Collision Induced Dissociation (CID)
In MS/MS and MS
n studies, the precursor ion is selected
and
fragmented in a collision cell or chamber before a mass spectrum of
fragments is
acquired. This spectrum is the product ion spectrum for the
particular, specific precursor
ion. This type of analysis is commonly used for sequencing (peptides
and sugars),
structural elucidation and analyte identification through fragment
fingerprinting. The most common method of fragmentation is collision
induced dissociation (CID). The figure shows a cartoon schematic of
the processes involved in CID. The precursor ion enters the collision
cell (or in the case of ion-traps or FT-MS, the precursor ion is
isolated in
the trap) containing a high pressure of an energised, chemically inert
collision gas (e.g. Ar, He, N
2, CO
2 etc).
The precursor ion
undergoes repeated collisions with the collision gas, building
up potential energy in the molecule, until eventually the
fragmentation
threshold is reached and the precursor ion fragments into product
ions. The types of
fragmentation that occur vary considerably with the type of precursor
species and the amount of energy involved. At lower energies (close to
the
threshold),
fragmentation reactions are often limited to neutral losses (H
2O,
MeOH,
CO, CO
2,
MeCN etc.) depending on the nature of the precursor ion. These neutral
losses are often not considered structurally significant, although
they
can be used to obtain information about functional groups. At higher
energies the carbon skeleton of the molecule can be broken by
retro-synthetic type reactions. These are
much more structurally significant, and often result in cleavage of
the
molecule at characteristic positions. If the energy is too high,
uncontrolled fragmentation can occur, this
should be avoided. Often it is desirable to work at around the
fragmentation threshold, or just above, to maintain most control over
the fragmentation processes. Ion-trap
and FT-MS instruments allow for the
most control over CID, but also tend to produce less energetic
reactions. Triple quadrupole and QTof instruments tend to produce more
energetic CID with more fragmentation, but less operator control.
Ion-trap and FT-MS allow multistage fragmentation experiments to be
conducted though, which is essential for structural elucidation
studies. More details on CID, MS/MS and structural elucidation is in
the
application section of this web site.