Electron capture dissociation

In mass spectrometry, 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.

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


 * $$[M + nH]^{n+} + e^- \to \bigg[ [M + nH]^{(n-1)+} \bigg]^* \to fragments$$.

Liberation of the electric potential energy results in fragmentation of the product ion.

ECD produces significantly different types of fragment ions than other fragmentation methods such as collision-induced dissociation and infrared multiphoton dissociation. These other methods introduce internal vibrational energy in some way or another while ECD does not. The unique (and complementary) fragments observed and the ability to fragment whole macromolecules effectively has been promising. The low efficiencies and other experimental difficulties have prevented wide spread use. ECD is primarily used in Fourier transform ion cyclotron resonance mass spectrometry.

ECD is a recently introduced technique and is still being investigated. The mechanism of ECD is still under debate but appears 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. Suggestions have been made that radical reactions initiated by the electron may be responsible for the action of ECD.

In a similar technique called electron transfer dissociation the electrons are transferred by collision between the analyte cations and anions.