Instrumentation
Laser 0ptoacoustic SpectroscopyA New Technique of Gas Analysis L. B. Kreuzer DIAX Corp. 250 Sobrante W a y Sunnyvale. C a l i f . 94086
Laser Optoacoustic S p e c troscopJVgas analysis offers a single s y s t e m which c a n automatically annljlze a m i x t u r e of gases w i t h high rejection ratios and sensitiuity. O n e particularly p r o m ising field of application is gaseous air pollution detection, where n single L O S S y s t e m can rcplace t h e collection of single gas tznalyzers which is currently required to prmlide a complete analysis of a m b i e n t air The availability of laser sources of infrared light has greatly increased the range of analytical problems which can he solved by infrared spectroscopy. The optoacoustic effect has proved an effective method, in combination with a laser infrared source, for detecting weak IR absorptions in gases ( I , 2). With the combination of a laser source and an optoacoust,ic method of detecting IR absorption, a new technique of gas analysis called Laser Optoacoust ic Spectroscopy ( L O S ) has been developed. A prototype gas analyzer using LOS built by the author is able to detect gas concentrations as small as 1 part per billion ( p p b ) . It is able to analyze automatically mixtures of gases and differentiate between gases with overlapping infrared absorption spectra. The purpose of this paper is to describe the new technique of LOS gas analysis to potential users. Since many potential users may not have knowledge of laser infrared sources, a
discussion of lasers and the properties of the light they generate is presented in the next section. This is followed by sectinns which describe LOS gas analysis. present experimental results, describe the DIAX Model 100 LOS gas analyzer. and discuss applications. Laser Source of Infrared Laser action was first demonstrated at 6943 in ruby by Maiman in 1960. Since that time, the types of lasers
and the number of observed emission wavelengths have increased rapidly. Today, it takes 200 pages of the “Handbook of Lasers” ( 3 )to list and describe the more common lasers and their emission wavelengths. Emission wavelengths are now available from the ultraviolet to the far infrared. The current state of laser development suggests that lasers will become increasingly more common as light snurces in spectroscopy. f , r ~ n t i r i ~ i e d ipl no y ~ i l j l / . l j
Figure 1. Gas laser system Laser tube IS excited by high-voltage discharge between electrodes represented as bulbs at each end (a) When optical cavity consists of two mirrors laser emission occurs only at lines of high gain (b) Discretely tunable laser results wtien one mirror i s replaced by properly rciled and blazed diffraction grating This configuration generates a siugle wavelength at a time Rotation of grating cause? laser to emit both lines of high and low gain
(a) S I M P L E GAS LASER
R
-.R = io0 *A
H I G H VOLTAGE SLPPLY
.
f
RClOO%‘
COOLING WATER
(b)
GAS LASER WITH DIFFRACT~ON GRATING WAVELENGTH SELECTtQN
235 A COOLING WATER
Gas lasers are particularly well suited as sources for LOS gas analysis. Lasers such as the CO and C 0 2 molecular gas lasers provide practical discretely tunable sources. To understand how these lasers are used as effective light sources for LOS, it is not necessary to understand in detail how they operate. It is, however, important to understand those features of laser operation and the characteristics of laser light produced by these lasers which make them good LOS sources. Gas lasers are often regarded as emitting one wavelength which depends on t h e composition of t h e laser gas. T h e C 0 2 laser is commonly said to be a 10.6-pm laser. This is because the laser configuration most conimonly used, consisting of a laser discharge tube and two highly reflecting mirrors (Figure l a ) , generates radiation a t 10.6 pm.If one of the highly reflecting mirrors is replaced with a properly ruled and blazed diffraction grating, then the laser can be tuned to emit other wavelengths in addition to 10.6 pm by rotating the grating (Figure I b ) . The wavelengths that such a laser can be tuned to emit correspond to transitions between energy levels of the C 0 2 laser gas. A properly adjusted laser of this type will emit a single wavelength for a particular grating orientation. As the grating is rotated from this orientation, the wavelength will remain fixed until the grating has rotated a finite amount, a t which point the emission wavelength will jump discontinuously to a new value. This discontinuous tuning property of the C 0 2 laser is common to many other gas lasers such as the CO and the HeNe laser. At first glance, the tuning characteristics of gas lasers may seem undesirable for spectroscopic applications. Their discontinuous tuning characteristics do not allow them to be used as light sources to record spectra in the usual manner t h a t IR spectroscopists employ. This discontinuous tuning is a distinct advantage, however, when these sources are used to measure gas absorption in a LOS gas analysis system. This advantage comes from the great degree of frequency stability and reproducibility t h a t this type of laser possesses. The exact wavelength emitted by a gas laser of this type depends on the energy levels of the laser gas and not on the exact grating orientation. The grating orientation need only be set exactly enough to select the proper laser emission line. This means that it is possible to build sources that will emit a sequence of exactly reproducible and known wavelengths. The value of such a source will become clear in the following sections which describe LOS and how it is used for gas analysis. 240 A
