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www.pfeiffer-vacuum.net<br />
Inlet<br />
System<br />
Figure 4.2 shows the typical structure of a mass spectrometer system:<br />
The substances to be analyzed are admitted into a vacuum chamber through the<br />
inlet system via a capillary or metering valve, for example, and then<br />
Partially pumped down to the system’s working pressure<br />
The actual analyzer is located in the vacuum and consists of the following components:<br />
The ion source ionizes neutral gas particles, which are then<br />
Sorted in the mass filter on the basis of their mass-to-charge ratio m / e<br />
The ion current is measured using a Faraday detector or a secondary electron multiplier<br />
(SEM) after the ions have left the separating system. The measured current is a parameter<br />
of the partial pressure of the respective gas molecules or a parameter of fractals that may<br />
possibly have been generated in the ion source<br />
A data analysis system processes the ion currents measured with the aid of the detector<br />
and presents these currents in various forms. Today, data analysis software programs are<br />
capable of supporting the user in interpreting mass spectra.<br />
Mass spectrometers differ as a result of the wide variety of available versions. The main<br />
difference consists of the separating systems. The following four types of mass filters are in<br />
widespread use today:<br />
Sector field devices use the deflection effect of a magnetic field on moving<br />
charge carriers<br />
Time-of-flight mass (TOF) spectrometers utilize the differing velocities of molecules of<br />
equal energy for separation<br />
In ion traps, the trajectories of the ions are influenced by a high-frequency field<br />
Quadrupole mass spectrometers utilize the resonance of moving ions in a<br />
high-frequency field (similar to ion traps)<br />
Ion Source Mass Filter Detector Data Analysis<br />
System<br />
<strong>Vacuum</strong> System<br />
Figure 4.2: Components of a mass spectrometer<br />
Our discussion will be confined to sector field mass spectrometers and quadrupole mass<br />
spectrometers, as these are the mass spectrometers that are most widely used in the field of<br />
vacuum technology.<br />
4.1.1 Sector field mass spectrometers<br />
Because of their simple, robust design, sector field mass spectrometers are used for helium<br />
leak detectors, where only little demands are placed on resolution.<br />
Page 87<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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