Vanishing and re-appearing standard peak on GC-FID?

[cb23]

Hi all,

a lab user showed me two chromatograms both run on an Agilent 6890 GC-FID with an OV-17 column and aldononitrile acetate derivates (dissolved in ethylacetate:hexane, 1:1).

We are using FAME 13:0 as an internal standard for injection.

Now, in some of the chromatograms the FAME peak (RT 3 min) is missing (100x smaller than usually). I thought it was the user who had forgotten to add the internal standard, or the pipette was malfunctioning (blocked). When re-measuring the sample a day or two later, the FAME peak was there though.

The relative peak areas of the other analytes (RTs around 15 min) were more or less the same (Area of an analyte peak relative to the sum of all analyte peak areas).

I tested one sample myself but according to the user, it happened a few times.

I find this a bit difficult to explain. My only (weak) explanation would have been a leaky inlet and losses of the more volatile standard, so I retightend the nuts and replaced the septum and syringe a few days ago. I have received mixed reports on my maintenance so far.

Can anyone explain such a pattern?

Thanks and all the best,
cb23

[Steve Reimer]

So the FAME peak is the first peak out, correct? Is it a splitless injection? Is a pressure pulse used? If you do a split injection do you have this problem?

[cb23]

Yes, it is the first peak after the solvents.
It is actually a split injection (1:10).