Agilent Technologies 5975T LTM GC/MSD, Troubleshooting And Maintenance Manual

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Analyzer
13
Troubleshooting and Maintenance Guide 205
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 31 ) 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 ( Figure 31 ) 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.
Ion focus
The voltage on the ion focus lens ( Figure 31 ) can be varied from
0 to –127 VDC. A typical voltage is between –70 and –90 VDC. In
general:
• Increasing the ion focus voltage improves sensitivity at
lower masses.
• Decreasing the ion focus voltage improves sensitivity at
higher masses.
• Incorrect ion focus adjustment results in poor high mass
response.