Limitation to Optoacoustic Detection of Atmospheric Gases by Water Vapor Absorption Jerry Gelbwachs

The Aerospace Corporation, P.O. Box 92957, Los An­geles, California 90009. Received 23 November 1973.

Optoacoustic spectroscopy provides a highly sensitive method for the detection of molecules in the ambient air. Kreuzer and Patel have employed this technique to mea­sure nitric oxide concentrations of 10 parts/billion (ppb).1

In later work Kreuzer et al. detected 5 ppb of ethylene in air.2 Recently, Dewey et al. have demonstrated that acoustic gain can increase the detection sensitivity.3 If the sensitivity improves to the point that 1 ppb or less can be monitored, this method will become a prime candidate for the detection of trace gases of the atmosphere, drugs, and explosives. However, absorption of the laser excita­tion radiation by other constituents of the ambient air may limit the minimum molecular concentration that can be monitored. Indeed, water vapor has been identified as an interfering species.1 In this report, the sensitivity for optoacoustic detection of light and heavy gases in the presence of strong and weak water vapor bands has been calculated.

The existence of strong molecular absorption lines cou­pled with the availability of numerous laser sources render the 5-10-µm spectral range particularly suitable for op­toacoustic detection. In this range water vapor displays more than 4000 absorption lines.4 The average line width at atmospheric pressure is 3 GHz with little variation from line to line. Very little spectral space is left unoccu­pied by the pressure-broadened water vapor absorption lines. The strength of these lines extends from 10 – 4

cm – 1 / g / cm 2 to 10–4 cm – 1 / g / cm 2 . A strong band is char­acterized by a line strength of 10 cm – 1 /g /cm 2 ; average and weak bands exhibit line strengths of 10–1 c m – 1 / g / cm2 and 10 – 3 cm – 1 /g /cm 2 , respectively. At less than 15-Torr total pressure the water vapor lines are predomi­nantly Doppler broadened with full widths at half-maxi­mum intensity of ~0.15 GHz. Therefore, at low pressure, interference from water vapor is reduced at excitation wavelengths that do not coincide with the centers of water vapor lines.

The influence of water vapor absorption bands on the minimum detectable concentration of trace gases is strongly dependent on the character of the trace gas ab­sorption coefficient. For light gases, such as atmospheric pollutants, the peak absorption coefficient α0 is indepen­dent of pressure over the range from 1 atm to 15 Torr; this is a consequence of the discrete nature of the absorption spectra of light molecules. Heavy gases, such as vapors of drugs and explosives, exhibit unresolved continuum spec­tra (over several wavenumbers) in the same pressure range. Consequently, the α0 for heavy molecules varies directly with pressure and is not critically dependent on wavelength.

The absorption coefficients for light molecules, heavy molecules, and water vapor can be calculated as a func­tion of pressure and wavelength displacements from line center. Absorption of laser radiation by the above molec­ular species produces signals at the detector output. Comparison of these signals to the noise background de­termines the sensitivity of the optoacoustic technique. Background signals arising from absorptions by the win­dows and walls of the optoacoustic chamber have been shown to limit the sensitivity to a level much higher than

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Fig. 1. Sensitivity for optoacoustic monitoring of light gases (upper graph) and heavy gases (lower graph) at 20°C and 40% relative humidity vs minimum detectable absorption coefficient. Locus of curves S and s represents detection in the presence of strong water vapor bands at 1 atm and 15 Torr, respectively. The corresponding locus of curves for detection near weak bands is regions W and ω. Within a given region, one moves vertically downward as the displacement between the center of the absorp­tion line of the trace gas and the water vapor line center increases.

that imposed by the Brownian motion of the microphone diaphragm.5 In this case, the minimum detectable ab­sorption coefficient is determined by the fluctuations of the background signal.

In order to perform a meaningful comparison, it is nec­essary to select typical values for various parameters. At 20°C, 40% relative humidity and 1-atm pressure, the cal­culated line-center absorption coefficient is 10 – 3 c m – 1 for strong water vapor bands and 10 – 7 c m – 1 for weak ones. The same absorption coefficient per unit concentration αo/c at 1-atm total pressure is assumed for both heavy and light gases. A value representative of strong ir tran­sitions, a0/c = 1 0 8 cm – 1 / ppb , 6 is selected. The mea­surement uncertainty (noise level) has been observed to be 1% of the background signal in an operational system.2

Consequently, this value has been chosen here. Using these parameters, the sensitivity (for a SNR of

unity) for monitoring light and heavy gases has been cal­culated as a function of minimum detectable absorption coefficient. The latter quantity is a characteristic of a particular optoacoustic system. The excitation wave­length has been chosen to be coincident with the center of

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the absorption line of the trace gas. The results appear in Fig. 1. Regions S and s are the locus of curves repre­senting detection in the presence of strong water vapor bands at 1 atm and 15 Torr, respectively. The corre­sponding regions for detection in the vicinity of weak water bands are W and ω. As the spacing between the center of the water vapor line and the line center of the trace gas increases, the sensitivity improves, i.e., one moves vertically downward within each region. The upper edge of a region corresponds to the two absorption lines overlapping; at the lower edge the absorption lines are separated by 4.5 GHz. Less than 20% of the spectral intervals between adjacent water vapor absorption lines exceed 9 GHz in the 5-7-µm range.4

From these curves we conclude that water vapor limits equally the sensitivity at 1 atm for detection of heavy and light gases. The limiting values are ~ 1 0 3 ppb and ~ 0 . l ppb in the proximity of strong and weak water vapor bands, respectively. These values are directly proportion­al to water vapor concentration. At low pressure, the limiting values for the detection of light molecules in the presence of water vapor can be reduced by a hundredfold, whereas the anticipated improvement for the detection of heavy molecules is minimal. Caution should therefore be exercised in extrapolating from the sensitivity attained for monitoring light gases (pollutants) to the sensitivity for heavy gases (vapors of drugs and explosives).

This work was supported by the Law Enforcement As­sistance Administration under contract J-LEAA-025-73.

References 1. L. B. Kreuzer and C. K. N. Patel, Science 173, 45 (1971). 2. L. B. Kreuzer, N. D. Kenyon, and C. K. N. Patel, Science 177,

347(1972). 3. C. F. Dewey, Jr., R. D. Kamm, and C. E. Hackett, Appl.

Phys. Lett. 23,633(1973). 4. W. S. Benedict and R. F. Calfree, Line Parameters for the 1.9

and 6.3 Micron Water Vapor Bands, ESSA Prof. Paper 2 (U.S. Dept. of Commerce, Washington, D.C., 1967).

5. L. B. Kreuzer, J. Appl. Phys. 42, 2934 (1971). 6. E. D. Hinkley and P. L. Kelly, Science 171, 635 (1971).