Data were obtained which will provide a test of the accuracy of the differential absorption method for trace contaminant detection in many-component gas mixtures. The necessary accurate absorption coefficient determinations were carried out for several gases; acetonitrile, 1,2-dichloroethane, Freon-113, furan, methyl ethyl ketone, and t-butyl alcohol. The absorption coefficients are displayed graphically. An opto-acoustic method was tested for measuring absorbance, similar to the system described by Dewey

Steinfeld, J. I.

English

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https://ntrs.nasa.gov/search.jsp?R=19750020487 2020-03-22T20:19:16+00:00Z
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_	 '
1	 01
(NASA-CR-143261) MONITCRING SPACECRAFT
	 N75-28560;
ATMOSPHERE CONTAMINANTS BY LASER ARSOEPTION
SPECTROSCOPY Seeiannnril Progress Report,
Jan. - Jun. 1975 (Massachusetts Inst. of
	 Unclas	 {k'	 Tech.)	 13 P HC $3.25
	 CSCL 06K G3/45 29069
	 {
SWIATUNAL 11RQURPw5 RT:,MRT	 I
Sanuary-J'unc 1975
i
,^23 9 56^NASA Research Grant No. NGR-22-009-766
CP
c^mn .^ G Loj
140I, nTORING SPACECRAFT A'TMo'PPHPRE CONTPMINArrRS	
026le^ti^^
' BY IA91?R ABSORPTION SPLCTROGCUPX
F	 ra . 
)1
P?'bJ'esnor J. I. Steinfel.d., Principal Investigator
D. D. Green, Research Assistant
Department of Chemistry } Massachusetts Institute of Tecl,nolo^--
Cambridge,'Massachusetts 021,39
1
{
The NASA Technical. Officer for this grant fs,;
I^
)
^ F
k^ l
Phillip D. Qaattr9ne
Environmental Control Research }ranch
NASA Ames Research Center
Moffett Field, California 	 t
t
_
_
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A
^; t
-2-
1. Since the last progress report, our aim has been to gbtain data which
will provide a test of the accuracy of the differential absorption method for
trace contaminant detection in many-component gas mixtures. The necessary
accurate absorption coefficient determinations were carried out for seieral
more gases, namely, acetonitrile, 1,2-dichloroethane, ireon-1].3, furan, methyl
ethyl ketone, and t-butyl alcohol. The absorption coefficients are displayed
graphically in Pigs. 1-6. This brings to fifi>een the total number of gases
for which we have catalyzed CO2-laser line absorption coefficients to an accu-
racy of ± 1%.
2. We have tested an opto-acoustic method of measuring absorbance, sim-
ilar to the system described by Dewey [Opt. Eng. .^3, 463 (1971)]. litwever,
since no improvement in sensitivity over a diroct-absorption measurement was"
obtained, this method was not pursued.
e
3. A series of measurements have been carried out on known gas mixtures,
in order to furnish data for testing calculational procedures. The runs for
mixtures of trace gases were performed. using 'che absorption method. The mix-
tures were chosen in such a way as to determine the effect of the following
factors on the accuracy of the determination of contaminant concentrations:
i
1) The 'variation of the absorption for similar mixtures, i.e., mixtures
with the same relative concentrations of the gases, but different
total pressures of the contaminant mixture;
.i
2) The presence of a relatively high concentration of a broadly
t	 .,
i
b	 -
t
absorbing gas;
3) The variation of the relative concentrations of two gases with
very sir(_ lar absorption spectra.
-3-
The following method was used in the preparation of the mixtures. The
ratio of the volume of the absorption cell to the volume of a sidearm bulb
was determinett as accurately as possible (t 0.5%). Each gas to be used in
a mixture was first purified by the freeze-pump-thaw cycle, then fraction-
'
	
	 ally, distilled over to another cold finger for storage. All gas pressures
were measured on a dibutyl phthalate manometer; all manometer readings were
taken with a cathetometer. A predetermined amount of each gas was let into
4
j
	
