The sensitivity of an opto-acoustic absorption detector is increased to make it possible to measure trace amounts of constituent gases. A second beam radiation path is created through the sample cell identical to a first path except as to length, alternating the beam through the two paths and minimizing the detected pressure difference for the two paths while the beam wavelength is tuned away from the absorption lines of the sample. Then with the beam wavelength tuned to the absorption line of any constituent of interest, the pressure difference is a measure of trace amounts of the constituent. The same improved detector may also be used for measuring the absorption coefficient of known concentrations of absorbing gases

Rosengren, L. G.

English

NASA Technical Reports Server

United States Patent [I91 
Fletcher et al. 
[ I  1 3  3,995,960 
[45] Dec. 7, 1976 
[ 541 METHOD AND APPARATUS FOR 
BACKGROUND SIGNAL REDUCTION IN 
MEASUREMENT 
Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Lars-Gran Rosengren, 
Kungsbacka, Sweden 
OPTO-ACOUSTIC ABSORPTION 
[ 761 
[22] Filed: July 25, 1975 
[ 2 1 ] Appl. No.: 599,284 
[52] U.S. CI. ............................... 3561204; 2501343; 
356197; 3561201 
[51] Int. CL2 ..................... GOlJ 3142; GOlN 21/24 
[58] Field of Search .............. 356/51, 97, 201, 204, 
3561205, 206; 2501343, 345 
[561 References Cited 
UNITED STATES PATENTS 
3,820,901 6/1974 Kreuzer ............................... 356/97 
OTHER PUBLICATIONS 
Delany, M. E., “The Optic-Acoustic Effect in Gases;’’ 
Science Progress, July 1959, vol. 47, No. 187, pp. 
459-467. 
Primary Examiner-John K. Corbin 
Assistant Examiner-Richard A. Rosenberger 
Attorney, Agent, or Firm-Monte F. Mott; Paul F. 
McCaul; John R. Manning 
[571 ABSTRACT 
The sensitivity of an opto-acoustic absorption detector 
is increased to make it possible to measure trace 
amounts of constituent gases (approaching 0.01 parts 
per billion) in a sample by creating a second beam 
radiation path through the sample cell identical to a 
first path except as to length, alternating the beam 
through the two paths and minimizing the detected 
pressure difference for the two paths while the beam 
wavelength is tuned away from the absorption lines of 
the sample. Then with the beam wavelength tuned to 
the absorption line of any constituent of interest, the 
pressure difference is a measure of trace amounts of 
the constituent. The same improved detector may also 
be used for measuring the absorption coefficient of 
known concentrations of absorbing gases. 
10 Claims, 2 Drawing Figures 
TUNABLE 1 LASER 1- ADJUSTABLE 
L2  8 
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U.S. Patent Dec. 7, 1976 3,995,960 
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3,995,960 
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The novel features that are cnsidered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention will best be under- 
stood from the following description when read in con- 
5 nection with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
METHOD AND APPARATUS FOR BACKGROUND 
SIGNAL REDUCTION IN OPTO-ACOUSTIC 
ABSORPTION MEASUREMENT 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under asNASA contract and is sub- 
ject to the providions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 10 
85-568 (72 Stat. 435; 42 USC 2457). 
FIG. 1 is a schematic diagram of an exemplary em- 
FIG. 2 is a diagram of waveforms useful in under- 
bodiment of the present invention. 
standing the present invention. 
BACKGROUND OF THE INVENTION DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
Referring now to FIG. 1, a sample cell 10 and tunable 
laser 11 is shown in an arrangement for opto-acoustic 
measurement of trace amounts (approaching 0.0 1 
parts per billion) of atmospheric pollutant gases in an 
air sample. The cell is provided with an inlet valve loa 
This invention relates to opto-acoustic absorption 
detectors commonly referred to as sPectroPhones, and 15 
more particularly to a method and apparatus for mea- 
suring the concentrations of absorbing gases in a sam- 
ple cell using opto-acoustic detection of absorption. 
