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GC/MS vs GC/FID(as Toluene Equivalents)

Posted: Wed Jan 11, 2017 6:40 am
by Jaehwan Lee
Dear friends in the world,
I have questions regarding measurement method difference between GC/FID and GC/MS.
For the quantitation of ambient VOCs, we use GC/FID type. GC/MS also has qequipped with thermal desorption type like GC/FID system.
Our GC/FID sytem has preconcentration type(Tenax TA, 20mg) + GC/FID sytem to monitor VOCs in ambient air. Calibration ranges are 5~20ppbv(5, 20, 50ppbv)using diluter with 10ppm standard gas cylinder.
When I compare the individual concentration(e.q. Toluene, m,p-Xylene) between GC/FID and GC/MS, toluene concentrations are very similar each other.
But m,p-xylene concentrations are too much different. Usually, GC/MS concentration has 1.6times(n=5) higher than GC/FID type.
Would anybody tell me this difference or recommend something?
Best regards.
From Lee

Re: GC/MS vs GC/FID(as Toluene Equivalents)

Posted: Wed Jan 11, 2017 8:36 am
by dblux_
Jaehwan Lee wrote:
... But m,p-xylene concentrations are too much different. Usually, GC/MS concentration has 1.6times(n=5) higher than GC/FID type.
...

Which method gives results closer to true concentrations ?
Does your column resolve p-xylene from m-xylene ?
How do you quantitate GC/MS signals ?
Knowledge gained during method validation may help you understand reasons of differences.

Re: GC/MS vs GC/FID(as Toluene Equivalents)

Posted: Thu Jan 12, 2017 3:39 am
by Jaehwan Lee
Thanks.
GC/FID concentration is more closer than GC/MS.
In this case, we tested accuracy at 10ppb concentration.
GC/FID method is dilution adsorption from 10ppm stdnadard cylinder gas.
GC/MS method is adsorption from liquid standard using adsorption tube.
Quantitation is based on Toluene equivalents for toluene, and m,p-xylene.
Unfortunately, our column(HP-5) can't resolve m,p-xylene.
Best regards.
Lee

Re: GC/MS vs GC/FID(as Toluene Equivalents)

Posted: Thu Jan 12, 2017 7:05 am
by dblux_
These two methods have different detectors and different adsorption.
IMHO you should determine desorption coefficients for both methods and correct results with them.
If you do not resolve p- and m-xylene then you determine them as a sum. May be p- and m-xylenes have slightly different responses on FID and the composition of mixture in air is different then this in calibration mixture.