Monopole mass spectrometer uses sweep method to detect vacuum system contaminants. Method requires little increase of total scanning time

Herzog, R. F.

English

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NASA TECH BRIEF 
July 1970
	 Brief 70-10057 
NASA Tech Briefs announce new 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 Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Mass Spectrometer Detects High Molecular Weight Components 
A novel sweep method for monopole mass spec-
trometers permits the detection of vacuum system 
contaminants of very high molecular weight with 
little increase in scanning time. 
The basic monopole mass spectrometer uses a 
radio frequency electric field between a two-electrode 
configuration for analyzing ions of different e/m. 
One electrode has a right-angle cross section; the 
other has a circular cross section whose center is 
located on the bisector of the right angle. It employs 
a field configuration similar to that of the better 
known quadrupole mass spectrometer. In this case, 
however, the three rods are replaced by grounded 
symmetry planes between the rods. The two planes 
form the so-called V-electrode which is on ground 
potential so that only a single rod is required to pro-
duce the characteristic field. Eon orbits in both instru-
ments are identical and consist of two independ-
ent oscillations in a direction y from the apex of 
the V-electrode to the center of the rod, as well as in 
an orthogonal direction x. The oscillation ampli-
tudes are relatively small for ions of one particular 
mass associated with the superimposed ac and dc 
voltages supplied to the rod electrode. These ions 
pass the filter and are detected thereafter by a Faraday 
cage or an electron multiplier. Relatively high oscil-
lation amplitudes result when ions of larger masses 
enter the same field; these ions impinge on the rod 
electrode or on the exit aperture. Ions of smaller 
masses impinge on the V-electrode. 
The novel sweep method for monopole mass spec-
trometers consists of two parts: (I) A low mass 
range is swept in. the usual manner by increasing the 
ac voltage and keeping the dc to ac voltage ratio coh-
stant and rather high to obtain good resolution
and sensitivity. (The highest mass which can be 
measured in this way depends primarily on the maxi-
mum ac voltage available from the power supply.) 
(2) An adjacent high mass range is swept by reduc-
ing the dc voltage and keeping the ac voltage con-
stant at its maximum value. The mass is inversely 
proportional to the dc voltage. In this regard the 
mass scale is identical to the one obtained in a con-
ventional magnetic mass spectrometer, if the mag-
netic field strength is kept constant and the scanning 
is performed with the acceleration voltage. The 
monopole mass spectrometer provides much better 
transmission and sensitivity for high masses than a 
small magnetic vacuum analyzer. Thisspectrometer 
is free of the hysteresis errors of a magnetic spectrom-
eter. 
The mass range of the monopole mass spectrometer 
can be extended to infinity if the ac rod-voltage is 
kept constant and the dc rod-voltage is reduced. The 
mass is inversely proportional to this dc voltage. 
If the ion energy is kept constant, the sensitivity stays 
essentially constant over the whole mass range and 
the resolution is inversely proportional to the mass. 
If the ion energy is reduced in proportion to the dc 
voltage, Xhe resolution stays constant and the sensi-
tivity is lessened. This method of mass scanning per-
mits the detection of vacuum system contaminants 
of very high molecular weight with little increase of 
total scanning time. No ions, regardless of how heavy, 
can escape detection. 
Notes: 
1. Laboratory equipment companies may be inter-
ested in this novel sweep method, which would be 
of use in monopole mass spectrometers used for gas 
analysis, leak detection, and air pollution studies. 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19700000053 2020-03-17T00:03:37+00:00Z
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: TSP70- 10057
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: R. F. Herzog of

GCA Corporation
under contract to

NASA Headquarters 
(HQN-10477) 
Brief 70-10057	 CategoryOl


Mass spectrometer detects high molecular weight components

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