There is an increasing interest in detecting trace 
amounts (approaching 0.01 parts per billion) of atmo- 20 and an outlet valve lob for sampling the air at Some 
’pheric pollutant gases in an air sample. However, the convenient pressure using a suitable pump (not shown) 
as that produced by for first purging the cell while both valves are open, background 
the heating of the cell windows, limits the sensitivity of charging the cell with only valve open and then 
opto-acoustic absorption detectors. Terrence F. Des- closing the valve loa when the desired static pressure is 
David A. Depatie and Thomas w. report 25 reached as measured (by means not shown). 
their efforts to overcome this problem of sensitivity 
with a differential cell in a paper titled “Absorption 
Coefficient Measurements of Nitrous Oxide and Meth- 
The normal operation of an opto-acoustic detector 
requires tuning the laser wavelength to the absorption 
line of the molecules expected in the gas sample in the ane at DF Laser Wavelengths,” Physics Let- cell. The energy absorbed by the molecules increases ter% vel. 2-57 No. 6, 300-303 (1975). Two identical 3o the pressure in the The increase in pressure is cells were used in tandem. Both were first filled with detected through a transducer 12, such as a capacitor non-absorbing gas, and the pressure differential signal microphone, and displayed on a meter 13 or on a cath- between the cells was then minimized while the laser 
ode ray tube (CRT) 14 via an amplifier 15 imple- beam was transmitted through both cells to “zero” the 
mented as a lock-in amplifier for sensitive, narrow instrument. Absorption measurements were then made 
35 bandwidth amplification in order to eliminate noise. by filling one cell with a sample. The pressure differen- 
That pressure is then a measure of the amount of the tial signal between the two cells represents the absorp- 
tion of the test gas. Residual pressures molecules, i.e., the concentration of the trace constitu- 
which are present as background signals due to absorp- ent of interest. The problem with this normal operation 
tion by the windows of the cell is effectively balanced is that windows and l7 for the laser beam path 
lem that extreme care must be exercised in filling both crease in pressure within the cell. The resulting back- 
dure and during the measuring procedure. Still another dows ‘s Often large as to the absorption by 
approach to the problem referred to, but not discussed trace constituents Of a sample, making it impossible to 
in their paper, is the development of a “windowless 45 use opto-acoustic techniques for detecting the pres- 
resonant cell” by David A. Depatie. The problem with ence Of trace constituents* 
that approach is said in that paper to be that ‘‘the To significantly improve the sensitivity of the opto- 
pickup of ambient acoustic noise increases.” acoustic detector, a reflecting chopper 20 is employed 
to direct the laser beam via mirrors 21 and 22 through 
SUMMARY OF THE INVENTION 50 a second pair of windows 23 and 24. Lenses 25 and 26 
In accordance with the present invention, the resid- are simply for focusing the laser beam through the cell 
ual background signal in an opto-acoustic pressure windows. The chopper alternates the laser beam into 
detector is reduced by alternately diverting a beam of the two Paths, Preferably with equal time through each 
light to a second path through a second pair of windows Path, although that is not necessary. BY adjusting the 
in a sample cell. The lengths of the two paths are signif- 5 5  light beam power through attenuators 27 and 28, it is 
icantly different. The beam intensity in the two paths is Possible to compensate for any time difference. 
then adjusted through suitable means to minimize the The alternate beam path through the second pair of 
detected pressure difference between the two paths windows is employed to balance out residual pressures 
while the beam wavelength is tuned away from the due to absorption by the first pair of windows 16 and 
absorption lines of the sample in the cell, thereby re- 60 17, although all windows are made of the same material 
ducing any residual background signal due to absorp- and thickness, but that is not a requirement. 
