FID air ratio nitrogen/oxygen

[a54mb]

Dear all
I have some experience in chromatography but I am an absolute beginner concerning classic detectors like the FID. (I did a lot of stable isotope work using IRMS in the last years). Now I am in the position to re-establish a lab to measure CH4 in lowest amounts (4 mL of air STP, lowest concentrations: 350 ppb CH4).

What I found in that lab is a bottle with N2/O2 = 60/40. From my manual I expect AIR = N2/O2 = 80/20.
Furthermore I reconstructed that my forerunners (not available) used an old Varian GC (approx. late 1980s) with the following flow rates for the FID:
H2 = 30 mL/min
60/40 N2/O2 = 330 mL/min
N2 = 20 mL/min
Carrier = He = 5-10 mL/min

I assume N2 was used as make up gas.

Can anyone explain, why someone would use N2/O2 = 60/40?
Could it be that this increases sensitivity for lowest amounts of CH4?
Could it fade out a disturbing species (like CO)?
Could it be that N2 was not used as make up but to dilute the 60/40 mix?

Any hints are welcome. My only knowledge on this is from textbooks and the manual (saying: air, no specs on N2/O2 ratio). Please forgive if essential info is missing here, please tell me what I forgot.

My new GC will be a: Agilent 7890B equipped with a FID for capillary columns only.

Thanks!
Michael

[CE Instruments]

Never heard of anyone using a 60/40 mix. FIDs usually work best with a slight increase in Nitrogen over the standard Air mix, see Special Gases "FID Air" specifications. Your new GC will come with recommended settings including a different set of values for ignition controlled by the GC. If you can afford a new 7890 then it is probably sensible to junk the 60/40 mix and get some standard air.

[a54mb]

Thanks CE Instruments

Money for the air bottle is not an issue. Actually, if I go for 80/20, I might use a zero air generator that is not needed by a colleague.

You state

"FIDs usually work best with a slight increase in Nitrogen over the standard Air mix"

.
The question is what is the "usual" application? I speculate that the specific needs here for lowest trace amounts of CH4 was the reason to go for 60/40 (N2/O2).
However, if the latter is something odd, and no more information is around, clearly I will start with the usual 80/20 mix and see how far I come in terms of sensitivity...

Best
Michael