LOD for GC-FID

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

5 posts Page 1 of 1
HI
I want to measure the detection limit of the GC-FID , which i analyse the gas samples like propane and ethylene.
please help me to measure the LOD and LOQ.
also i have just one standard mix gas( all component about 18 ppm).
thank you.
Given what you have, I would measure the signal for the analyte peaks in my standard. I would measure the noise in the baseline just prior to the peaks. From that data, I can calculate the S/N ratio at 18 ppm. Then, I would asssume linearity of the detector down to undetectable (with the FID, not a bad assumption). From a simple proportion (taking LOD as S/N = 3), you can estimate the LOD.

18/(S/N at 18) = LOD/3

LOD = 18*3/(S/N at 18)

The most accurate way to do it is to keep injecting less until you reach S/N = 3. That may not be easy to do in your situation.
rb6banjo wrote:
Given what you have, I would measure the signal for the analyte peaks in my standard. I would measure the noise in the baseline just prior to the peaks. From that data, I can calculate the S/N ratio at 18 ppm. Then, I would asssume linearity of the detector down to undetectable (with the FID, not a bad assumption). From a simple proportion (taking LOD as S/N = 3), you can estimate the LOD.

18/(S/N at 18) = LOD/3

LOD = 18*3/(S/N at 18)

The most accurate way to do it is to keep injecting less until you reach S/N = 3. That may not be easy to do in your situation.


thanks a million
i use DANI GC master 1000 and my software is clarity, i could measure S/N by software priorto the peaks.but i have about 20 peaks and some of them appear in succession (without distance). also i have to refer may calculation to an standard because of ISO 17025...
If your standard is too complicated, just run a blank vial (air) or just run the GC (no injection but collect the data). You can measure your noise with a blank run (no peaks). What I offer above allows you to do it in one injection.

The most consistent LOQ that I've seen in all of the discussions I've read on this topic takes it as when the S/N is 10. I've had instances (with an MS detector) where I have been sure of what I'm detecting at S/N less than 10 but for an FID where you don't know for sure what you're detecting and you're just relying on the retention time, S/N = 10 is probably pretty good. You could say less. To me, all of this LOD/LOQ stuff comes down to "what is the risk in being wrong?". If you're measuring an impurity in a system that might spoil your catalyst, any blip in the baseline might be an indicator that your process is going to be compromised, you might want to make the S/N = 2 or 1.5 as your LOD. After injection of standards/samples where the S/N is repeatedly 5 and you get precision that is acceptable to you, perhaps your LOQ is when S/N is 5. You need to verify it experimentally though.

Those values of 3 and 10 are sort of "accepted" values and this comes from reading lots of printed accounts on this subject as well as discussion threads on this forum. Those values seem to have most of us in some sort of agreeement. Just my $0.02. It comes up frequently. You can search for the threads in the search box above.
sana70 wrote:
HI
I want to measure the detection limit of the GC-FID , which i analyse the gas samples like propane and ethylene.
please help me to measure the LOD and LOQ.
also i have just one standard mix gas( all component about 18 ppm).
thank you.


I don't use LOD, no one asks for it and to me (ISO 17025 as well) the only value that counts is LOQ.

My LOQ is always my lowest calibration point, i put it in these terms: if i can't calibrate it, i can't quantify it.
If you can calibrate it and R2 is good and avg RSD% is less than 25% i'm done.
Probably it won't be the lowest possible LOQ you can get, but as soon as it's ok for your job (for example, at least 10 times less than maximum allowed concentration for your regulatory limit) it's way more accurate than using S/N.

Just my 2 cents.
Davide Balbo from Italy
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