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5.5 Vacuum Systems

Almost everything about mass spectrometry requires that the experiment be conducted in vacuum. The filament used to generate the electron beam for ionization will rapidly oxidize in the present of air; the ion signal is reduced by collisions with ambient gas molecules; some instruments utilize electrostatic lenses that operate using sufficiently high electric fields that can discharge via ambient gas; the electron multiplier detector is essentially a capacitor carrying 1000-3000 volts, and a discharge would destroy it. The electron multiplier can also detect neutral collisions, and its operational life depends on the number of collisions it detects; the number density of gas at ambient pressure is sufficiently large that the detector lifetime is exceeded in a very short time. Pressures less than one atmosphere (760 torr) are described as rough vacuum, high vacuum and ultra high vacuum. Analytical mass spectrometers operate in the high vacuum pressure range.


John S. Riley, DSB Scientific Consulting