A study of the quadrupole mass spectrometer for operation in hostile environments.
The aim of the project described in this thesis was
to develop an analytical measuring instrument for use as a
residual gas analyser in industrial vacuum systems. The
quadrupole mass spectrometer was chosen operating in a new
and as yet not fully tested mode. In this novel mode, only
a. c. voltages are applied to the rods and a special
collection system using energy and spacial filtering is
employed (so called the a. c. mode). This is in contrast to
the normal mode, where a. c. and d. c. voltages are applied
to the rods, and a simple plate or an electron multiplier
collector is used.
This thesis describes a series of experiments to
investigate the properties of the quadrupole mass filter
in the a. c. mode when operating in hostile environments.
To carry out this investigation a small vacuum furnace was
built and charged with workshop swarf and viton gaskets.
The sensitivity of a 50 mm quadrupole mass filter operating
in the a. c. mode was observed to fall by 40% after
100 hours of operation at 10-3 mbar in the above gases.
However, the relative sensitivity over the mass range 10-
100 a. m. u. remained the same. In a separate set of experiments
the relative sensitivity of an 85 mm quadrupole mass
filter was examined in the a. c. mode. The ratio of singly
to doubly charged ions for argon and krypton changed by
15% or less after 50 hours contamination. This is in
complete contrast with that of the normal mode. In the normal mode instability caused by even minor comtamination
was always observed. There is shown in the thesis to be a
theoretical basis for this difference. It was verified
that heating the source to 200°C and sweeping the ion
energy to relatively high value for high mass successfully
aided the development.
In addition, the use of a Nier type source, high
pressure limitation, and multi-peaks were also investigated.
The Nier source was found to provide an improved
ion beam, enabling good resolution to be achieved. The
high pressure limit of a quadrupole mass filter in the
a. c. mode was found to be 10-4 mbar. The multi-peaks were
shown to be a fundamental phenomenon which could be
avoided when operating with normal ion energy. The resolution
of a quadrupole mass filter in the a. c. mode was
found to be proportional to the power 2.3 of the number of
cycles ions spent in quadrupole field.