We have seen peptides like this. There are actually 2 in the "Rat Heart Failure" dataset I like to teach at courses. I usually tell participants that these peptides would be extremely challenging to quantify without a heavy labeled peptide standard, and even then it is probably better to think of other peptides you might measure to get at the biology that is interesting you. If you absolutely need to measure this peptide form, then you should order an isotope labeled internal standard that will co-elute with it and you can use the ratio between the unlabeled and labeled forms as your proxy for quantity.
I wouldn't recommend using this peptide in a label-free experiment.
--Brendan