Hi Wolfram,
Thank you for replying. I have answered your questions as best I can below. Our GC is a Thermo Trace Ultra, The combustion interface is the Finnigan III, and the IRMS is the Delta V Plus.

1. What is the temperature setting for the injector? 260C
2. What's the average concentration per FAME in your FAME mix (in nmol/microL)?
Unsure. Samples were extracted from both seagrass and mangrove sediments, but I only received them ready for methylation. I was told that the total lipid weight was between 1-2mg.
3. Have you looked what the chromatogram looks like for a dummy run (no injection or neat solvent only)?
I have run hexane only with similar results (can send you the Isodat run file if that would help).
4. Same as point 3 but for single compound sample, say 10:0 or 12:0.
I have not done this, but will do so now.
5. Is your system fitted with a FID and a cross-piece prior to the oxidation recator?
Yes, FID and cross-piece.
King regards,