Mass Spectrometers:
A mass spectrometer for use on vacuum systems is variously referred to as a partial 16 COMMON CONCEPTS
pressure analyzer (PPA), residual gas analyzer (RGA), or quad. These are indispensable for monitoring the composition of the gas in a vacuum system, for troubleshooting, and for detecting leaks. It is however a relatively difficult procedure for obtaining quantitative readings.
Magnetic Sector Mass Spectrometers: Commercial instruments were originally developed for analytical purposes but were generally too large for general application to vacuum systems. Smaller versions provided excellent performance, but the presence of a large permanent magnet or electromagnet remained a serious limitation. Such instruments are now mainly used in helium leak detectors, where a compact instrument using a small magnet is perfectly satisfactory for resolving the helium-4 peak from the only common adjacent peak, that due to hydrogen-2.
Quadrupole Mass Filter: Compact instruments are available in many versions, combined with control units
that are extremely versatile (Dawson, 1995). Mass selection is accomplished through the application of a combined DC and RF electrical voltage to two pairs of quadrupole rods. The mass peaks are selected, in turn, by changing the voltage. High sensitivity is achieved using a variety of ion sources, all employing electron bombardment ionization, and by an electron-multiplier-based ion detector with a gain as high as 105. By adjusting the ratio of the DC and RF electrical potentials the quadrupole resolution can be changed to provide either high sensitivity to detect very low partial pressures, at the expense of the ability to completely
resolve adjacent mass peaks, or low sensitivity, with more complete mass resolution.
A major problem with the quadrupole spectrometers is that not all compact versions possess the stability for
quantitative applications. Very wide variations in detection sensitivity require frequent recalibration, and spurious peaks are sometimes present (Lieszkovszky et al.,1990; Tilford, 1994).
Despite such concerns, rapidly scanning the ion peaks from the gas in a system can often determine the gas composition.Interpretation is simplified by the limited number of residual gases commonly found in a system
(Drinkwine and Lichtman, 1980).
A mass spectrometer for use on vacuum systems is variously referred to as a partial 16 COMMON CONCEPTS
pressure analyzer (PPA), residual gas analyzer (RGA), or quad. These are indispensable for monitoring the composition of the gas in a vacuum system, for troubleshooting, and for detecting leaks. It is however a relatively difficult procedure for obtaining quantitative readings.
Magnetic Sector Mass Spectrometers: Commercial instruments were originally developed for analytical purposes but were generally too large for general application to vacuum systems. Smaller versions provided excellent performance, but the presence of a large permanent magnet or electromagnet remained a serious limitation. Such instruments are now mainly used in helium leak detectors, where a compact instrument using a small magnet is perfectly satisfactory for resolving the helium-4 peak from the only common adjacent peak, that due to hydrogen-2.
Quadrupole Mass Filter: Compact instruments are available in many versions, combined with control units
that are extremely versatile (Dawson, 1995). Mass selection is accomplished through the application of a combined DC and RF electrical voltage to two pairs of quadrupole rods. The mass peaks are selected, in turn, by changing the voltage. High sensitivity is achieved using a variety of ion sources, all employing electron bombardment ionization, and by an electron-multiplier-based ion detector with a gain as high as 105. By adjusting the ratio of the DC and RF electrical potentials the quadrupole resolution can be changed to provide either high sensitivity to detect very low partial pressures, at the expense of the ability to completely
resolve adjacent mass peaks, or low sensitivity, with more complete mass resolution.
A major problem with the quadrupole spectrometers is that not all compact versions possess the stability for
quantitative applications. Very wide variations in detection sensitivity require frequent recalibration, and spurious peaks are sometimes present (Lieszkovszky et al.,1990; Tilford, 1994).
Despite such concerns, rapidly scanning the ion peaks from the gas in a system can often determine the gas composition.Interpretation is simplified by the limited number of residual gases commonly found in a system
(Drinkwine and Lichtman, 1980).
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