Biology Reference
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Ion
Source
Mass analyzer
Detector
e.g.
Quadrupole (Q)
TOF
Ion Trap
Multi
Channel
Plate
ESI,
MALDI
Fig. 1.
Basic modular structure of a mass spectrometer.
ionization methods. This ground-breaking advance was recognized in
2002 with a Nobel Prize in Chemistry.
Jargon in MS can be daunting, but much can be simplified when one
bears in mind the modular structure of the instrument, i.e. ion source,
mass analyzer, detector (Fig. 1). Examples of ion sources as mentioned
above are ESI and MALDI, whereas types of mass analyzers include
quadrupoles (Q), ion traps (IT), and time-of-flight (ToF), among others.
The combination of ion source and mass analyzer(s) gives rise to the
name of the configuration of a particular instrument. A MALDI-ToF is
a time of flight instrument with a MALDI source whereas an ESI-Q-
ToF (Fig. 2) is a tandem mass spectrometer made up of two analyzers:
quadrupole and ToF with an electrospray ion source. Technical descrip-
tions of these analyzers are beyond the scope of this chapter, thus the
reader is referred to an excellent introductory topic (Siuzdak, 1996).
Briefly, a ToF analyzer measures the mass as a function of the
time it takes for the ions to get from the source to the detector. Heavier
ions will take a longer time than lighter ions. The important thing to
remember with quadrupoles is that this can be used in transmission
mode, where one can let all ions through, or as a mass filter, wherein
one can select ions of a particular mass ( m / z ) and subject these to col-
lisional activated dissociation (CAD) in order to be fragmented (or dis-
sociated) and the products analyzed, giving structural information. This
technique using CAD is referred to as tandem mass spectrometry,
MS/MS or MS 2 . Finally, ion traps like quadrupoles can perform
MS/MS but can do so in more than one cycle, which gives rise to MS n
capabilities resulting in greater structural information. A detector then
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