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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>