The detection of a test gas using mass spectrometers is far and away the
most sensitive leak detection method and the one most widely used in
industry. The MS leak detectors developed for this purpose make possible
quantitative measurement of leak rates in a range extending across many
powers of ten (see Section 5.2) whereby the lower limit ≈ 10-12 mbar · l/s,
thus making it possible to demonstrate the inherent gas permeability of
solids where helium is used as the test gas. It is actually possible in principle
to detect all gases using mass spectrometry. Of all the available
options, the use of helium as a tracer gas has proved to be especially practical.
The detection of helium using the mass spectrometer is absolutely (!)
unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present
in normal air in a concentration of only 5 ppm and is quite economical. Two
types of mass spectrometer are used in commercially available MSLD’s:
a) The quadrupole mass spectrometer, although this is used less frequently
due to the more elaborate and complex design (above all due to the
electrical supply for the sensor), or
b) the 180° magnetic sector field mass spectrometer, primarily due to the
relatively simple design.
Regardless of the functional principle employed, every mass spectrometer
comprises three physically important sub-systems: the ion source, separation
system and ion trap. The ions must be able to travel along the path
from the ion source and through the separation system to the ion trap, to
the greatest possible extent without colliding with gas molecules. This path
amounts to about 15 cm for all types of spectrometers and thus requires a
medium free path length of at least 60 cm, corresponding to pressure of
about 1 · 10-4 mbar; in other words, a mass spectrometer will operate only
in a vacuum. Due to the minimum vacuum level of 1 · 10-4 mbar, a high
vacuum will be required. Turbomolecular pumps and suitable roughing
pumps are used in modern leak detectors. Associated with the individual
component groups are the required electrical- and electronic supply sys-