use. Possible inlet leak if not?

Could be the spring at the end of the detector board probe isn't touching the side of the collector inside the detector. If that spring is bent or broken and doesn't touch the collector it won't pick up the current generated by the detector.

Also what is the makeup gas and flow?

Air to Hydrogen ratio also affects detector sensitivity and signal.

I used to run mine anywhere from about 7 to 30 on baseline signal depending on the instrument a application. The 30 signal was much more sensitive but also had more background. After it is lit you can adjust the air flow and hydrogen flow to help with signal levels. Too much makeup gas flow will also drop the signal, I usually set column and makeup combined to 30-40ml/min measured at the detector with the fuel and air gas turned off.

The fact that both detectors are showing the same problem, suggests to me something common to both FID's. I would start with checking if you have not reversed the air and hydrogen supplies to the GC. If these are reversed, it is possible that the flame is actually burning at the top of the detector, which would give you your condensation, but no signal at the collector.

Gasman

Also check if the detectors are '' on ''.
Strike the keys '' Det1'' then '' on '' , on the keyboard , and check the signal.