Fault Diagnosis
Leak Detection
9-4
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
Warning! High Voltages!
To avoid flash over, exercise extreme
caution when probing with helium in the vicinity of the ion source
feed through. Helium is extremely susceptible to “break down” when
exposed to high electric fields.
Abnormally high atmospheric peaks and a slight rise in the
background pressure, as displayed by the Penning gauge reading,
may indicate a small leak in the instrument. In general, the most
convenient method for leak detection is to use the extreme sensitivity
of the instrument itself. Tune the mass spectrometer to a suitable gas
(helium, mass 4, is particularly effective). Then probe around the
vacuum chamber using a fine collimated stream of the same gas. A
sharp increase in the ion current of the monitored peak indicates that
the search gas has entered the vacuum system. Finer gas flows may
be required to help pinpoint the exact location of the leak.
Leaks may sometimes occur at elastomer seals, particularly after they
have been disturbed. First check that the seal fixings are not loose. If
they are, tightening may cure the problem; however, care should be
taken not to over tighten as damage to the seal or fixings may result.
In most instances, this is due to contamination of the sealing surface
by dust, other particulates, or fibres. These can normally be removed
by use of a clean, filtered compressed gas or wiping the surface with
a clean, lint free cloth. A small amount of solvent, such as IPA,
applied to the cloth may assist in the process.
A larger leak which results in a pressure rise above 3 x 10
-5
mbar will
cause the emission to trip or prevent the system from attaining the
trip pressure on pumping down from atmosphere. This would prevent
switch on of the mass spectrometer. In this case, the leak can
potentially be located by probing around the vacuum system with
helium and looking for slight changes in the Penning gauge reading.
Leak
Detection
