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9.3.3 Ion trap mass spectrometry

Ion trap mass spectrometers use ESI to produce ions, all of which are transferred into

and subsequently measured almost simultaneously (within milliseconds) in a device

called anion trap(Fig. 9.7). The trap must then be refilled with the ions that are

arriving from the source. Therefore, although the trap does not measure 100% of all

ions produced (it depends on thecycle timeto refill the trap then analyse the ions) this

results nevertheless in a great improvement in sensitivity relative to quadrupole mass

spectrometers where at any given moment only ions of one particular m/z are

detected. ESI–ion trap mass spectrometers have found wide application for analysis

of peptides and small biomolecules such as in protein identification by tandem MS;

liquid chromatography/mass spectrometry (LC/MS); combinatorial libraries and rapid

analysis in drug discovery and drug development.

End electrodes

Ring electrode

From ion source

and quadrupole

1

4

Detector

2

35

Detector

Fig. 9.7Diagram of an ion trap. The ion trap contains three hyperbolic electrodes which form a cavity in a

cylindrical device of around 5 cm diameter in which the ions are trapped (stored) and subsequently analysed.

Each end-cap electrode has a small hole in the centre. Ions produced from the source enter the trap through the

quadrupole and the entrance end-cap electrode. Potentials are applied to the electrodes to trap the ions

(diagrams 1 and 2). The ring electrode has an alternating potential of constant radio frequency but variable

amplitude. This results in a three-dimensional electrical field within the cavity. The ions are trapped in stable

oscillating trajectories that depend on the potentials and them/zof the ions. To detect these ions, the

potentials are varied, resulting in the ion trajectories becoming unstable and the ions are ejected in the

axial direction out of the trap in order of increasingm/zinto the detector. A very low pressure of helium is

maintained in the trap, which ‘cools’ the ions into the centre of the trap by low-speed collisions that

normally do not result in fragmentation. These collisions merely slow the ions down so that during scanning,

the ions leave quickly in a compact packet, producing narrower peaks with better resolution. In sequencing,

all the ions are ejected except those of a particularm/zratio that has been selected for fragmentation (see

diagrams 3, 4 and 5). The steps are: (3) selection of precursor ion, (4) collision-induced dissociation of this

ion, and (5) ejection and detection of the fragment ions.

362 Mass spectrometric techniques