Aside from the overall structure, the linkage of the Neu5Ac can also differ, presenting further complications in the complete characterization of a glycoform. The added complexity of linkage heterogeneity convolutes characterization and is often left ambiguous based on MS/MS analysis. Moreover, glycans with the same composition but different structures can exist for the same glycosite. For example, the glycopeptide TTLQGAIRREP has two possible adjacent O-glycosites (both T) and a range of possible glycoforms for each site. Figure showcases the characterization of this glycopeptide by UVPD which produces both peptide sequence ions and glycan compositional ions. A disialylated core 1 glycan is located at T1; the b1, a2 2+ , b2 2+ , and z11 ions help localize the glycan. The spectrum shows sequential loss of saccharides, further confirming the glycan composition, along with a few fragment ions that help confirm the linkages of the two Neu5Acs. The first Neu5Ac is α-linked between the second carbon on the Neu5Ac and the third carbon on the Gal. The detection of the A2 ion supports this assignment, as the product only retains the third and fourth carbons of Gal. The second Neu5Ac is normally assumed to be α-linked between the second carbon on the Neu5Ac and the sixth carbon on the GalNAc, should a Gal already be linked to the GalNAc. The combination of a cross-ring cleavage and glycosidic cleavage, yielding the product 2,5 A3Z2, helps confirm the α2,6 linkage between Neu5Ac and GalNAc, increasing confidence in the exact structure of the glycan.