A neutral mass spectrometer, flown as part of the Induced Environment Contamination Monitor (IECM), is briefly described. Results from STS-2, -3, -4, and Spacelab 1 are qualitatively summarized. The gases observed were, for the most part, those with molecular weights below 45 amu with sources attributable to instrument background, shuttle-induced environment, and the ambient atmosphere. The most abundant gases were H2O, N2, and He. Heavier gases consisted primarily of fluorocarbons

Carignan, G.

Miller, E.

English

NASA Technical Reports Server

NEUTRAL MASS SPECTROMETER MEAsUREMl'2NTS I N  THE SHUTTLE BAY 
ENVIRONMENT 
E. Miller 
Space Science Laboratory 
NASA Marshall Space F l i g h t  Center, Huntsv i l le ,  AL 
G. Carignan 
Space Physics Research Laboratory 
University of Michigan, Ann Arbor 
Abstract. A neutral  mss spectrometer, flown as part of t he  Induced 
Environment Contamination Monitor (IECM) , is b r i e f l y  described. 
s u l t s  from STS-2, -3, -4, and Spacelab 1 are q u a l i t a t i v e l y  summarized. 
The gases observed were, f o r  t h e  most p a r t ,  those  with molecular 
weights below 45 am with sources a t t r i b u t a b l e  t o  instrument back- 
ground, shuttle-induced environment, and t h e  ambier;t atmosphere. The 
most abundant gases were H20, N2, and He. Heavier gases cons is ted  
pr imar i ly  of fluorocarbons. 
Re- 
In t roduct ion  
A n e u t r a l  mass spectrometer w a s  flown as p a r t  of t h e  Induced Contam- 
i n a t i o n  Monitor (IECM) on s h u t t l e  f l i g h t s  STS-2, -3, -4, and Spacelab 1 
[Miller, 1983a,b, 19841 (Figure 1). It w i l l  a l s o  fly on Spacelab 2. The 
mass spectrometer covered t h e  mss range from 2-150 amu with a nominal 
r e so lu t ion  of 1 amu over t h e  range. The instrument field-of-view was 
collimated by a three-stage skimmer t o  0.1 sr (Figure 2) .  
at angles g r e a t e r  than loo t o  t h e  v i e w  a x i s  are skimmed i n t o  a m t r i x  
of zirconium oxide g e t t e r s  where a c t i v e  atoms and molecules are pumped 
providing t h e  coll imation. Nonreactive gases such as helium and argon 
are not pumped by these  g e t t e r s  and thus are not collimated. 
I n  normal opera t ion ,  a 600-s cycle format was employed during which 
each mass number from 2 t o  150 am w a s  sampled f o r  2 s and during t h e  
subsequent 300 s ,  mass 18 (H20) w a s  sampled continuously with a 2-s 
i n t e g r a t i o n  period. Other formats, s e l e c t a b l e  by t h e  a s t ronau t s ,  re- 
duced t h e  in t eg ra t ion  period t o  0.2 s and/or t runca ted  t h e  mss range 
t o  2-50 am. With both options selected t h e  mass range from 2-50 w a s  
sampled each 10 S. The instrument was sealed i n  an u l t ra -c lean  ncuum 
p r i o r  t o  launch and opened t o  t h e  environment after t h e  s h u t t l e  w a s  i n  
o rb i t .  
The IECM was mounted i n  t h e  payload bay such t h a t  the v i e w  a x i s  of 
t h e  mass spectrometer was  along t h e  -Z axis .  
wi th  the bay doors open, t h e r e  was very l i t t l e  s h u t t l e  surface i n  t h e  
e n t i r e  2 n  f r o n t  hemisphere, so t h a t  v i r t u a l l y  a l l  a r r i v i n g  contaminant 
gases were s c a t t e r e d  from atmospheric gases. I n t e r p r e t a t i o n  of t h e  