Figure 2. Sampling of gas absorption
3
at COn laser emission wavelengths Solid curve representing absorption spectrum of hypothetical gas is made up of superposition of a number of collision broadened lines Tails of these lines are represented by interrupted lines Vertical lines represent narrow COZ laser emission wavelengths and their height represents the absorption measured at each wavelength Near coincidence between laser emission wavelength and gas absorption wavelength is indicated at point P
Laser Optoacoustic Spectrometer Infrared absorption properties of gases can be measured with discretely tunable gas lasers as sources by taking advantage of the many near coincidences t h a t exist between laser emission wavelengths and gas absorption wavelengths (4-6). The infrared absorption lines which constitute the IR absorption spectrum of a gas are not infinitely narrow. Doppler and collisional effects broaden them to a finite width. The ability to measure gas absorption with a discretely tunable laser source depends on the existence of these broadening mecha-
COMPUTER
nisms. The light beam generated by a discretely tunable laser has a n extremely narrow spectral spread or line width and may be regarded from a practical point of view as possessing a n infinitely narrow line width. Since such a laser tunes discontinuously, it cannot be tuned so that its emission wavelength coincides with the center of an absorption line. Fortunately. however, since the absorption spectrum of a pure gas has many lines, which will be collisionally broadened a t sufficient pressure, laser emission wavelengths are found that fall inside the line width of broadened absorption lines (Figure 2). The abili-
DATA OWPUS CP"'"^ "I" I nuL COMMANDS INPUT
TOTAL POWER-
SIGNAL DETECTION
PHASE REFERENCE
/ MICROPHONE
VALVE CaNTROL
GAS SAMPLE FLOW CONTROL VALVES I I TO PUMP SAMPLe IN
LASER STABILIZATION
I
CHOPPER
Figure 3. Prototype Laser Optoacoustic Spectrometer (LOS) gas analysis system
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 2, FEBRUARY 1974
ty to use a particular type of laser, for example, a COz laser, to measure a selected gas such as ethylene depends on the existence of COz laser emission wavelengths which are absorbed by ethylene. The 10.5321-~rnline of the COz laser is one example of a line which fits this situation. Figure 3 depicts the prototype LOS system built by the author. The infrared source is a COz laser which can be tuned to any of 50 wavelengths between 9.2 and 10.8 pm. The laser beam, with a power of about 100 mW, is chopped a t 24 Hz before it passes through the sample cell. Energy absorbed from the beam results in gas molecules making transitions from lower to higher rotation-vibration energy levels. These excited molecules decay through nonradiative processes, and the energy absorbed from the laser beam by the gas results in heating of the gas. Since the laser beam is interrupted by the light chopper a t 24
Hz, this heating of the gas will have a 2 4 - H ~modulation. This periodic temperature variation results in a periodic pressure variation. Infrared absorption is detected in a LOS system by placing a pressure transducer or microphone in the absorption chamber to measure this periodic pressure variation. This effect is called the “optoacoustic effect” and was discovered by Bell (7), Tyndall(8), and Rontgen (9) in 1881. It has been used for many years in nondispersive-type IR gas analyzers (10). An optoacoustic effect detector consisting of an absorption chamber and a microphone is an extremely sensitive way to detect weak absorptions. I t is possible to build a microphone and preamplifier combination which has a low noise level by using modern “state-of-the-art” components. In such a system, the main noise source is the Brownian motion of the microphone diaphragm. This
A
0 S
0 R B
A N C E D
10.72
9.24
WAVELENGTH (micrometers)
Figure 4. Laser optoacoustic absorption spectra taken with prototype LOS system at COz laser emission wavelengths A , ammonia; B. ethanol, C, ethylene; D, methanol; E, trichloroethylene
Brownian motion sets the lower limit of pressure variation that the microphone can detect. The power of the laser beam after it has passed through the absorption cell is measured by an infrared detector. Signals from the microphone in the cell and the IR detector are detected with standard “lock-in amplifier” techniques. These signals are then fed into a small minicomputer via a multiplexer and an analog-todigital convertor. The laser is tuned from one emission wavelength to another by a stepping motor which rotates the diffraction grating. This motor is controlled by the computer. The gas sample is changed by opening a set of valves and drawing in a new sample with a suction pump. These valves are also controlled by the computer.
