Page 117 - Tandem Techniques

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detector than for use as a tandem instrument for structure elucidation and solute identification. The
electrode orientation of the quadrupole ion trap mass spectrometer is shown in Figure 2.35.

Figure 2.35
Pole Arrangement for the Quadrupole
and Ion Trap Mass Spectrometers

It was shown in Figure 30, that the quadrupole spectrometer contains four rod electrodes. The ion trap
mass spectrometer has a quite different electrode arrangement and consists of three cylindrically
symmetrical electrodes comprised of two end caps and a ring. The device is small, the opposite internal
electrode faces being only 2 cm apart. Each electrode has accurately machined hyperbolic internal
faces. In a similar manner to the quadrupole spectrometer, an rf voltage together with an additional DC
voltage is applied to the ring and the end caps are grounded. In the same way as the quadrupole mass
spectrometer, the rf voltage causes rapid reversals of field direction, so any ions are alternately
accelerated and decelerated in the axial direction and vice versa in the radial direction. There are
operating parameters, (a), and (q), that define the conditions of oscillation which are analogous to those
for the quadrupole mass spectrometer but, in this case, (r0) is the internal radius of the ring electrode.
The ion trap is small and (r0) is typically about 1 cm. At a given voltage, ions of a specific mass range
are held oscillating in the trap. Initially, the electron beam is used to produce ions and after a given time
the beam is turned off. All the ions, except those selected by the magnitude