Chromatography Forum

was just wondering when is the background output from the FID considered to be too high.
currently running a method and the output used to be around 16. this is with 45/450ml/min hydrogen-air flow and helium as carrier gas.
now the output (with no modifications to the parameters)has crept up to 24. running on an agilent 7890N GC and reading the manual the normal output should be below <20pa.
I was reducing the flows to the detector and realised that when the hydrogen flow is reduced to say 30ml/min(from 45ml/min) the output would drop to around 16 again.
is it possible the hydrogen lines are contaminated?

It's possible that the hydrogen is contaminated, but far more likely that starving the flame of fuel is simply reducing its output. The manufacturer's recommended settings are well established to give the optimum signal:noise, so stick to them.

By far the most likely source for a higher background is column bleed, or continuous elution of heavy contaminants from samples. To check this, turn the carrier gas off (with the oven below 50C).

Peter

Ok thank you but I should have said that the output on all the GC's is higher than normal.

Now I suppose it's not ridiculously high but if it gets progressively worse then does it point to contamination or a leak or something?

It's also possible that your FID air is contaminated.

have you changed helium tanks recently? Contaminated tanks could cause that to jump, as I've seen happen a few times.

Ok thank you but I should have said that the output on all the GC's is higher than normal. Yes you should have, because it immediately eliminates anything to do with the individual GC or its column.

Now I suppose it's not ridiculously high but if it gets progressively worse then does it point to contamination or a leak or something? You almost certainly have contaminated hydrogen or air (what about make-up gas ?). Do you have scrubbers on the gas lines, and when were they last changed ?

Peter

I probably should have mentioned that I have recently been using a very narrow I.d liner on one of the headspace gc's and have been noticing a lot of carryover into future injections.

Have I caused contamination through backflash?

I probably should have mentioned that I have recently been using a very narrow I.d liner on one of the headspace gc's and have been noticing a lot of carryover into future injections.

Have I caused contamination through backflash?

Unlikely with headspace unless you have a very rapid injection form a syringe, or a very high residual pressure in a loop injector. And if this was the problem it would affect only that particular instrument - I would bet hard cash money that it is impossible to backflash all the way into the gas lines.

It's probably a good idea to sit down and list all the things that you need to mention - then we can look at the problem as a whole rather than bit by bit.

Peter

It's also possible that your FID air is contaminated.

I agree with this. A hydrocarbon filter could solve this problem.

What kind of air are you using? UHP air is synthetic, typically very low in methane. Compressed air will have methane. Methane will not get scrubbed at least not for very long.

Best regards,

AICMM