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How an ion trap works - 2
| The ion trap has a stability diagram not unlike that of the single quadrupole, but it's not symmetrical above and below the x-axis because of the slightly different geometry. Unlike single quadrupoles, ion trap instruments tend to operate AC-only, holding their ions, in effect, along the x-axis of this diagram. |  |
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As for the single quadrupole, large ions have large stability regions, small ions small ones, but rather than draw it that way, it is easier to draw one stability region, and represent the big ions to the left of the small ions. This is the same as imagining that we have a particular AC voltage applied, and at this voltage, the big ions will find themselves to the left of their stability diagram, while the small ions will be to the right of their (smaller) stability region. |
Therefore at a very simplistic level, all that is necessary to convert an ion-trap into a mass selective system is to ramp the AC voltage gradually upwards, and the ions will fly out of the trap into the detector in order of mass, smallest first.
There are a few (a lot!) of extras. Firstly, if the trap operated this way and with a good vacuum, the ions would tend to accelerate into wider and wider orbits, and leave the trap anyway. Therefore a dampening gas, Helium, is allowed to leak very slowly into the trap to maintain a slightly raised pressure. Secondly, resolution of the trap can be vastly improved by bringing the ions into more coordinated orbits by adding a small AC voltage between the two end-cap electrodes. If this is not done, then as the main voltage forces an ion to become unstable, the ion may have a long, or a short, distance to travel to get out of the trap. There would be a difficulty in telling the difference between ions that had a long way to go, and slightly heavier ions that started to leave the trap later, but happened to have a shorter path to the nearest exit.
The end-cap AC voltage is also ramped as the main voltage is ramped, to encourage the right size of ion to leave.
The real forté of the ion-trap however is fragmentation.