Collision-induced dissociation

In mass spectrometry, collision-induced dissociation (CID), referred to by some as collisionally activated dissociation (CAD), is a mechanism by which to fragment molecular ions in the gas phase. The molecular ions are usually accelerated by some electrical potential to high kinetic energy in the vacuum of a mass spectrometer and then allowed to collide with neutral gas molecules (often helium, nitrogen or argon). In the collision some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by a mass spectrometer.

CID and the fragment ions produced by CID are used for several purposes. Partial or complete structural determination can be achieved. In some cases identity can be established based on previous knowledge without determining structure. Another use is in simply achieving more sensitive and specific detection. By looking for a unique fragment ion you can detect a given molecule in the presence of other molecules of the same nominal molecular mass, essentially reducing the background and increasing the limit of detection.

CID is used as part of tandem mass spectrometry for many experiments in proteomics. While CID is currently the most popular method for standard tandem mass spectrometry, there are other also other fragmentation methods for special purposes, for example electron transfer dissociation (ETD), electron capture dissociation (ECD) and Infrared multiphoton dissociation (IRMPD).

CID in a triple quadrupole
In a triple quadrupole mass spectrometer there are three quadrupole mass analyzers. The first quadrupole termed "Q1" can act as a mass filter and transmits a selected ion and accelerates it towards "Q2" which is termed a collision cell. The pressure in Q2 is higher and the ions collide with neutral gas in the collision cell and fragments by CID. The fragments are then accelerated out of the collision cell and enter Q3 which scans through the mass range, analyzing the resulting fragments (as they hit a detector). This produces a mass spectrum of the CID fragments from which structural information or identity can be gained. Many other experiments using CID on a triple quadrupole exist such as precursor ion scans that determines where a specific fragment came from rather than what fragments are produced by a given molecule.

CID in an FTMS ICR cell (SORI-CID)
SORI-CID (sustained off-resonance irradiation collision-induced dissociation) is a CID technique used in Fourier transform ion cyclotron resonance mass spectrometry which involves accelerating the ions in cyclotron motion (in a circle inside of an ion trap) and then increasing the pressure resulting in collisions that produce CID fragments. After the SORI-CID process is complete the pressure is reduced back to high vacuum and the analysis of the fragment ions is performed as usual.