The design and use of mass spectrometers to measure and identify atmospheric gas compositions of planets are discussed. Special emphasis during the design phase was given to dynamic range that the instrument must have, mass range the instrument must cover, and precision of the measurements that are necessary, for example to confirm isotope ratios. Some problems encountered in the use of the instrument are illustrated using data from the Pioneer-Venus program

Spencer, N. W.

English

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A MASS SPECTROMETER CONCEPT FOR IDENTIFYING PLANETARY
ATMOSPHERE COMPOSITION
Dr. Nelson W. Spencer _-" 7_NASA Goddard Space Flight Center N 20400
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DR. SPENCER: Professor Nier has introduced the subject very
nicely and told you a lot of details of these systems. I'd like
to use my few minutes to speak to a few principles pertinent to
some of the considerations which guide people in using mass
spectrometers for atmospheric measurements.
The basic problem is not a new one. It is to try to get a
sample of the atmosphere and measure it without modifying it.
Atomic oxygen is a good example. In most instruments it recom-
bines on the surfaces and is measured as 02, which is acceptable
if the ambient 02 is negligible (true for most cases). Thus, if
you didn't know that there was atomic oxygen up there in the
first place, you might conclude that molecular oxygen was pre-
sent until your fundamental physics told you otherwise. That's
fine for the earth, but when we go into other atmospheres, we
don't really know what is there, and then it is not quite so
obvious. I think that the discussions yesterday, particularly
those concerning Jupiter and the trace constituents emphasized
the point and illustrate the situation that we find ourselves in,
and that is how do we really analyze a sample of the atmosphere
in a rather brutal way, which is what the mass spectrometer does,
without changing its composition. So, getting a sample is a
challenging task, a concern, a consideration that one must be
aware of. Obviously, the other things that are a little more
apparent in considering a design are the dynamic range that the
instrument must have, the mass range that the instrument must
cover, the precision of the measurements that are necessary for
example, to confirm isotope rations.
A number of systems have evolved, and I want to use some of
our more recent work on Pioneer Venus as an example to illustrate
some of the problems.
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This diagram may not surprise you very much, but it is quite
fundamental. (Figure 8-7). Basically, we need some arrangement
to sample the atmosphere. We use the term "sample" in the very
broadest sense; whether you take a parcel of gas and bring it into
the instrument and analyze it or whether it flows through the in-
strument in the sense that Professor Nier was speaking about
really depends on the particular application. Fortunately, in
the upper atmosphere, in satellite usage, one can take a sample
directly into the ionization region of the mass spectrometer with-
out it having experienced surface collisions and analyze it with
perhaps what you might consider the minimum amount of modifica-
tion. However, the atmospheric sample is not the only gas ob-
served in the source because the surfaces produce gases as well.
If you can use the energy of the particles as a differentiator,
then you have a very nice tool for differentiating between the
particles which are of spacecraft origin or mass spectrometer
origin and atmospheric origin. When one goes to lower atmospheres,
and I am going to speak generally about more dense atmospheres,
then that tool is not available and the chemical effects in the
ion source are more difficult to avoid. The sample inlet system
that is represented in this block diagram reflects those portions
of the system which Conduct a sample of the atmosphere, whether
it be a batch or a continuous flowing gas, into the ion source of
the instrument.
These systems will in general have pumps. There are a vari-
ety available, the kind to be used depending upon the particular
atmosphere. For Venus, where the atmosphere is dominated by
other than inerts, Getter pumps are very handy devices. Ion
pumps are useful as well for controlling the inerts.
Most of our activities concentrate on guadrupole analyzers
as shown. The rest of the figure should be quite familiar to you.
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Figure 8-7
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The next figure (Figure 8-8), shows what a typical system
may look like. The quadrupole and the other elements of the
analyzer are shown. In general, the analyzer portion is separa-
ted from the ion source region by a relatively low conductance
ion orifice. It has its own pumping system to maintain the back-
ground gas at a suitably low level. The left portion of the slide
shows the ion source and the inlets which are closely associated
physically because it's desireable to minimize the amount of sur-
face that is exposed to the gas. The pump (and leaks) are sized
to maintain an adequate flow of gas through the ion source. Also
shown are three inlets which will be discussed later.
The next slide shows typical weights corresponding to the
block diagram (Figure 8-9 )- It must be noted, however, that
these weights are mission dependent. For example, the structure
that is required to support the various elements of the instru-
ment will vary from mission to mission and is necessarily close-
ly associated with the sample system.
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Although the next figure _Figure 8-10) is a rather poor
reproduction, it illustrates a typical instrument installation
