154 Troubleshooting and Maintenance Manual
7 Analyzers and Collision Cell
Other Source Elements
Magnet
The field created by the magnet directs the electrons emitted by the filament
into and across the ionization chamber. The magnet assembly is a permanent
magnet with a charge of 350 gauss in the center of the field.
Repeller
The repeller (Figure 27) forms one wall of the ionization chamber. A positive
charge on the repeller pushes positively charged ions out of the source
through a series of lenses. The repeller voltage is also known as the ion energy,
although the ions only receive about 20% of the repeller energy. The repeller
voltage can be varied from 0 to +42.8 VDC. Some tune programs use a fixed
repeller voltage. Others ramp the repeller voltage to find the optimum setting.
• Setting repeller voltage too low results in poor sensitivity and poor high
mass response.
• Setting repeller voltage too high results in precursors (poor mass filtering)
and poor low mass resolution.
Drawout plate and cylinder
The drawout plate on a standard EI ion source (Figure 27) forms another wall
of the ionization chamber. The ion beam passes through the hole in the
drawout plate and into the drawout cylinder. The drawout cylinder is slotted.
The slots correspond to slots in the source body. These slots allow carrier gas
and un-ionized sample molecules or fragments to be pulled away by the
vacuum system. The drawout plate and drawout cylinder are both at ground
potential.
Extractor lens
The extractor lens is a part of the EI extractor ion source. This lens replaces
the drawout plate and cylinder on a standard EI ion source, but performs a
similar function. A voltage is applied to the extractor lens to increase focusing
in the EI ion source.
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