Tandem Mass Spectrometry (MS/MS)
In the low energy (soft) ionisation methods (ESI or MALDI) spectra
often only contain peaks representing the intact molecular species
with very little fragmentation. Consequently the spectra are of little
use for structural characterisation. In these cases, induced
fragmentation is required using a technique like collision induced
dissociation (CID) and tandem mass spectrometry (MS/MS). One of the
most commonly available tandem mass spectrometers is the triple
quadrupole (QQQ) instrument. There are many varieties and
configurations of tandem instrument, this page will introduce the
modes of operation of the triple quadrupole as used for ESI-MS/MS -
most of what is described here also applies to other techniques and
instruments with little modification.
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(A) Product
ion scanning.
The precursor ion is focussed in Quadrupole 1 (MS1)
and transferred into Quadrupole 2 - the collision
cell. Here it undergoes CID and fragments. The
fragments are then transferred into Quadruole 3 (MS2)
which is scanned to produce a spectrum of fragments -
the product ion
spectrum. This results in a specific MS/MS
spectrum of one single precursor ion independently of
any other ions present in the spectrum and is the most
commonly employed method.
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(B) Precursor
ion scanning.
Quadrupole 3 is held static at the m/z
of a specific product ion only and quadrupole 1 is
scanned across the desired m/z
range. This experiment results in a spectrum of
precursor ions that produce that particular product
ion during CID fragmentation. This experiment is used
in metabolite profiling to search for common
structural cores (the product ion) - that maybe
characteristic of a particular compound class (in a
complex mixture).
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(C) Neutral
loss scanning.
Quadrupole 1 is scanned as well as Quadrupole 3. The
combination of scanning is used to detect precursor
ions that fragment via a specific neutral loss during
CID fragmentation. This mode of operation is also
often used in metabolite profiling to search for
common functional groups (the neutral lost) - that
maybe characteristic of a particular compound class
(in a complex mixture).
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