quadrapole mass filter
The instrumentation for mass spectroscopy that allows selection of ions of a specific m/e- ratio. As the desorbed material travels down an electric field toward the detector, it passes through a space boarded by four parallel rods, which are electrodes. All four electrodes have the same DC current, onto which is superimposed an AC current that oscillates at a rate calculated to retain within the four rods only ions of the desired m/e- ratio and to deflect all other ions. Thus, the signal-to-noise ratio is kept high. By varying the charges on the electrodes, the spectrum of ions can be sampled. An alternative design is the ion-trap, in which the desorbed ions enter a cavity bounded front and back by circular entrance and exit endcap electrodes, and on the sides by a ring electrode. The voltages on all three electrodes are varied so that in-coming ions, passing through aperture in the entrance endcap electrode are directed into a circular orbit inside the ring electrode, until a sufficient number are accumulated, at which time, they are directed through an aperture in the exit endcap electrode toward the detector in a sequence that reflects their m/e- ratio.
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