Monopole mass spectrometer is increased by nearly an order of magnitude when a weak external magnetic field is applied so that the ion beam is deflected towards the rod. This magnetic field eliminates background noise at the low end of the mass scale

Herzog, R. F.

English

NASA Technical Reports Server

December 1969	 Brief 69-10666 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Monopole Mass Spectrometer with Improved Sensitivity
and Reduced Background 
A monopole mass spectrometer with improved 
sensitivity and reduced background has been devel-
oped by a NASA contractor, It has been determined 
that the sensitivity of such a spectrometer can be in-
creased by almost an order of magnitude, when a weak 
external magnetic field is applied in such a direction 
that the ion beam is deflected towards the rod. This 
magnetic field also eliminates the background noise 
at the low end of the mass scale. In addition, there 
is the added advantage that the voltage adjustment 
is less critical for a monopole spectrometer than for a 
quadrupole one. 
In the monopole mass spectrometer a magnetic 
field configuration is used similar to that of the well-
known quadrupole mass spectrometer. In the monopole 
one, however, the three rods are replaced by the 
grounded symmetry planes between the rods, the two 
planes forming the so-called v-electrode that is on 
ground potential, making it possible for a single rod to 
produce the characteristic field. The ion orbits in both 
instruments are identical and are comprised of two 
independent oscillations in a y-direction from the apex 
of the v-electrode to the center of the rod, as well as 
in an orthogonal direction. In the quadrupole design, 
the ions can oscillate several times about the instru-
mental axis without being removed unless the ampli-
tude is so large that the ions strike any of the four rods. 
The ions in the monopole design are also removed if 
they strike the v-electrode, preventing oscillations 
about the instrumental axis, since all ions face removal 
after about a half cycle of oscillation in the y-direction. 
As a result, the monopole voltage adjustment is sub-
stantially less critical than that needed for the quadru-
pole device, and the voltages required to analyze a
specific mass range are reduced significantly. Although 
this is a definite advantage of the monopole device, 
only a small number of the incoming ions can be 
utilized in it. That portion of any slightly diverging 
incoming beam that is directed to the v-electrode is 
removed immediately. Also, a fraction of the ions that 
are originally directed away from the v-electrode, but 
which enter the field during the time interval when the 
field is directed toward the v-electrode, are removed 
by this electrode. Therefore, the transmission coeffi-
cient of the monopole filter is significantly less than 
that for the quadrupole filter. 
In the development of the monopole mass spec-
trometer, it was assumed that its sensitivity could be 
increased by deflecting the ion beam away from the 
v-electrode using a weak, external magnetic field 
normal to the axis of the monopole spectrometer. The 
direction of this field was such that the beam was de-
flected both away from the apex of the v-electrode and 
towards the center of the rod electrode. In an experi-
ment, it was observed that the imposition of such a 
magnetic field increased the peak heights by a fraction 
of 5 to 10. This observed effect was attributable pri-
marily to the ion beam deflection where it had to be 
expected that a deflection towards the v-electrode 
would reduce the sensitivity. As a matter of fact, a 
relatively slight beam deflection is sufficient to pro-
duce the desired effects. 
The imposition of the magnetic deflection field had 
another beneficial effect. Since voltages on the rod 
are relatively low in the vicinity of hydrogen I and 2, 
heavier ions are hardly deflected and have a high 
probability of being transmitted through the monopole 
filter. This causes an increased background at the low 
(con(inued overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
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end of the mass scale, which can be eliminated by the 
application of the transversal magnetic field. 
Notes: 
I. This innovation should be of particular interest to 
research and development agencies of scientific 
equipment manufacturers, the oil industry, and the 
steel industry; it should also be useful in geological 
explorations. 
2. The following documentation may be obtained 
from:
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65)
Reference: NASA CR-101445 (N69-28972), 
Mass Spectrometer for D-Region Studies 
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: Richard F. Herzog of

Technology Division
GCA Corporation
under contract to 
NASA Headquarters 
(HQN- 10476)
. 
Brief 69-10666	 Category 01


Monopole mass spectrometer with improved sensitivity and reduced background

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