Vacuum Technology Know How - Triumf

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The higher the mass range, and if differentiation between adjacent masses is still
to be made, the stricter will be the requirements relating to the relative accuracy of the
mass filter. In this simple picture, the required relative accuracy is scaled linearly with
the mass range
Summary
A quadrupole mass filter is a dynamic mass filter for positive and negative ions. The mass
scale is linear to the applied amplitude of the HF voltage. Mass resolution can be conveniently
and electrically set by means of the ratio between the DC voltage U and the high-frequency
voltage amplitude V. Due to their small dimensions and light weight, quadrupole mass
spectrometers are suitable both as pure residual gas analyzers and, in higher-quality design,
as sensors for gas analysis.
4.1.2.2 Ion sources
Before gases can be analyzed in a mass filter, they must first be ionized in an ion source by
means of electron bombardment.
Electrons are emitted from an electrically heated cathode (filament). A voltage is applied
between anode and cathode, which accelerates the electrons. Neutral gas molecules that are
present in the formation space between anode and cathode are ionized by collisions between
electrons, forming single and multiple positive ions. The energy of the colliding ions exerts a
significant influence on both the number and type of ions that will be formed.
Indicated pressure
10 1
cm -1 mbar -1
1
10 -1
10 -2
10 1 10 2 10 3 10 4 10 5 10 6
Figure 4.6: Ion density as a function of electron energy
Source: Pupp / Hartmann, Vakuumtechnik, Grundlagen und Anwendungen, Hanser Verlag
Electron energy
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