In this condition, where only metastable fragment ions are detected (Figure ), it is evident that all backbone fragment ions as well (P1+71) and (P2+80), have a similar contribution of a metastable origin. In particular, (P1+71) linker fragment ion has an intensity 0.61 times lower than the signal (m/z 1684.54) assigned to the C-terminal rearrangement of peptides (b10 α +H2O and b10 β +H2O) generated mainly by a metastable fragmentation as reported previously by Gaskell et al. .