FID and MS Peak Areas correlation

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

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Hi,

The peak area in GC-FID correlates to the amount of analyte present in the injected sample, but is this also the case for the peak area in GC-MS?

Furthermore, are the peak areas in GC-FID and GC-MS comparable if the same method has been used?

Thanks in advance :)
Hi, Phil!
Of course, no. You can’t compare signals (integrals) from the different sources. Area of MS will depend on the number of factors (Scanning mode, Scan rate, Mass Range, DwellTime (in case that you will use SIM mode) etc.). On the other hand, area of the peak in FID will depend on the structure of the compound.
You can compare only signal from the MS with MS data and signal from the FID with the same FID signal.
When the path is over and passed successfully, it's nice to remember even the mistakes ...
phil1150 wrote:
Hi,

The peak area in GC-FID correlates to the amount of analyte present in the injected sample, but is this also the case for the peak area in GC-MS?

Furthermore, are the peak areas in GC-FID and GC-MS comparable if the same method has been used?

Thanks in advance :)


Both correlate to the amount of analyte injected, but the response of each detector to a given analyte or analyte class must be taken into account.

Inject a long chain hydrocarbon like dodecane and the FID will respond with more sensitivity than the MS will. The FID senses the analyte as it is ionized in the flame and since dodecane burns in an oxygen rich flame very well the response is very good. But when dodecane is fragmented by the MS, it will generate multiple ionized fragments as different masses, the MS will read each fragment as an individual response, each less responsive than the total. If you measure the peak on MS using the Total Ion Chromatogram method then the response will be more in line with the response of the FID, but most of the time with MS you measure one selected mass fragment while using others as qualifiers so if there are 30 fragments you are only measuring 1 of the 30 fragments and the response will be much lower.

On the opposite side of things we can look at Carbon Disulfide. This analyte does not ionize well in the flame of the FID and gives almost no response, but in the MS it fragments very little producing an ion at m/z 76 which has a relative abundance of 80% of all the fragments combined therefore when measuring mass 76 for carbon disulfide you get a very good response on MS versus the poor response on FID.

A third example is Benzene. Benzene ionizes very well in the FID flame and on MS it forms only a few mass fragments with m/z 78 being at about 75% abundance relative to the total fragments so it responds well on both detectors.

Analytes with multiple chlorines on them can respond poorly to both detectors because they do not ionize well in the flame and they give a multitude of fragments due to the 35/37 mass distribution of chlorine. These are analytes such has pesticides and require the ECD detector for best sensitivity.
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