tion pressure of the window material. When the beam It should be noted that the alternate (reflected) beam 
wavelength is thereafter tuned to the absorption line of path has a length I, through the cell much shorter than 
any trace constituent of a gas sample in the cell, the the primary (direct) path which has a length I d .  Since 
pressure difference between the two paths through the 65 the pressure, P, in the cell is proportional to the absorp- 
two cells is due only to absorption by the specific con- tion coefficient, cy, and the path length, I, for a fixed cell 
stituent of interest in the cell having a spectral line at volume, v ,  the direct path pressure (Pd=kld,  where 
the wavelength of the laser. h / v )  will be much greater than the reflected path 
out by this double cell arrangement, but it has the prob- 40 through the absorb energy and an in- 
cells to the Same pressure during the balancing proce- ground due to absorption losses in the win- 
3,995,960 
3 4 
(P ,=kl , ) .  Consequently, as the laser beam is switched nized that modifications and equivalents may readily 
through the two paths, it modulates the cell pressure as occur to those skilled in the art. For example, both sets 
shown in waveform C of FIG. 2. While the laser wave- of windows could be set at Brewster's angle to minimize 
length is turned off resonance of an absorption line of reflection of light by the window material, particularly 
the sample gas, the attenuators 27 and 28 are adjusted 5 reflection of light from inside surfaces back through the 
so that the rms value of the first harmonic of the pres- sample in the cell. Use of Brewster's angle will, of 
sure difference (variations in amplitude of the wave- course, permit arrangements for the two light paths 
form C) is minimum. The waveform displayed on the other than at right angles to each other. Also light 
CRT will then be as shown by the dotted line waveform sources other than lasers could be used, but for sensi- 
C in FIG. 2, assuming only the attenuator 28 is adjusted 10 tivity on the order of 0.01 parts per billion, a laser is 
in this example. If both are adjusted, the dotted line required. Other suitable light sources might be ordinary 
waveform will have substantially the same form (ap- white light sources directed in a beam through filters 
proaching a straight line), but at a lower amplitude. that Pass a selected narrow band of wavelengths as 
Both attenuators may be conventional crystals for required. Consequently, it is intended that the claims 
transmitting the polarized light of the laser with 15 be interpreted to cover such modifications and equiva- 
for rotating the axis of the crystal relative to the axis of lents. 
polarization of the light to attenuate the light transmit- I 
ted. 1. In an opto-acoustic absorption detector for mea- 
operation, the cell is charged with a sample of suring the concentration of constituent gases in a sam- 
mixed gases and the laser wavelength is tuned away 20 ple contained in a cell by transmitting a beam of light 
from the absorption line of the trace constituent sus- wavelength Source through primary 
petted to be present in the sample. The relative beam Of said and producing as 
from a 
windows On Opposite 
intensity of the two transverse radiation paths is then a measure of the concentration of said constituent, a 
adjusted to balance out the background signal due to 
absorption by the window material. That is done by 25 energy transfer to said 
operating on one or the other, or both, of the attenua- 
signal proportional to any increase in pressure due to 
through absorption Of 
said beam at a selected wavelength On the absorption 
2, and 28 in the beam paths, and observing the 
as shown in waveforms C of FIG. 2. The solid line 
waveform C is indicative of the windows 23 and 24 30 
for passing a beam through said cell in a path of a but any unbalance may be opposite to that. By so ad- 
justing the attenuators as to minimize the amplitude of length different from the path between said pri- 
mary windows, any modulation present in that waveform C, any resid- 
ual background in signal due to absorption by the win- 35 for alternately directing said beam of light 
dows 16 and 17 is balanced out. through said primary windows and said second set 
Once the balancing procedure has been completed, of windows, 
the attenuators are locked in their adjusted position . for adjusting the relative power of said beam 
and the laser wavelength is tuned to the absorption in said two paths for minimum absorption pressure 
lines of the constituent gases of interest. The pressure 40 difference indicated by said signal when said beam 
difference (amplitude of modulation) in the detector is selected to be away from the absorption lines of 
output displayed on the CRT as shown in waveform D constituents of said sample, and 
iS a tlleaSUre Of the amount (Concentration) Of a Con- means for displaying the absorption pressure differ- 
stituent associated with the wavelength of the beam. ence indicated by said signal for a selected wave- 
For example, if three pollutants are suspected Of bein 45 length of said beam as a measure of the concentfa- 
present in an air sample, the laser is tuned to the wave- tion of a constituent in said sample when the wave- 
length of the absorption line of each in sequence, not- length of said beam selected is on the absorption 
ing at each wavelength the amplitude of the modulation line of said constituent. 
on the detector Output, i.e., on the output of the pres- 2. Apparatus as defined by claim 1 wherein said 
sure transducer. The detected pressure amplitude is the 50 means for alternately directing said beam comprises a 
measure of the amount of each pollutant present. Trace reflecting chopper. 
gas concentrations as low as 0.01 parts per million can 3. Apparatus as defined by claim 2 wherein said sec- 
be detected with this differential-path cell because it ond set of windows are on the sides of said cell in a path 
significantly reduces the background signal. The ad- normal to the path through said primary windows on 
justment of the relative power in the two beam paths 5 5  the ends of said cell, and said reflecting chopper alter- 
need not be readjusted if the power output of the laser nately passes said beam directly through one of said 
varies. This is an important advantage. No special ref- paths and reflects said beam 90" to a first 90" reflecting 
erence gas is required for this balancing procedure. mirror and from there to a second 90" reflecting mirror 
That is another important advantage. which reflects the beam through the other of said paths. 
