| 摘要 |
A mass filter apparatus for filtering a beam of ions is described. The apparatus comprises an ion beam source for emitting the beam, and first and second mass filter stages in series to receive the ion beam from the source. A vacuum system maintains at least the second filter stage at an operable pressure below 10-3 torr. The vacuum system is arranged to maintain both the first and second filter stages at substantially the same operating pressure. The first mass filter stage is arranged for transmitting only ions having a sub-range of mass to charge ratios which includes a selected mass to charge ratio. The second filter is arranged for transmitting only ions of the selected mass to charge ratio. Hence, the second mass filter can achieve hig h accuracy detection and is not subjected to the problems experienced in the prior art, such as build up of material on quadrupole rods which results in a distorted electric field close to the rods. The first mass filter acts as a coarse filter which typically transmits 1% of ions received from the ion source. Thus, the detection accuracy and lifetime of mass spectrometers embodying this invention are greatly improved.
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