Principle of the Quadrupole Ion Trap

For the QIT, the electric field has to be considered in three dimensions. Let the potential to be applied to the ring electrode (jcy-plane) while - to is applied to the hyperbolic end caps. Then, the field can be described in cylindrical coordinates by the expression [137,141] 

Solving these differential equations which are again of the Mathieu type yields the parameters and 

The way the three-dimensional quadmpole field acts to keep ions within a certain volume, i.e., within a potential well some electron volts in depth, can be illustrated by a mechanical analogue A ball has to be prevented from rolling from a saddle by rotating the saddle just right to bring the ball back to the middle before it can leave the surface via one of the steeply falling sides (Fig. 4.43). Paul demonstrated the dynamic stabilization of up to three steel balls by such a device in his Noble lecture. [103,104] 

The trajectories of low-mass ions in a QIT were shown to be similar to those observed for charged aluminum dust particles. [146-149] Wuerker recorded Lis-sajous trajectories, superimposed by the RF drive frequency, as a photomicrograph (Fig. 4.44). [146] The complex motion of the ions is the result of the two superimposed secular oscillations in r and r direction. 

The use of a light buffer gas (0.1 Pa He) to dampen the ion motion towards the center of the trap significantly enhances resolution and sensitivity of the QIT. [150] 

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