How an ion trap works

An ion trap has electrodes like a single quadrupole, but wrapped into a circle. There are thus two convex end-cap electrodes, and a ring electrode shaped like a dough-nut. The ions enter and leave through the end-caps.

A voltage is applied between the ring electrode and the two end-caps, so an ion inside the trap will find itself in a potential well. The wood and cardboard model to the left is an inaccurate model of this potential well. The red areas represent a cross-section of the end-caps at a positive voltage, the blue bits a cross-section of the ring electrode, which in this model is negative.

A positive ion will tend to run down the slope towards the blue, negative electrode.

However, the voltage applied to the trap is an alternating one. As the ion falls down towards the negative electrode, the field changes, and 90 degrees later this electrode is actually the positive one. If the ion were still moving in the same direction, it would now be "falling" up hill.

The only truly stable point in this field is right in the middle, where the potential never moves up or down as the field rotates.

The ions will find themselves moving round in little circles in the trap, the largest tending to end up right in the middle at the "dead point" because of their inertia. The smaller ions will always be dragged around a bit more in the field.

Although the wooden model above is inaccurate, it needn't have been. In 1989 Wolfgang Paul was awarded the Nobel prize (shared) for his part in the discovery of the ion trap, and he was able to demonstrate the principle using a ball-bearing on an accurate model of a potential well, made up in perspex, and set to rotate on an overhead projector!

Of course, merely stabilizing ions doesn't make a mass-selective detector. How the ion trap achieves this requires a bit of consideration of the stability diagram of an ion in the trap.