Technical Description: System
Ion Source
B-8
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
Thoriated Iridium
The emissive material is a thoria (thorium oxide) coating on an
iridium wire support. This has a lower work function than other
common filament materials such as tungsten and rhenium.
Thoriated iridium filaments are recommended for operation at
low pressures and low pumping speeds since less out gassing
occurs due to the lower working temperature of the filament.
This also makes the filament more tolerant of operation under
conditions of overpressure and is less likely to “burn out” and
fail. Thoriated iridium filaments are used for the majority of
applications on the Prima PRO and Sentinel PRO.
Tungsten Thoria Alloy
This material is a 1% ThO / 99% W alloy and is formed into a
coiled wire, which gives prolonged life for hydrocarbon
applications, particularly where there is greater than 20% of
hydrocarbon compounds of molecular weight higher than 40.
Filament Current Limit
The filament current limits for each of these materials are given
in the table below.
Table B
–3.
Filament current limits
Filament
Current Limit (Amps)
Thoriated iridium
3.1
Tungsten / ThO alloy
3.0
shows how the filament is mounted to the source
block.
Figure B
–5.
Mounting of the filament onto source block
