Chromatography Forum

In the GC/MS run of certain amines, I see a mass of 70 Daltons greater than the molecular ion (molecular ion itself is negligible). Is it possible and why?

This does not sound to be right. If you have severly overloaded peaks, one might be able to postulate ion-molecule reactions to give ions greater than the molecular weight of the compunds. But even in that case, one should see the expected spectrum for the compounds at the very beginning and very end of the peak -- where the concentration would be typical of what one sees in a GC/MS run. And, severly overloaded peaks tend to give severly distorted spectra due to overloading of the detector.

If you have severly overloaded peaks, dilute and shoot again to get good spectra. If the peaks are not overloaded, you are most likely looking at something other than what you expected.

thanks for the reply. The peak is not overloaded but I do beleive I'm seeing something else (despite AMDIS 100% library hit). How about another compund of interest which does have large molecular ion peak (and other ions per NIST are present as well) but there is still even a larger peak 70 Daltons greater than the molecular ion.

This does not sound to be right. If you have severly overloaded peaks, one might be able to postulate ion-molecule reactions to give ions greater than the molecular weight of the compunds. But even in that case, one should see the expected spectrum for the compounds at the very beginning and very end of the peak -- where the concentration would be typical of what one sees in a GC/MS run. And, severly overloaded peaks tend to give severly distorted spectra due to overloading of the detector.

If you have severly overloaded peaks, dilute and shoot again to get good spectra. If the peaks are not overloaded, you are most likely looking at something other than what you expected.

If these "certain amines" are drugs, and you are using an alcohol to extract them then there is some discussion inthe archives somewhere that might be relevant - apparently they can react with amides to form heavier compounds.

Peter