In untargeted metabolomic (LCMS) my analytical standard is not in the same retention time as my annotation. The annotation was done through isotope-labeling to annotate the elemental formula, followed by MS/MS all-ion fragmentation data analysis. The fragmentation pattern of Phe-Glu was matching my MS/MS data for this peak (except minor alterations of the ratios between fragments). Is there another reason for that to happen, except of course that it is just not the right annotation?
Thank you very much for replying
Most likely it is not the correct annotation. PubChem lists close to 10k entries for the sum formula C14H18N2O5, so you have a good chance one of those yields a highly similar MS/MS.
If the standard is correct and the LC conditions constant, I am not aware of any matrix effects that could shift the RT by the amount shown in your figure.
Have you checked the 149 in your standard derived MS/MS? PubChem reports that it should be a 148.
Also, you only show relative intensities in your figure. In your pool+standard sample -- do you observe a peak at 5' of similar intensity as in the pool sample alone? You should. And if you find it, this would confirm that the initial annotation was wrong.
Bests, Jan
Great!
I figured the msms will give me high certainty as it being referred to as high degree of annotation.
Yes there was the original peak in this sample and dipeptides has a lot of shared fragments, which i did not realized before.
So Its just not the correct annotation. However im convinced its a dipeptide very close to Phe-Glu so I will get other isomers standard for that.
Thank you very much Jan
- Badges
- Science topic
- Similar topics
- Philosophy
- Esthetics
- Interpretation