Missing isotope in Fire Debris

[JSheffPhD]

I've been a fire debris analyst for some time; however, recently it was noted by another analyst that we started missing an isotope in our MS. Specifically we typically see a high abundance for the C3 compounds such as trimethyl benzene. For some reason the 104 isotope peak for these compounds which shows up when ion profiling the 104 and 118 ion, has just disappeared from the Mass spect. No other ion has appeared to have the same issues yet. Has anyone experienced any issues like this using Agilent 6890/5973 GCMS? If so how did you resolve the strange issue?

[Amirav]

JSheff
Something is not understood, m/z=104 and/or 118 are fragments and not isotope peaks. The molecular weight of trimethyl benzene is m/z=120 and its abundant isotopes are m/z=121 and 122, please explain.
Lost weak fragments such as m/z=118 near a strong m/z=120 could be due to lost MSD resolution at the low mass side which you may check in tune.
We have a new method for fuel characterization based on isomer distributions. If you wish me to send it please write me.
Amirav
amirav@tau.ac.il