Types of instruments studied were magnetic sector, omegatron, quadrupole, and monopole. Experimental results obtained included absolute sensitivity to argon, relative sensitivity to ten gases (hydrogen, helium, neon, nitrogen, carbon monoxide, oxygen, argon, carbon dioxide, krypton, and xenon), and cracking patterns for these gases

Hultzman, W. W.

English

NASA Technical Reports Server

September 1975	 B75-10185 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service. Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Characteristics and Performance Study
of Mass Spectrometer Residual Gas Analyzers 
The operation and properties of various mass-
spectrometer residual gas analyzers for use in vacuum 
measurements were analyzed in terms of efficiencies of 
ion extraction, ion separation and transmission, and ion 
collection. Types of instruments studied were magnetic 
sector, omegatron, quadrupole, and monopole. Experi-
mental results obtained included absolute sensitivity to 
argon, relative sensitivity to 10 gases (hydrogen, helium, 
neon, nitrogen, carbon monoxide, oxygen, argon, carbon 
dioxide, krypton, and xenon), and cracking patterns for 
these gases. Tests were performed at a pressure level of 
about 2x101 toff. 
In many high vacuum applications, it is important to 
identify the gas species present as well as to know the 
approximate pressure level. Examples where such informa-
tion is useful are materials research, spacecraft and space 
power system tests, thin-film processing, and outgassing, 
gettering, and pumping studies. Such measurements are 
commonly made with mass-spectrometer residual gas 
analyzers (RGAs). To determine pressures and identities 
of individual gases, knowledge of three major RGA 
characteristics is required. These are absolute sensitivity to 
a reference gas, relative sensitivity to other gases, and the 
cracking pattern of the measured gases. The cracking 
pattern denotes the total spectrum of ions that appear 
when a particular gas is ionized, expressed as percentages 
of the principal ion. The cracking phenomenon results 
from the presence of isotopes and from the processes of 
dissociation and single and multiple ionizations. Absolute 
sensitivity is the ratio of indicated output current 
contributed by either the principal ion, or the sum of all 
ions in the gas cracking pattern, to the partial pressure of 
the gas (amperes/torr). 
Relative sensitivity is the ratio of absolute sensitivity 
for a particular gas to that for a standard gas, such as 
nitrogen or argon. Argon was used in the present work. 
For total-pressure measurements made with ionization 
gages, relative sensitivity is substantially invariant with 
gage type and closely correlated with gas ionization cross 
section. However, for RGAs, relative sensitivity also 
depends on efficiencies of ion separation and collection. 
These properties are unique to individual RGA design and 
operating parameters.
Much information, is available on absolute and relative 
gas sensitivity as a function of upstream. or sample 
reservoir pressure in analytical chemistry applications. 
However, it is difficult to use this information to measure 
absolute partial pressures in a vacuum environment unless 
the flow characteristics of both the gas inlet and the 
vacuum system under consideration are accurately known. 
Only a limited amount . of information exists in the 
literature on RGA relative sensitivities for partial-pressure 
measurement. This is especially true for the intercompari-
son of different types of. RGAs on the same vacuum 
system. 
Test results showed a day-to-day variation in absolute 
sensitivity to argon was about 10 percent, even when 
voltage and current settings were repeated as closely as the 
standard control units allowed. Shift in absolute 
sensitivity after prolonged exposure to oxygen was 
inconsistent among the RGAs, and varied from 30 to 300 
percent; some instruments recovered slowly after 
exposure, and others retained a permanent shift. Absolute 
gas sensitivity can change considerably for the same RGA 
when the mass range is changed. The change in argon 
sensitivity varied from 0 to 800 percent for the four 
RGAs which had multiple ranges. 
Overall collection efficiency for the omegatron, as 
measured, was considerably higher than the others, due to 
the low electron emission current and absence of beam-
forming slits or apertures. No single parameter was found 
that would permit the prediction of an instrument's 
relative sensitivity for various gases. However, the values 
of relative sensitivity, as measured, may be used with an 
in-place calibration with a reference gas such as argon or 
nitrogen to obtain absolute sensitivity to other gases. A 
calibration ion gage may be used as a standard in such a 
calibration with a reference gas. 
The accuracy obtainable for relative sensitivity may be 
judged from the variation in nitrogen-to-argon relative 
sensitivity obtained over the time period of investigation 
(165 days). This ranged from ±2 to ±8 percent for RGAs 
with Faraday cup detectors, to about ±15 percent for 
those with electron-multiplier detectors. 
(continued overleaf) 
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The cracking pattern dita emphasize the fact that 
cracking patterns differ widely for different types of 
RGAs and even differ moderately for different ranges of 
the same RGA. Both sensitivity and cracking pattern 
depend on the type of RGA, operating parameters of ion 
sources, transmission efficiencies of the different 
separators, and relative gain of electron multiplier 
detectors. 
Notes: 
1. Further information is available in the following 
report: 
NASA TN-D-7554 (N74-19084), Characteristics 
and Performance of Several Mass Spectrometer 
Residual Gas Analyzers 
Copies may be obtained at cost from: 
Aerospace Research Applications Center 
Indiana University 
400 East Seventh Street 
Bloomington, Indiana 47401 
Telephone: 812-337-7833 
Reference: B75-10185 
2. Specific technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B75-10185
Source: William W. Hultzman
Lewis Research Center
(LEW.12393) 
B75-10185	 Category 03


Characteristics and performance study of mass spectrometer residual gas analyzers

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