Technical Description: System
Ion Source
Thermo Fisher Scientific
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
B-5
Component
Description
Foreline trap
Sentinel PRO systems include a foreline trap fitted
immediately above the mechanical rotary pump to
prevent oil vapour back-streaming due to low
backing line pressures. On Prima PRO systems this
is not required due to the viscous flow of sample
gas into the rotary pump, from the inlet bypass line,
which suppresses vapour back-streaming.
Atoms or molecules of gas must carry an electrical charge before
they can be analysed, i.e. they must be ionised. Ionisation takes place
in the ion source where a beam of electrons is passed through the
sample gas. Electrons that either collide with or pass in close
proximity to the sample gas molecules cause electrons to become
detached from or adhere to the molecule. In the former case, the ion
formed will be positively charged; in the latter, negatively charged.
Positive ions are used exclusively for the analysis.
The electron beam is derived from a hot wire filament and is focused
magnetically through the ion source. The effect of the magnetic field
on the electron beam is similar to that experienced by ions in the
magnetic sector and they will describe a circular path. Due to the
smaller mass of the electron, however, the paths have a much smaller
radius and the beam tends to spiral along the lines of magnetic field.
Ion Source