a r r i v i n g  f l u x  i n  terms of its source s t r eng th  or column dens i ty  thus  
involves cons idera t ion  of t h e  s c a t t e r i n g  cross  sec t ion  of t h e  8 km/s 
c o l l i s i o n s  between contaminants and t h e  atmosphere. On STS-4, t h e  IECM 
w a s  grappled by t h e  Remote Manipulator Arm and a survey was m d e  with 
G a s  en t e r ing  
On STS-2, -3, and -4, 
74 
https://ntrs.nasa.gov/search.jsp?R=19860003777 2020-03-20T15:59:07+00:00Z
the instrument a x i s  looking back i n  toward the bay and wing surfaces.  
On Spacelab 1 the  IECM w a s  loca ted  w e l l  below t h e  s i l l  l i n e  so t h a t  
much of t h e  aft bulkhead and instrument sur faces  w e r e  w i th in  t h e  uncol- 
l imated field-of-view. 
The IECM and t h e  mass spectrometer functioned w e l l  on all fou r  
f l i g h t s .  There were some anomalies and minor problems, but f o r  t he  
most part, opera t ion  was nominal. The q u a n t i t a t i v e  i n t e r p r e t a t i o n  of 
t h e  mass spectrometer measurements w a s  expected t o  be d i f f i c u l t  and it 
has been. The mjor l i m i t a t i o n  of the spectrometer measurements has 
been t h e  r e l a t i v e l y  l a r g e  background of previously pumped gases over 
t h e  zirconium oxide g e t t e r s .  Despite the d i f f i c u l t i e s  and the l i m i t a -  
t i o n s  of the  system, some success has been achieved i n  cha rac t e r i z ing  
t h e  gaseous environment i n  the v i c i n i t y  of the s h u t t l e .  
Results 
A q u a l i t a t i v e  summary of the measurements will be presented here 
descr ib ing  the  i n t e r p r e t a t i o n  of t h e  gaseous con t r ibu to r s  t o  each of 
t h e  mss numbers from 2-44 and a few o thers  at g r e a t e r  mss number. 
Mass 2--Molecular hydrogen t h a t  is almost entirely background of t h e  
zirconium oxide g e t t e r s .  No s ign i f i cance  can be a t tached  t o  the mss 2 
measurement. 
Mass 3 - - S t a t i s t i c a l l y  i n s i g n i f i c a n t  (SI).  
Mass 4--Helium. This is t h e  best measurement made by t h e  i n s t r u -  
ment. 
Helium is not coll imated or otherwise a f f ec t ed  by the g e t t e r s  and so 
t h e  instrument f ield-of -view is 2 T sr. Atmospheric helium is observed 
and i ts  q u a n t i f i c a t i o n  is i n  good agreement w i t h  model values on all 
f l i g h t s .  
t o r r  on Spacelab I) and is observed even i n  t h e  wake [Naumann e t  al., 
19851 = 
There is v i r t u a l l y  zero background cont r ibu t ion  at mss 4. 
Helium is a l s o  a major contaminant of the s h u t t l e  (10-9 
Mass T--SI. 
Mass 6--s1. 
Mass "--SI. 
Mass 8--SI. 
Mass g--SI. 
Mass 10--SI. 
Mass 11--Small but s t a t i s t i c a l l y  s i g n i f i c a n t  ; poss ib ly  borane. 
Mass 12--All carbon bear ing molecules con t r ibu te  t o  t h i s  peak. 
75 
Mass 13--Mostly a product of d i s s o c i a t i v e  ion iza t ion  of CH4. This 
peak toge the r  with 14,  15, and 16 is used t o  sepa ra t e  methane from 
o the r  con t r ibu to r s  t o  these  mss numbers. 
Mass 14--A complex sum of doubly and d i s s o c i a t i v e l y  ionized N2, 
doubly ionized CO, and d i s s o c i a t i v e l y  ionized CH4. Its abundance is 
