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Pfeiffer <strong>Vacuum</strong><br />
Page 88<br />
Formula 4-1<br />
Quadrupole<br />
deflection voltage<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
The operating principle of sector field mass spectrometers is shown in Figure 4.3.<br />
Ion Source<br />
Figure 4.3: Operating principle of a sector field mass spectrometer<br />
Neutral gas molecules are ionized in an ion source through electron bombardment. The electrons<br />
thus generated are accelerated into the magnetic sector field with the aid of an electrical<br />
voltage. The magnetic field is homogeneous in the area of the trajectories of the ions and is<br />
positioned perpendicular to the image plane. Helium ions having a mass of 4 amu are able to<br />
pass through a slot to reach the detector. All other molecules are unable to pass through the<br />
slot and are re-neutralized. The ion current measured for helium is proportional to the helium<br />
partial pressure.<br />
4.1.2 Quadrupole mass spectrometers (QMS)<br />
4.1.2.1 Quadrupole mass filter<br />
The filter system of a quadrupole mass spectrometer consists of four parallel rods arranged<br />
in the form of a square. Each pair of opposite rods in Figure 4.4, designated (+) or (-), is<br />
connected with the other. Voltage<br />
U 1 = U + V . cos �t<br />
is applied between the two pairs of rods.<br />
Ion Detector<br />
Magnetic Sector<br />
Field<br />
At this point, only a brief phenomenological description of the operating principle will be<br />
provided. Reference is made to the literature for a detailed presentation [14, 15, 16].<br />
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