The detection of a test gas using mass spectrometers is by far 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 Leak types and rates) 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 systems and software which, via a microprocessor, allow for the greatest possible degree of automation in the operating sequence, including all adjustment and calibration routines and measured value display.