All the 18 O-labelling is retained in P3+98 linker fragment ion, specifically in the succinic anhydride group (Figure ) that is alkylating the Cys (Figure ) while P1+71 shows its natural isotopic ion distribution (Figure ). This difference agrees with the fragmentation mechanism depicted in Figure and the structures proposed for both linker fragment ions. The reduced P3 peptide (P3-SH, m/z 629.20) does not contain the linker in their structure in consequence showed its natural isotopic distribution (Figure ). Other linker fragment ions generated by the thioether bond fragmentation detected at m/z 984.34 and m/z 1018.38 and assigned as P3+169 and P3+203, respectively, showed isotopic ion distributions that evidenced the incorporation of 18 O like the ion detected at m/z 940.32 assigned as b2 α (Figure ). The linker fragment ions P3+169 and P3+203 contain the hydrolyzed thiosuccinimide linker (Fig. and).