9 Mass spectrometric techniques
A. AITKEN
9.1 Introduction
9.2 Ionisation
9.3 Mass analysers
9.4 Detectors
9.5 Structural information by tandem mass spectrometry
9.6 Analysing protein complexes
9.7 Computing and database analysis
9.8 Suggestions for further reading
9.1 INTRODUCTION
9.1.1 General
Mass spectrometry(MS) is an extremely valuable analytical technique in which the
molecules in a test sample are converted to gaseous ions that are subsequently separated
in a mass spectrometer according to theirmass-to-charge(m/z)ratioand detected.
Themass spectrumis a plot of the (relative) abundance of the ions at eachm/zratio.
Note that it is the mass to charge ratios of ions (m/z) and not the actual mass that is
measured. If for example, a biomolecule is ionised by the addition of one or more protons
(Hþions) the instrument measures them/zafter addition of 1 Da for each proton if the
instrument is measuring positive ions orm/zminus 1 Da for each proton lost if measuring
negative ions.
The development of two ionisation techniques,electrospray(ESI) andmatrix-assisted
laser desorption/ionisation(MALDI), has enabled the accurate mass determination of
high-molecular-mass compounds as well as low-molecular-mass molecules and has
revolutionised the applicability of mass spectrometry to almost any biological molecule.
Applications include the new scienceofproteomicsas well asindrug discovery.The latter
includes combinatorial chemistry where a large number of similar molecules (combina-
torial libraries) are produced and analysed to find the most effective compounds from a
group of related organic chemicals.
Mris sometimes used to designate relative molar mass. Molecular weight (which is a
force not a mass) is also frequently and incorrectly used.Mris a relative measure and
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