Different peak structure for MSD and FID

[Tandem]

The method: determination of three different Tri-n-alkylphosphates with alkyl = C3, C4 or C5.
The problem: Peaks with the same amount of alkylphosphate detected by FID do have the same area and height, BUT with a detection by MSD the C3 is twice as big (area and height) as the peak of the C4 and four times bigger (area and height) than the one of C5.
All method parameters except the detector are the same.
What could be the reason for this effect???

[Amirav]

What is the transfer line and ion source temperatures?

[WK]

Tandem,
Why expect the TIC response to be the same as that for FID?
Are the mass spectra the same for all three?
I would recommend you quantitate by FID response.
WK

[sg]

That is true, the working principle for FID and MSD are different, how you expect they give same area and height. Even for FID, if you use different intrument, it may also give different value...