Peaks in FID but not MS?

Discussions about GC and other "gas phase" separation techniques.

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I’m really confused by some residual solvent data I acquired from headspace gas analysis of samples by GC-FID-MS.

I set up the GC-MS-FID with an Agilent splitter plate and was very pleased when it worked first time. I’ve got really good alignment of the FID and MS signals verified by a heptane signal in DMAC which is used as the internal standard. I can see a nice sharp peak in the FID and MS and a nice spectrum to confirm the heptane so I'm pretty happy that the instrument is working properly.

I'm using a DB-624 column, starting at 30C. When I run the real samples I've been asked to characterise two small impurity peaks which are eluting really early in the run (the samples are made up in DMAC) they even elute before acetone. They are showing up very clearly in the FID but there is no trace of them in the MS TIC. I’ve tried EI and CI sources, I’ve mined down into the data and done back ground subtraction but no matter what I try I can’t see anything like a spectrum at the same retention time as the peaks in the FID.

I’ve tried researching residual solvent analysis and artefact peaks but I’m completely stumped. Clearly the impurities contain carbon and they are retained on the column (DB-624). They are from headspace samples so are volatile in nature but don’t produce a signal in EI or CI?

Has anyone come across anything like this before?

Many thanks in advance for any advice you can give,
What masses are you scanning from?
I'm scanning from m/z 20 - 400. We realised that the peaks must be from something really small so took the m/z down to take into account any small fragment ions produced in EI.
Could it be something like Ammonia? I think it can respond on the FID but would not show up on MS since its molecular weight is 17.
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