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of the applied rf voltage, are lost to the walls of the trap, and the remainder continue oscillating in the
trap. The potential of the applied rf voltage is then increased, and the ions sequentially assume unstable
trajectories and leave the trap via the aperture to the sensor. The ions exit the trap in order of their
increasing m/z values. The first ion trap mass spectrometers were not very efficient, but it was found
that the introduction traces of helium to the ion trap significantly improved the quality of the spectra.
The improvement appeared to result from ion-helium collisions that reduced the energy of the ions and
allow them to concentrate in the center of the trap. The spectra produced are quite satisfactory for solute
identification by comparison with reference spectra. However, the spectrum produced for a given
substance will probably differ considerably from that produced by the normal quadrupole mass
spectrometer.

The Time of Flight Mass Spectrometer

The time of flight mass spectrometer was invented many years ago but, due to the factors controlling
resolution not being clearly recognized and also due to certain design defects that occurred in the first
models, it exhibited limited performance and was rapidly eclipsed by other developing mass
spectrometer techniques. However, with improved design, modern fabrication methods and the
introduction of Fourier transform techniques, the performance has been vastly improved. As a result,
there has been a resurgence of interest in this particular form of mass spectrometry. A diagram of the
time of flight mass spectrometer is shown in Figure 2.36.

In a time of flight mass spectrometer the following relationship holds,

where (t) is the time taken for the ion to travel a distance (L)
(V) is the accelerating voltage applied to the ion,
and (L) is the distance traveled by the ion to the ion sensor.