Electron Transfer Dissociation (ETD)
ETD spectra are generated by the transfer of an electron from a radical ion to the target molecule. The radical ion is generated from an aromatic hydrocarbon such as fluoranthene that has accepted the electron thermalized by a methane buffer gas. Unlike CID spectra of glycopeptides, ETD causes fragmentation of the peptide chain by cleavage of the N-Ca bonds to produce c- and z-type ions, unlike the b and y ions that dominate the CID spectra [58-60]. Little or no loss of the glycans is observed. The method requires the production of multiply charged ions and is, thus, particularly appropriate for ESI- generated glycopeptide spectra. Glycans, on the other hand, typically produce only singly charged ions such as [M+Na]+ or [M+H]+ but can be induced to produce doubly charged ions, which is suitable for ETD reactions by adducting them with cations such as Ca2+ or Mg2+ . Fragmentation of such species generates extensive cross-ring fragmentation, mainly of the X type, with Mg2+ reported to be the best cation. ECD is similar to ETD but is performed in an ICR cell.