heav i ly  modulated by angle of a t t a c k  because of t h e  N2 con t r ibu t ion  
and is a l s o  modulated by methane producing events. 
Mass 15--Almost exc lus ive ly  from methane; a very small f r a c t i o n  from 
t h e  1 5  i so tope  i n  n i t rogen  compounds. It i s  t h e  m a s s  of choice f o r  
quant i fy ing  t h e  methane source dens i ty  because it is uncontaminated by 
atomic oxygen and is almost equal ly  s e n s i t i v e  of methane as t h e  mass 16 
peak. 
Mass 16--Methane and atomic oxygen. Ambient atomic oxygen does not 
surv ive  the many su r face  c o l l i s i o n s  before ion iza t ion .  It combines t o  
form CO,  C02, and 02. 
i n d i r e c t  through d i s s o c i a t i v e  ion iza t ion  of oxygen bearing molecules. 
Methane is a s u b s t a n t i a l  background gas over t h e  zirconium oxide get-  
ters. It a l s o  appears t o  be c a t a l y t i c a l l y  produced i n  t h e  instrument 
from some e f f l u e n t  a s soc ia t ed  with t h r u s t e r  f i r i n g s .  It is pos tu la ted  
t h a t  it is formed on t h e  g e t t e r  sur faces  by conversion of unoxidized o r  
p a r t i a l l y  oxidized t h r u s t e r  f u e l ,  monomethyl hydrazine. 
The cont r ibu t ion  t o  t h e  mss 16 peak is thus 
Mass 17--Dissociatively ionized H20, i,e., OH+. The amplitude of 
t h e  mass 17 peak is  about 40 percent of t h e  mass 18 peak due t o  water. 
Mass 18--Water. The dens i ty  of H20; i n  t h e  ion source is  t h e  sum 
of t h e  instrument background and contaminant H20 from various s h u t t l e  
sources. I n  most observation geometries, t he  contaminant H20 obser- 
ved is t h e  consequence of s c a t t e r i n g  by t h e  ambient atmosphere i n t o  t h e  
instrument o r i f i c e ,  g iv ing  source d e n s i t i e s  of about 107/cm3 and 
column d e n s i t i e s  of 1012/cm2, 
Mass &-A s t a t i s t i c a l l y  s i g n i f i c a n t  but unknown contaminant. It is  
poss ib ly  an  ion iza t ion  product of a f l u o r i n e  compound, but t h e  assoc i -  
a t e d  s p e c t r a l  peaks have not been iden t i f i ed .  
Mass 20--Mostly doubly ionized argon with a small con t r ibu t ion  from 
t h e  oxygen 18 i so tope  i n  H20. 
f o r  c a l i b r a t i o n  is observed during ca l ib ra t ions .  
A small impurity i n  t h e  neon 22 used 
Mass 21- -S ta t i s t i ca l ly  i n s i g n i f i c a n t  except during c a l i b r a t i o n  using 
i s o t o p i c a l l y  l abe led  neon. Neon 22 w a s  used for  t h e  c a l i b r a t i o n ;  con- 
t r i b u t i o n s  at msses 20 and 21 r e su l t ed  from impur i t ies  and, i n  t h e  
case of mass 21, a s l i g h t  c ross - ta lk  from t h e  large mass 22 peak. 
Mass 22--Doubly ionized C02, except during c a l i b r a t i o n  when t h e  
i s o t o p i c a l l y  labe led  neon 22 caused count rates i n  excess of 2 X 10 6 / 
2 s at mass 22. 
Mass 23--SI. 
76 
Masses 24, 25, 26, and 27--Small, but measurable, products of an 
ins t rument  contaminant of un iden t i f i ed  or igin.  
Mass 28--Molecular n i t rogen  (ambient and contaminant ) and carbon 
monoxide which is a r e l a t i v e l y  l a r g e  instrument background over t h e  
