Hi,

Even though I do not have an explanation for the mass difference you observe on your peptide readily available, a few things come to mind when looking at your data:

What peak from the UV trace did you select to generate the mass spectrum provided? What masses do you detect in the other peaks observed?

A serine doesn't seem obvious, as there are no other serines in your sequence, so I assume no serine was available during synthesis?

How accurate is your measurement? It could also be a combination of things, e.g. an additional alanine and oxidation of your methionine (although that would result in a slighlty larger mass difference..)

Were data acquired on a Waters TOF or Q-TOF? In case of the latter, I would definitely advise to run MS/MS of your peptide, preferably of the doubly charged precursor (at m/z 690.26).

Hope this helps, good luck with the puzzle!

First, knowing which mass spectrometer was used here would help in finding an explanation. I assume the ionization source was electrospray, which gives leads to an expected m/z of 1292.6 for [M+H]+. Your peak is 1378.65, which means your molecule weighs 1377.6: uneven mass, so uneven amount of nitrogen atoms -> hence it cannot be one amino acid residue that was added (unless it would contain 2 nitrogens, but this does not make sense experimentally or based on these data).

I am not sure if the 690 peak is your double charges species, because this would give you a mass of 689.86.

How was the peptide synthesis performed? Manually? Automated? What coupling reagents? Capping with Ac2O after each coupling step?