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Isabelle,
I agree with Willi - D31 palmitate is ~100% deuterium. After pyrolysis you get mostly D2 (m/z=4), a little HD (m/z=3), and virtually no H2 (m/z=2). You see only a mass 3 signal because there's no detectable 2, some detectable 3, and no detected 4 (IRMS is not tuned to 4). 
-Bill 

> On May 20, 2016, at 7:54 AM, Isabelle Chery <[log in to unmask]> wrote:
> 
> Hello all,
> 
> I would like to analyse a uniformly labelled fatty acid with Deuterium (D31 Palmitc Acid) after derivatization to be volatile as methyl ester by Gc pyrolyse IRMS. 
> I use a Zebron ZB Wax capillary GC column to separate different Fames (C14:0 to C22:0). The injector is at 220°C and the high temperature conversion reactor is at 1420°C.
> The D31-Palmitic acid is totally separated from the natural Palmitic acid by GC. 
> I have beautiful peaks for natural abundance of FAMEs (mass 2 and mass 3) but for the D31-Palmitc acid, the peak shape (only mass 3 is present) is trailing and is not symmetric.
> I have changed the insert, the syringe, cut the column, and put a new reactor tube. But the problem is always being.
> Does anyone could help me?
> Thanks
> Isabelle
> 
> <Test.pdf>