In that example, 129+216+327 does not equal the parent's mass of 492.
My understanding is that during MS, the protein is broken down into peptide masses of varying length of approx. 5 to 20 amino acids. They also lose ions such as water or ammonia like in the example., the experimental spectra I thought would show the masses of these "children" that can be summed up to the parent. But that is not the case. Where am I wrong in my understanding?