Varian 4000 GC, Руководство по эксплуатации оборудования

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Captive Screw
03-931696-01
Magnet Structure
03-931677-02
03-931696-01
Belleville Washer
14-998260-00, 3 pcs
Retaining Plate
03-931736-01
Screw
12-168001-00
Spring
21-709281-00
Filament Disk
03-920174-01
Base
03-931738-01
Gate
03-931739-01
Center Ring
03-931737-01
Isolator
03-931611-01
Screw
12-168105-00
Internal Ionization Assembly
The filament assembly consists of two filaments and a repeller plate. The two
filaments are mounted side-by-side, with each filament approximately equidistant
from the entrance hole of the oven’s electron focusing lens. Note that the 4000
MS only uses one filament at any given time; the extra filament is provided as a
backup in case the first one burns out. The repeller plate is a stainless steel plate
that is held at a lower potential than the filament to repel the electrons into the
trap.
Each filament is a rhenium ribbon. When sufficiently heated by electric current,
the filament produces electrons by thermionic emission. The filament emission
current refers to the flow of emitted electrons from the filament. The filament
emission current is set in the Internal EI or (CI) Properties tab dialog in the 4000
MS Method. Emission current settings range from 5 to 100
µ
A.
NOTE: It is unlikely that two filaments will have the same net flow of electrons
into the ion trap. Thus, the signal amplitudes from two different filaments will
probably not be the same. A typical difference is 2:1, but it may be as high as
5:1.
The electron gate is a cylindrical electrode that controls the entry of electrons into
the ion trap cavity. When electrons emitted from the heated filament are not
needed for ionization, the electron gate is held at a -150 Vdc potential. An
anodization layer insulates the electron gate from the filament end cap.
When the ion trap requires electrons, the electron gate potential changes from
-120 to +120 Vdc. The gate potential remains positive for a variable length of
time, e.g., from 10
μ
sec to 65 ms. During this interval, the electrons are focused
into the ion trap cavity with sufficient energy to achieve electron ionization of the
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