Fault Diagnosis
Mass Spectrometer Faults
Thermo Fisher Scientific
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
9-7
There can also be changes to the spatial distribution of electron
trajectories and in the electron energies following impact with the
sample gas molecules. The nature and efficiency of ionization, as
well as the energies and trajectories of ions exiting the source, can all
be affected. As a result of all these effects, there can be changes in
the absolute sensitivities, relative sensitivities, and cracking patterns.
The magnitude of the effects depends very much on the types of
gases being analyzed –the more overlaps or the greater the variation
in ion types (e.g. mass and multiplicity of charges) the greater the
potential impact.
Contamination can result in sample reacting with or being absorbed
on the walls of the ionization chamber, which can modify the surface
potentials due to localized charging effects.
Check the calibration method has been correctly configured. The
general “rules” for configuring calibration gases are as follows:
Fragmentation:
Can only be selected when there are no overlaps
in the spectra of the gas component, at the selected masses, with
those of any other gas in the cylinder.
Background:
Enable for background measurement only when
there are no contributions at any of the selected peaks in the
cracking pattern for the gas component in the calibration
cylinder.
Sensitivity:
Can be calibrated only when there are no
overlaps at the principle peak.
Linearity:
Can always be calibrated provided the component is
present at a significant and accurately known level in a gas
mixture.
Check that there is flow when the calibration gases are selected. If a
non-hazardous calibration gas is selected and no hazardous sample
gases are flowing through the RMS, removal of the RMS cover
allows for verification that the RMS is selecting the correct
calibration gas stream.
Check that the Penning pressure reading is consistent with that
obtained during original installation. Depending on the molecular
leak type used, the Penning pressures for nitrogen or air are typically:
70 micron pinhole leak
ca. 5 x 10
-6
mbar
50 micron pinhole leak
ca. 3 x 10
-6
mbar
30 micron pinhole leak
ca. 1 x 10
-6
mbar
Note these pressure readings are gas dependent.
Poor Results
Following
Calibration
