What is ECD?

Electron Capture Dissociation (ECD)

Electron-capture dissociation (ECD) is a method of fragmenting gas phase ions for tandem mass spectrometric analysis (structural elucidation). ECD involves the direct introduction of low energy electrons to trapped gas phase ions. It was developed by Roman Zubarev and Neil Kelleher while in Fred McLafferty’s lab at Cornell University.

Principles

Electron-capture dissociation typically involves a multiply protonated molecule M interacting with a free electron to form an odd-electron ion

Liberation of the electron affinity energy results in fragmentation of the product ion.
ECD produces significantly different types of fragment ions (although primarily c- and z-type, b-ions have been identified in ECD) than other MS/MS fragmentation methods such as electron-detachment dissociation (EDD) (primarily a and x type), collision-induced dissociation (CID) (primarily b and y type) and infrared multiphoton dissociation. CID and IRMPD introduce internal vibrational energy in some way or another, causing loss of post-translational modifications during fragmentation.
In ECD (and in EDD as well), fragments retain post-translational modifications such as phosphorylation and O-glycosylation. In ECD, unique fragments (and complementary to CID) are observed and the ability to fragment whole macromolecules effectively has been promising. The low fragmentation efficiencies and other experimental difficulties have prevented widespread use.
The mechanism of ECD does not to necessarily break the weakest bond and is therefore thought to be a fast process (nonergodic) where energy is not free to relax intramolecularly.