Assuming that you have separate GC's for GC-FID and GC-C-Irms, perhaps your heavy compound is getting left behind in the injector - try increasing splitless time. I have seen mass discrimination for a mixture decrease stepwise as I increased splitless time.
All the best
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of Kit-sum LEUNG
> Sent: Thursday, 31 January 2013 7:45 p.m.
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] GC-Pyrolysis-IRMS of long-chained alkanes
> Hi all,
> I would like to study the hydrogen isotope ratio of long-chained
> alkanes (C>35) by GC-pyrolysis-IRMS, but it seems that the alkane cannot
> undergo pyrolysis (as only a sharp peak is found in GC-FID
> chromatogram). The temperature of pyrolysis furance is set to 1450
> degree C and successful pyrolysis of alkane with no. of carbon equals to
> 30 is observed. Do anyone have experience of long-chained alkane
> pyrolysis and knows what the problem is.