Controlled fragmentation of glycans is normally produced by CID in a collision cell, but fragments can also be formed elsewhere in the instrument. Fragments can arise from decomposition in the ion source (in-source decay
Table 3.3 Residue masses of common monosaccharides.
a Top figure = monoisotopic mass (based on C = 12.000000, H = 1.007825, N = 14.003074,
O = 15.994915). Lower figure = average mass (based on C = 12.011, H = 1.00794, N = 14.0067,
O = 15.9994. The masses of the intact glycans can be obtained by the addition of the residue masses given above, plus the mass of the terminal group (H2O for an unmodified glycan)—18.011 (monoisotopic) and 18.152 (average)—and the mass of any reducing-terminal or other derivative.
(ISD) or prompt fragmentation) and within the flight tube of time-of-flight (TOF) instruments (PSD) . High-quality fragmentation spectra can now be obtained from both electrosprayed and MALDI-generated ions [246, 257-259] by use of Q-TOF-type instruments. The review by Zaia  provides a good overview of carbohydrate fragmentation. Fragmentation of carbohydrates occurs by two main pathways: glycosidic cleavage involving the breaking of a bond between sugar rings and cross-ring cleavage involving the bonds comprising the rings. Glycosidic cleavages from even-electron ions of the type [M+Na]+ result in the loss of neutral molecules and are accompanied by hydrogen migrations. A third type of fragmentation that was proposed to involve a six-membered transition state and the transfer of a carbon-attached hydrogen atom has recently been suggested ; the fragment ions effectively eliminate two oxygen functions to leave the expelled neutral particle with a carbonyl group.