In contrast to the fragmentation of [M+H]+ ions, fragmentation of [M+alkali metal]+ ions (Figure 3.7b) generally requires more energy, but the rearrangements reported from the [M+H]+ ions are absent . The ease with which various ions decompose follows the order H = Li+ > Na+ > K+ > Cs+. [M+Cs]+ ions do not fragment other than to give Cs+ [244, 292, 293]. Internal fragments (losses from two or more sites) are common and can cause difficulties with spectral interpretation. Cross-ring fragments, usually of the A type [244, 294], are often present, but when low energies are used for fragmentation, as with CID on Q-TOF-type instruments [150, 244, 246], these ions are not usually abundant. More abundant cross-ring fragment ions, particularly X type, can be produced at higher energies of the type found in TOF/TOF-type mass spectrometers [266, 295-298] or formed by high-energy photodissociation . The [M+Na]+ ions of permethylated carbohydrates have been examined with TOF/TOF instruments. Their spectra also contained many A- and X-type cross-ring fragments, but, in general B and Y ions formed by cleavage adjacent to GlcNAc residues dominated the spectra [151, 152, 300].
For some monosaccharides, [M+Ag]+ ions have been used to produce characteristically different spectra relative to adducts of group I metals allowing them to be differentiated, whereas [M+Cu]+ ions did not . With N-linked glycans, silver appears to be particularly good at cleaving glycosidic bonds but does not offer any particular advantage for structural determination . Among doubly charged cations, calcium appears to be particularly effective at fragmenting carbohydrates and for producing high sensitivity detection. Doubly charged ions are the major products formed with these metals although copper has a tendency to form singly charged ions .