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Filaments
Thermo Fisher Scientific
a positive potential relative to the ion source helps move positive
ions out of the source and improves the ion extraction efficiency.
The number of ions formed depends upon both the quantity of
molecules in the ion source and the electron current. The electron
current is monitored on the trap electrode and a feedback circuit in
the analyser supplies unit (ASU) adjusts the filament current so that
electron flux is maintained at a constant level.
The electron beam and ion exit apertures are the only openings in the
ion source which is otherwise fully enclosed. Introduction of a
sample gas to this enclosure results in a higher pressure environment
inside the source enclosure compared with that of the surrounding
vacuum chamber where the filament assemblies are positioned and
operate. This pressure differential helps suppress instrument
backgrounds, inherent in the vacuum chamber, and reduce effects
due to sample / (hot) filament interactions.
Figure B–4
illustrates the arrangement of the Prima PRO ion source
assembly. This is of the Nier type, in which ionisation is achieved by
collision of the sample molecules with a collimated electron beam;
the source materials are primarily ceramic (alumina) and stainless
steel. All the source components are aligned by four precision-
ground ceramic rods fixed to the source mounting flange and the
assembly is held in place by the source clamp.
Source mounting flange
Ion exit plate
Half plates
Figure B–4. Prima PRO ion source assembly
Selection of a filament
There are two commonly used filament materials on the Prima
PRO (others may occasionally be used for special applications).
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Technical Description: System
Source block
Source slit
Z-Restrictor
Source clamp
Alpha slit
Ion Source
B-7
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