zirconium oxide g e t t e r s  and o the r  metal surfaces.  A t  small angles of 
a t t a c k  t h e  mass 28 peak is dominated by streaming ambient molecular 
n i t rogen ,  and t h e  q u a n t i t i a t i v e  i n t e r p r e t a t i o n  of t h e  peak y i e l d s  
reasonable values f o r  ambient density.  During t h e  contamination survey 
taken on STS-4 wi th  t h e  IECM looking i n t o  t h e  payload bay t h e  f luxes  of 
molecular nitrogen appear t o  be about ha l f  t h a t  of water. This would 
lead t o  a column dens i ty  f o r  N2 contaminant of 1.0 X 1012/cm2. 
survey a l s o  showed t h a t  t h e  major N2 contamination source was i n  t h e  
v i c i n i t y  of t h e  aft  bulkhead (F igure  3). 
The 
Mass 29--Mostly N2 with one atom of nitrogen 15. 
Mass 30--Probably NO with l a r g e  peaks observed during t h r u s t e r  
f i r i n g s ,  
Mass 31--Not analyzed. 
Mass 32--Molecular oxygen mostly of atomspheric o r i g i n  through t h e  
recombination of atomic oxygen. The level of molecular oxygen contami- 
na t ion  is very low, probably below t h e  instrument background. 
Mass 33--SI. 
Mass 34--Not analyzed; p a r t l y  016-018. 
Mass 35--SI. 
Mass 36--~n unknown minor contaminant, poss ib ly  HC1, plus a small 
component of argon 36. 
Masses 37, 38, 39, 41, and 42--Small but s t a t i s t i c a l l y  s i g n i f i c a n t  
peaks c h a r a c t e r i s t i c  of propene. 
Mass 40--Argon. Analysis of t h e  mss 40 peak at low angles of 
a t t a c k  y i e l d s  reasonable v a l u e s ' f o r  ambient argon dens i t i e s .  There i s  
an argon contaminant on a t  least some of t h e  f l i g h t s .  The STS-3 .door 
c los ing  event showed a s t eep  rise i n  m a s s  40. 
Mass 44--C02. Mostly instrument background with some s m a l l  con- 
t r i b u t i o n  from t h e  s h u t t l e  t h a t  is t o o  low with respec t  t o  background 
t o  quantify. 
Masses 50 , 81, and 100--Fractionation spectrum of C2F4 (tetra- 
f luoroethene) observed on Spacelab 1 f l i g h t .  
Masses 85, 87, 101, 103, 125, and 137--Fractionation spectrum of 
f reon  114, CzC12F4 from a known 6.2 cm3/day leak  i n  t h e  Spacelab 
cooling loop on Spacehb  1. 
77 
Masses 67 and 69--The p r inc  
Freon 21, CHC12F, are pres  
loop. Strong s i g n  
contamination surv 
Miller, E. R. ( e d i t o r ) ,  STS-2, -3, 
Miller, E. R., Update of Induced Environment Contamination Monitor 
Monitor (IECM) Summary 
Results, presented at AIAA S h u t t l e  Environments and Operations 
ing,  Washington, D.C., 1983b. 
E. R. ( e d i t o r ) ,  Induced Environment Comtamination Monitor - 
Preliminary resul ts  from t h e  Spacelab 1 f l i g h t ,  NASA TM-86461, 
1984. 
lar s c a t t e r i n g  and wake vacuum measurements, Journal of Vacuum 
Science and Technology, i n  p re s s ,  1985. 
Naumann, R. J., G. R. Carignan, and E. R. Miller, Space S h u t t l e  mlecu-  
78 
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STS 4 
GEOMETRY OF 
CONTAMINATION SURVEY 
POSITIONS 17, 18, 18 
I .  I I I I I I I I 1 I I I I 
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Fig .  3. Mass 28 during the remote contamination survey on STS-4. 
81 


Neutral mass spectrometer measurements in the shuttle bay environment

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