with sample tubes projecting through the probe wall. In Pioneer
Venus, there are some particular temperature and structural
problems which require special consideration. The acceleration
forces must be supported in some manner by the elements of the
system, and that's where some of the weight appears that is not
particularly defineable, but which I classify as mission depend-
ent weight.
(Figure 8-11). I mentioned that it is necessary to accom-
modate to a rather wide range of pressures in the instrument when
descending through an atmosphere to the surface. At the same
time, it is necessary to optimize, for dynamic range purposes,
the pressure in the ionization region. There are a number of
devices that can be used to reduce the atmospheric sample to
an acceptable pressure level for the mass spectrometer. At the
same time, one is concerned about the particular material that
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the glass is exposed to. Again, one has a choice. One can use
glass, quartz, ceramic devices or metals, but the basic consider-
ation is how do these materials react with the gases in the at-
mosphere.
The approach that we have taken for atmospheric probes is
to provide what we believe is a fair amount of redundancy. We
take a number of different batches of atmospheric gas during the
descent of a probe into the atmosphere and analyze each of them
individually. Three channels are shown in the slide, however,
for Venus, eight is an appropriate number. The amount of time
that the flow can take place for the particular sample really de-
pends upon the particular mission. It could be either nearly
continuous or brief. The system must also be very clean so one
can have confidence that gases are not being carried there which
will alter the analysis. The example illustrated here shows
three capillaries with an opener for uncovering and exposing
each. There is also .a device which terminates the sample by
sealing off the tube at the end of a selected flow period. Con-
sidering a number of sample tubes, the times of the various
samples can be spaced through the atmosphere to accommodate for
example the considerations that John Lewis was speaking about
yesterday where different strata in the atmosphere might prompt
one to look for different groups of gases.
Figure 8-12 illustrates one measurement scheme during a par-
ticular sample. The vertical scale represents the operating
pressure level in the ionization region. In general, it is not
constant, but for the purposes of this discussion, it makes lit-
tle difference. I think you can see essentially what happens;
at some time through an internally generated signal, the device
is exposed to the atmosphere and the gas permitted to flow into
the instrument. One can select, depending upon the particular
altitude range or the particular localized study, scans of selec-
ted mass numbers. Scans can be continuous, where you look at
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every mass, which I have labeled here non-adaptive, for purposes
of identification. You may want to study the altitude distribu-
tion of the gases. The data system can be used a little more
effectively by using an adaptive approach that looks at pre-
selected masses.
At the end of some period of time, the channel itself can be
closed and the instrument then sealed off from the atmosphere.
The capillary in this case, or whatever the leak happens to be,
is sealed off and the high-pressure gas that is now remnant in
the capillary and ion source is removed. The pump system is
thus able to reduce the remaining gas in the system to a back-
ground level. This is a particularly important concept, because
the surfaces of the instrument of the ion source do retain gases,
which must be expected, especially in an unknown and hostile at-
mosphere such as Venus; and presumably for other planets where
there may be a number of exotic components in one form or another.
They may react with and be retained by the surfaces of the ion
source. One Would like to know, for example, that one doesn't
carry a particular gas that may result from some surface chemical
reaction at one altitude to some lower altitude. This arrange-
ment permits one to iook at that background.
I included the last but didn't really intend to talk about
it (Figure 8-13 ); however, a talk about mass spectrometers would
not really be complete without showing a spectrum. People would
not think that you were being very honest. This is a nest spec-
trum from a laboratory study £hat we have done that illustrates
the capabilities of small quadrupoles. You can see the typical
things - the number of gases and the resolution. It gives you
a feel for the dynamic range of instruments and peak shapes.
I think I will close, then, with just one remark. I have
been speaking to you about things that are real in terms of in-
struments. We, collectively, have done a lot of development
over the years towards these instruments, and I think we have r
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come to quite an advanced state. I think we are ready for mis-
sions to the planets. These instruments are in many cases built
and operating. Many of them are tested. Many of the principles
have been tested and have been found lacking in some regards.
The test that Professor Nier speaks about on A.E. will be
carried forward also. We too will be doing a similar, but
somewhat more advanced experiment on the next A.E. satellite
with a system that is particularly designed for planetary upper
atmosphere use. We are not speaking about what might be, we
are speaking about what in fact can be, and what is being done.
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A mass spectrometer concept for identifying planetary atmosphere composition

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