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places could have different energies. This energy spread (and fragmentation occurring

during this initial extraction period) usually broadens the peak corresponding to any

particular ion which leads to lower mass accuracy. However, if extraction is delayed

until all ions have formed, this spread is minimised. The procedure is known as

delayed extraction(DE), whereby the ions are formed in either a weak field or no

field during a predetermined time delay, and then extracted by the application of a

high-voltage pulse. The degree of fragmentation of ions (Section 9.3.10) can also be

controlled, to some extent, by the length of the time delay. Delayed extraction is

illustrated in Fig. 9.15.

9.3.10 Post-source decay

Post-source decay(PSD) is the process of fragmentation that may occur after an ion

(theprecursorion) has been extracted from the source. Many biological molecules,

particularly peptides, give rise to ions that dissociate over a timespan of microseconds

and most precursor ions will have been extracted before this dissociation is complete.

The fragment ions generated will have the same velocity as the precursor and cause

peak broadening and loss of resolution in a linear TOF analyser (Fig. 9.16). The

problem is overcome by the use of a reflector.

28000 29000 30000

m/z

0.05

0.10

0.15

0.20

0.25

29281.7

28109.0

29434.4

28215.8

28313.2

28423.5

29873.2

29616.8

27872.1

27803.4

% Intensity

Fig. 9.14MALDI–TOF spectrum of protein isoforms. The spectrum is almost exclusively singly charged ions

representing the molecular ion species of the constituent proteins. Compare this spectrum with the electrospray

spectrum of another protein (Fig 9.6) where the multiply charged ions result in multiple peaks which would

make it harder to interpret masses of mixtures. (I acknowledge the assistance of Bruker Daltonics who

carried out the analysis.)

370 Mass spectrometric techniques