Experimental Results The prototype LOS gas analyzer system described in the last section was built to evaluate the performance of such a system. Particular attention was directed toward high sensitivity, the ability to analyze mixtures of gases, and automatic operations. This section describes the results achieved in these areas and presents a description of how the apparatus functioned. The prototype LOS system was used to measure the absorption of the 50 wavelengths which could be obtained from its COz laser in a variety of gases and vapors. A sample of the gas or vapor under study was drawn into the sample chamber, and the laser was tuned through each of its emission wavelengths, and the measured absorption recorded. Figure 4 shows the absorption properties measured for five gases. These data were collected with the gas samples diluted to a concentration of about 100 ppm (parts per million) in an artificial mixture of nitrogen and oxygen mixed to simulate clean air. The absorption properties of other gases have been measured. In each case, strong absorptions existed a t some COz laser emission wavelengths if the gas had an absorption band which was overlapped by the range of emission wavelengths of the COz laser. The absorption spectra measured for gases of this type showed significant absorption a t several different emission wavelengths. Each gas or vapor tested showed a spectrum significantly different from the others tested. These results indicate that LOS spectra of different gases often overlap, but that the spectra of different gases are unique and provide a good basis for the identification of components of a gas mixture. The ability of the system t o detect small concentrations of gases was
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 2, F E B R U A R Y 1974
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the absorbance A I may be replaced by the sum of the absorbances of each of the components. In Equation 4 the absorbance: E=l
of the g t h component is given as the product of the path length b in the sample chamber, the absorbtivity a I 4 at wavelength i, and t h e concentration Cg.Substitution of Equation 4 into 3 gives Equation 5:
0
2
4
6
3 IO 12 14 16 B 20 22 24 26 28 TIME (MINUTES)
Figure 5. Measurement of ethylene concentration in air samples by use of single CO2 emission wavelength
This equation is the key to using a LOS system to analyze a gas mixture. It shows t h a t the gas concentrations Cg can be calculated from the measured quantities Sf and TI by solving a set of simultaneous linear equations. The formal solution of Equation 5 is given by Equation 6. This equation
C,
= ;Ea,,-’ 1 A log(: !=I
evaluated theoretically and experimentally. By comparing the strongest absorption in a gas LOS spectrum to the system noise level, it is possible to calculate gas concentrations which would give a unity signal-to-noise ratio. Calculated sensitivities range from 3 to 0.1 ppb. The sensitivity depends on the absorption strength of the gas. Gases tested include ammonia, benzene, 1,3-butadiene, l - b u tene, 1,2 dichloroet hylene, ethanol, ethylene, ethyl ether, methanol, nitric oxide, nitrogen dioxide, propylene, and trichloroethylene. This sensitivity range applies to the detection of any infrared absorbing gas by a LOS system of this type if the range of laser emission wavelengths overlaps a gas absorption band. The ability of the system to detect a small concentration of ethylene in air was tested experimentally. The COz laser was tuned to emit a t 10.5321 pm. This wavelength is strongly absorbed by ethylene. This line corresponds to t h e ethylene a b sorption maximum in Figure 3. A small quantity of ethylene gas was released into t h e air of the laboratory room. Air samples were drawn into the sample cell at 2-min intervals, and the sample absorption was measured. The ethylene concentration in the room air decreased exponentially with time because of t h e forced ventilation of the room by an air conditioning system and air exhausted by a chemical hood. The ethylene concentration of each air sample was calculated from t h e measured infrared a b -
sorption. These data are recorded in Figure 5. T h e straight line fit of the data points on a semilog plot indicates that the room air was well mixed by ventilation and that the LOS system has a linear response. The good fit of the d a t a points a t the low concentration readings demonstrates the practical achievement of high sensitivity. Mixtures of gases can be analyzed by the LOS prototype system by taking advantage of the dependence of the microphone signal on gas absorbance. Suppose that the sample container contains a mixture of gases which has a n absorbance A l at laser emission wavelength XL. Then. according to Beer’s law, the signal S,, measured by the microphone in the sample container which is proportional to the power absorbed by the s a m ple, is given by Equation 1:
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In Equation 1, PLis the laser beam power a t wavelength i. The beam power TI,which is transmitted through the sample container, is measured by the IR detector placed behind the sample cell (Figure 3). This is given by Equation 2: Equations 1 and 2 can be combined to solve for sample absorbance:
If the sample contains N component gases, then, according to Beer’s law.
