Poor Sensitivity; Poor Stability On Startup - Thermo Scientific Prima PRO User Manual

Fault Diagnosis
Mass Spectrometer Faults

Poor Sensitivity

Poor Stability on
9-6 Prima PRO & Sentinel PRO Mass Spectrometers User Guide
For low or unstable sensitivity and / or poor peak shape:
Ensure that the source and lens tuning is optimized.

Check for contamination of the source or lens. If necessary,

remove and clean.
Ensure that an electrode in the source or collector assembly is not

disconnected and / or charging up. Normally, in this condition,
no peaks are observed.
Try the second filament. Sensitivity can be degraded after a

filament has been in use for a long period, often due to
contamination or physical movement of the filament wire.
Check that the Penning gauge pressure reading is normal.

An extremely low pressure may indicate blockage of the inlet –
blocked RMS and / or screen filter and / or capillary and / or
glass leak.
There are a number of factors that influence the initial stabilization
Startup (settling) of the mass spectrometer following startup or restart.
Filament emission characteristics are altered by reactions with the
sample gas and depend on the specific chemistry of the gas. Different
gas mixtures will have differing effects on the filament. Gas mixtures
that have a net oxidizing effect tend to be the reverse of mixtures that
are net reducing. Where an instrument has either been idle or
analyzing a radically different sample stream, there will be an initial
stabilization in the filament characteristics as they adjust to the
average composition of the samples. This will normally result in an
overall general drift in absolute peak heights. From cold startup,
there will be other factors contributing to this effect such as out-
gassing from the enclosure and various system elements as they
come up to working temperature.
If there is a very large change in the average gas composition with
time, for example from one day to the next, then it may be necessary
to recalibrate on a daily basis to maintain system accuracy within an
acceptable bandwidth. Most processes, however, involve a stable
average composition of process gas samples.
Reactions between the sample gases and filament result in changes in
the electron work function of the filament emissive material. The
electron flux through the ionization region is regulated by control of
the trap current, which will adjust filament current to affect any
necessary compensation. This in turn changes the filament
temperature and could affect the temperature distribution of the ion
source, even though the overall source temperature may be regulated
to ± 0.1°C by a source temperature controller.
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