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

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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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
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.
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
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.
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