4. Apparatus as defined in claim 3 wherein said pri- 
pollution monitoring because it is rugged, simple, and mary windows and said second set of windows are 
occupies minimal space. It may also be used to equal made of the same material. 
advantage in a laboratory for measuring the concentra- 5. An opto-acoustic absorption detector capable of 
tion of each gas in a mixture of known gases, or for measuring concentrations of constituent gases of a 
measuring absorption coefficients of known gas sam- 65 sample in trace amounts approaching 0.01 parts per 
ples. billion comprising 
Although particular embodiments of the invention means for producing a light beam of variable wave- 
have been described and illustrated herein, it is recog- 
line of a constituent of said sample, apparatus for re- 
result of an increase in pressure due to energy transfer 
to said sample through absorption by said windows 
comprising 
difference in pressure in the cell displayed on the CRT, ducing any background signal produced a' a 
absorbing Inore energy than the windows l6 and 17, a second set of windows on opposite walls of said cell 
This differential-path cell may be used in situ for air 60 
length, 
3,995,960 
5 6 
a cell for containing said sample, 
means for detecting the pressure of said sample in 
said cell, 
primary windows in said cell for providing a first path 
for said beam through said cell, 
a second set of windows in said cell for providing a 
second path for said beam through said cell, said 
second path being of a length substantially differ- 
ent from the length of said first path, 
said primary windows and said second set of win- 
dows, and 
means for adjusting the relative power of said beam 
directed through said primary windows and said 
second set of windows to minimize any difference 15 
detected by said pressure detecting means in the 
pressure of said sample in said cell due to transfer 
of energy by absorption of said beam while its 
wavelength is selected to be away from the absorp- 
tion lines of said constituents, thereby balancing 20 
out any residual background signal from said pres- 
sure detecting means when the wavelengths se- 
lected thereafter are on the absorption lines of said 
constituents. 
6. Apparatus as defined in claim 5 wherein said 25 
8. Apparatus as defined in claim 7 wherein said pri- 
mary windows and said second set of windows are 
made of the same material. 
9. In an opto-acoustic absorption detector for mea- 
5 suring the concentrations of constituent gases in a sam- 
ple contained in a cell, a method for reducing any resid- 
ual background signal due to absorption of energy by 
primary windows through which a beam of light is 
passed using a variable wavelength source of light com- 
means for alternately directing said beam through 10 prising the steps of 
filling said cell with said sample, 
adjusting said beam of light t3 a wavelength away 
from the absorption lines of said constituents in 
said sample, 
alternately diverting said beam of light through a 
second set of windows disposed for passing the 
beam through said cell in a path distinct from the 
path between said primary windows, said second 
set of windows being spaced a distance apart signif- 
icantly different from the distance between the 
primary windows, 
detecting any absorption pressure difference in said 
cell for the two paths, and adjusting the relative 
power of said beam in the two paths until the de- 
tected pressure difference is minimized, 
adjusting said beam of light to a particular wave- 
length on the absorption line of a constituent gas of 
interest in said sample, and 
detecting the absorption pressure difference in said 
cell for said beam at said particular wavelength as 
a measure of absorption by said constituent gas. 
10. The method of claim 9 using the same material 
for both the primary windows and the second set of 
windows. 
means for alternately directing said beam comprises a 
reflecting chopper. 
7. Apparatus as defined in claim 6 wherein said sec- 
ond set of windows are on the sides of said cell in a path 
normal to the path through said primary windows on 30 
the ends of said cell, and said reflecting chopper alter- 
nately passes said beam directly through one of said 
paths and reflects said beam 90" to a first 90" reflecting 
mirror and from there to a second 90" reflecting mirror 
which reflects the beam through the other of said paths. 35 * * * * *  
40 
45 
50 
55 
60 
65 


Method and apparatus for background signal reduction in opto-acoustic absorption measurement

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