	
the manifold. After the pressure in the manifold reached equilibrium, the
cell was closed off and the gas in the cell was frozen into the sidearm bulb.
The whole system was then pumped on for a few minutes. After all the gases
were stored in the bulb, it was brought to room temperature and the gases
were allowed to expand into the absorption cell and mix overnight. (This
was experimentally found to be sufficient mixing time.) The next day air
{	 was added to bring the mixture to atmospheric pressure.
The first few runs were mixtures of 6 1 9, 11, and 12 gases. They will
give an indication of the increasing inaccuracies in the results as the number
of contaminants is increased. The concentrations used were sufficient for
partial,but not total, absorption of the laser light. The pressures of some
of the gases were varied, notably benzene and cyclohexane which have similar
spectra, in order to see interference effects and the error in concentration
as a function of concentration. The next run was with 12 gases all in weakly
absorbing amounts. Nine gases were placed in the cell for the next run. They
all had large absorption coefficients,,and were present in moderate concentra-
tions. During preparation, the sidearm bulb was closed off before admitting
air to the cell. After the absorption measurements for this._^un were made,
i
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it
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the cell was evacuated and the contents of the sidearm bulb was again allowed
to expand into the cell and mix overnight. Thus, the relative concentrations
of the gases in these two runs should be the same, but the total pressure due
to the trace contaminants is reduced by a factor of about 15. For the next
two runs, a gas mixture with all gases present in strongly absorbing concen-
trations was made. An additional run was made on the mixture with the same
relative concentrations, but the total pressure was reduced by a factor of 15.
also ran -'\',he same mixture further reduced in total pressure by the same
factor with an 'amount of a broadly absorbing gas (isopropanol) added so that
absorption valuer for the mixture were significant. The resultant spectrum
G
should appear al^ the spectrum of the broadly absorbing gas with the absorption
of the mixture added on top. It is thought that the presence of an absorber
might improve sensitivity because small values of absorption have larger
error limits associated with them.
A 15-component mixture was then prepared, where 12 of the gases were
present in roughly the same absolute concentration as before, but once again
an amount of isopropanol was added. A mixture was also prepared of 15 gases,
all in concentrations which have moderate absorption of laser light.:- The
measurements were repeated three times, 11 hours apart, to check the repro-
ducibility of the data. The results are shown in the table below. No points
were discarded in the stendard deviation calculations.
4. A preliminary analysis of these data was carried out on the small
PDP-B/L computer in our laboratory. Mr. Green is currently carrying out more
extensive analysis at the computer facility at the NASA Ames Research Center.
In the previous Progress Report, it was noted (for a three-gas mixture) that
1, + i. r, .ice
I^
IT--
1/10
Number
 
of points	 c = (N11 n
2)1
/2
Typicali	
limitsfor w	 ^normal dist. (25)
0.5-0.3 27 0.013 2.00 1 0.026 (1,3%)
0.15-0,11 33 0.013 C oo ± 0.026 (0.7%)
0.07-0.04: 72 0.03 6.00 1 0.06 (1.0%)
0.025-0,007 13 0.03 9.00 ± 0.06 (0.7%)
using n wavelengths to determine the concentrations of n gases resulted in large
errors due to errors in experimental values. Use of a least-squares fitting
routine to use all experimental data to determine the concentrations was tried
first. For a system with n different gases, A different wavelength absorp-
tion measurements and c  estimated concentrations (j = 1, ...) n), the esti-
n
mated absorption would be y  = £ Eij cj , where Eij is the-absorption coefficient
of the j th gas at the •ith wavelength. The yi values differ from the experi-
mental absorptions Ai because the ,ci are not the exact values and because of
experimental measurement errors. Minidization of the sum of the squares of?
the residuals 1J(=	 (yi - Ai )2 ) with respect to variation in concentration
i=1
(aTvbcj 0 for all J) 'will give a set of simultaneous linear equations,
fjlcl + fj2 c2 a f0 3 + ... i•fjnen = ` dj for each j	 1, ..., n
X	 %
where	 f
^lc = i£1 Eij Eilc and d j = i£1 Ei . (Ai - yi )
in matrix form,'ZQ = P, Q = e lp, where Z = gTk, D = gT (A-X). The solutions
C  are functions of the errors in the input concentrations. They can be used
p)
VL 1 A
-s-
as correction values and added to the input c  values. From these new c^
estimates a new value of I is found. The process is repeated until t:.e
residuals no longer decrease significantly.
The P matrix is a symmetric positive definite matrix. This allows
elimination methods to be used which require non-vanishing positive pivots.
In particular, the triangular decomposition method of Cholesky is compact,
uses little memory space, and does not require inversion of the P matrix for
solution. This method decomposes iF into two triangular matrices (L, L T ) which
are transpositions of one another; thus only one need be calculated. Solu-
tion is then by back substitution. Here d^ = iElEijAi and no initial esti-
mates for the concentrations are required. Work is currently progressing at
NASA Ames in evaluating the mixture absorption data by both methods in order
to find the best method of solution. Various schemes of weighting absorption
lines are being tried also. Here the equation becomes jET - Z = Z Ta or
where J1 is the X x X weighting matrix.
5. The absorption coefficients of vinyl chlc:r,'.d. were measured in dif-
ferent=ji"ressures of air. Total pressures of the mixtures were 0.499, 0.748,
0.873, and 1.0 atmosphere, all with 4.1 Torr partial pressure of vinyl chloride.
The absorption coefficients at some frequencies decreased slightly with in-
creasing total pressure, at others it rose or remained constant. (For example)
in the 10.6 µmP branch, absorbance at P(22) rose 5%, P(8) rose 20%, P(36)
fell 15%, and P(30) fell less than l%). Use of a tunable diode laser to
observe these lineshapes as a function of pressure is pleaned. Stark tuning
of the molecular levels with the resultant absorption spectrum changes monitored
by the diode laser should allow identification of the molecular levels involved
.f
'i
and permit accurate tAlculation of the excited state dipole rioment. These
1%
techniques will also be applied to other molecules which e4f ibit strong
absorption in the 10 -11 µm region.
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Monitoring spacecraft atmosphere contaminants by laser absorption spectroscopy

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