+ 1)
(6)
shows t h a t the gas concentrations Cg can be calculated from the measured quantities SI and TI with the aid of the inverse of the matrix a l p .The m a trix a i g is composed of the absorption properties of each of the N components of the mixture. Determining the composition of a mixture by solving a set of simultaneous equations is a technique that has been used for many years in multicomponent analysis. The effectiveness with which Equation 6 can be applied to solve practical problems and analyze gas mixtures depends on the properties of the inverse matrix a g l - l and on the precision with which S, and TLcan be measured. If a R I - l is “badly behaved,” then small measurement errors in SIand TI may lead to a large uncertainty in the calculated gas concentrations. It is important to select the measurement wavelengths so that u p , is as “diagonal as possible.” Each measurement wavelength should be selected to be characteristic of one component and as free as possible from interference from other components. The prototype LOS system analyzes gas mixtures by measuring Sf and TI a t selected wavelengths and then calculating the gas concentrations. The minicomputer memory stores the matrix a l p and the programs necessary to solve Equation 6 and calculate gas concentrations. An important characteristic of this prototype system is the ability to detect a small quantity of one gas in the presence of a large concentration of‘another gas. This ability is described quantitatively by the rejection ratio between pairs of gases. 243 A
f a b l e I . Rejection Ratios INTERFER I NG COM PON ENT COMPONENT BEING MEASURED
Ethanol Methanol Ammonia Trichloroethylene
Ethanol
Methanol
Ammonia
...
270
760 1080 200
...
3200 1900
T h e rejection ratio between a pair of gases is the concentration of the first (interfering) gas which will give a signal equal to that produced by a unit concentration of the second gas (gas being measured). For example,l ppm of‘ethanol can just be detected in the presence of3200 ppm of ammonia. Rejection ratios have been c a l c u l a t e d by using measured absorption properties a n d assuming that the LOS system can measure S , to an accuracy of 1%.The calculated rejection ratios range from 200 to 107, depending on the degree of spectral overlap. The prototype LOS system was used to analyze mixtures of gases to evaluate how well this system was able to achieve the calculated rejection ratios. These tests were conducted with mixtures of ethanol, methanol, ammonia, and trichloroethylene vapor. T h e selected measurement wavelengths were 9.25, 10.16, 10.33. and 10.57 p m , respectively. The m e a surcd rejection ratios are presented in Table I. These values are in good agreement with the calculated values.
D I A X Model 100 G a s Analyzer The DIAX Model 100 gas analyzer (Figure 6 ) will be available after July 1974. It utilizes the LOS technique of gas analysis tested in the prototype system described above. Unlike the prototype system, which could only detect gases that absorb in the 9.2 to 10.8-pm region, the Model 100 will be able to detect all infrared absorbing gases. This results from the improved laser system in the Model 100 which produces over 200 different wavelengths over the range of 2-1 1 pm. The wavelengths are spread uniformly enough over this region to detect all IR absorbing gases. Like the prototype system, this system will have i ~ c om o d e s of operation. The first, the absorption mcnsiiremcnt m o d e . automatically measures and records a. sample’s absorption strength a t each of’the tem wavelengths. This mode is used to record the absorption properties of a gas sample containing a single chemical compound. It produces the absorption reference data needed for the second mode of operation which analyzes mixtures of gases. To analyze a mixture of gases, the system user must first store the absorption properties of each ANALYTICAL CHEMISTRY, VOL.
