In
MS/MS and
MSn studies, the precursor is
fragmented in a collision cell or chamber before the mass spectrum is
acquired. This produces a product ion scan for the particular precursor
ion. This type of data is 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) sometimes also called collisionally
activated decomposition (CAD). Figure 1 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 - Ar, He, N
2, CO
2 etc.
Fig 1. A cartoon schematic of CID
fragmentation.
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 product ions are formed. The types of
fragmentation that occur vary considerably with the type of product ion
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, retro-synthetic type reactions are often observed. These are
much more structurally significant, and often result in cleavage of the
molecule at characteristic positions. If the energy is too high, C-C
bong cleavage can occur leading to uncontrolled fragmentation - this
should be avoided. usually it is best 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 on CID fragmentation and structural elucidation is in the
application section of this web site.
©2005 Paul
Gates, University of Bristol
Last updated July 1st 2005.
