Collision-induced dissociation

Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of molecular ions in the gas phase. The molecular ions are usually accelerated by some electrical potential to high kinetic energy and then allowed to collide with neutral 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 tandem mass spectrometry.

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 detecting a unique fragment ion, the precursor ion can be detected in the presence of other ions of the mass to charge ratio, reducing the background and increasing the limit of detection.

In a triple quadrupole mass spectrometer there are three quadrupoles. 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 collides 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.

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