430 200
Trichloroethylene
16000
...
300 1080
1000
...
component of the mixture in t h e minicomputer memory. This can be done either by measuring these properties in the absorption measurement mode or by using previously measured LOS spectra. A library of LOS spectra is being built u p by DIAX Corp. To analyze a mixture of gases, the system is operated in the second or anal>sis m o d e Once t h e absorption properties of the components of the mixture are stored in the computer memory, the system can start automatic mixture analysis. More wavelengths than the number of gases present in the sample are selected. A sample is automatically drawn into the sample chamber. and t h e flow valves are closed. T h e laser source is then tuned in sequence to each of the selected wavelengths. The tuning pauses a t each wavelength to allow sample absorption and laser power to be measured. At the end of the wavelength scan. the minicomputer calculates the concentration of the gases present in t h e sample. This calculation, which is described above, requires a number of wavelengths equal t o t h e number of different component gases present in t h e sample. The system checks t h e consistency of the calculated gas concentrations by using measurements a t t h e extra wavelengths also to calculate concentrations. Agreement between these different calculations indicates t h a t t h e calculations accurately represent the mixture composition. This automatic consistency checking essentially eliminates t h e possibility t h a t absorption produced by a n unsuspected component of t h e mixture may cause measurement error. The results of the concentration and consistency calculations are typed out on the teletype unit. This information is also available at the digital output connector for recording on magnetic tape or transmission to a remote recording location. This system has been designed to analyze mixtures of 10 gases with a sensitivity of 1 ppb a n d a cycle time for a complete analysis of 5 min. The rejection ratio between gases with similar absorption spectra is greater t h a n 200; t h e rejection ratio between gases with different spectra runs as high as 108.
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Applications of LOS The technique of LOS gas analysis is expected to find a variety of applications. It offers a single system which can automatically analyze a mixture of gases with high rejection ratios and sensitivity. One particularly promising field of application is gaseous air pollution detection. A single LOS system can replace the collection of single gas analyzers which is currently required t o provide a complete analysis of ambient air. The LOS system should become a useful research tool. It can be adapted to many specialized research uses by changing the computer software. It can be interfaced to a gas chromatograph (GC) by feeding the effluent gas from the chromatographic column into the LOS system. The small sample volume t h a t can be used for LOS gas analysis makes this possible. I t is possible to detect 1 ppb of analyte in l cc of inactive carrier gas. This means that picogram sensitivity is possible in a combined GC-LO’S system. A combined GC-LOS system has some of the same properties as a GC-MS (mass spectrometer) combination. However, it may offer advantages of simplicity, sensitivity, and cost. Since most GC carrier gases do not absorb infrared, there is no need as there is in a MS, to remove the carrier gas. The spectrum recorded in a MS depends on the mass fragnientation pattern of the molecule, and the relative intensity of different peaks is not reproducible from one system to another. T h e LOS spectrum depends on laser wavelengths and IR absorption properties and is highly reproducible. I t provides an accurate method for identifying GC peaks. Industrial process control is another field of application. The LOS system provides real time data in a digital format t h a t can easily be interfaced to a process control computer.
References (1) Edwin Kerr and John Atwood, Appl Opt , 7,915 (1968). (2) L. B. Kreuzer. J A .D.D / Phxs 42,2934 ( 1971). (3) “Handbook of Lasers,” R. J. Pressley, E d . , Chemical Rubber Publ., Cleveland, Ohio. 1971. (4)- L. B. Kreuzer, N.D . Kenyon, and C. K. K.Patel, Science, 177,347 (1972). ( 5 ) P. L. Hanst, Appl. Spectrosc., 24, 161 (1970). (6) R. T. Menzies, N. George, and hi. K. Bhaumih, I E E E J . Quantum Electron., QE-6 (December 1970). ( 7 ) A . G . Bell, Phil. Mag., 11, 510(1881). ( 8 ) J. Tvndall. Proc. Ro\. Soc. (London). 31,30? (1881). (9) W. C. Rontgen. Phi/. Mag.. 11,308 (1881). (10) “Infra-Red Physics,” J . T. Houghton and S. D. Smith, pp 2i6--78, Oxford, England, 1966. 1
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