Experimental Analysis of Multi-Trace Gas Analyzer Based on Photoacoustic Spectroscopy for Manned Spacecraft

46th International Conference on Environmental Systems ICES-2016-235 10-14 July 2016, Vienna, Austria

Experimental Analysis of Multi-trace Gas Analyzer based on Photoacoustic Spectroscopy for Manned Spacecraft

Zhao, Hanqing1, Yang, Xiantao2, Meng, Gui3 Xi, Hongzhu4, Yu, Tao5, Zhou, Jianfa6, Peng, Yongqing7, Zou, Jiangbo8, Beijing Research Institute of Telemetry, Beijing, China, 100076

To ensure the spacecraft safety and the crew’s health for the long-term manned space missions, a real-time multi-trace gas monitoring instrument, based on the photoacoustic spectroscopy technology, was developed by BRIT in 2014. To assess the applicability of the instrument for space missions, a series of experiments have been carried out. 1) Compared with the target trace gases, the concentration of water vapor and carbon dioxide in the air is tremendously high. In order to make the quantitative analysis of their effects on the detection of other trace gases, the first group of experiments has been implemented. The experimental data show that the influence of water vapor could be eliminated easily by linear correction, while the influence of carbon dioxide must be corrected nonlinearly. 2) The cross interference between the trace gases with the same molecular functional group is very important to the applicability of the instrument. The second group of experiments is designed for the gases with the absorption spectrum overlapped or closed to each other. The results indicate that the selectivity of the instrument can be improved by using multiple characteristic peaks of the absorption spectrum in the algorithm. Through the data analysis, the detection limits of the instrument for most of the target gases can achieve the ppb level. For some sensitive gases, such as Bromotrifluoromethane, Chlorodifluoromethane, it could even reach the ppt level. 3) An effective operation method to replace the toxic target gas by the non-toxic substitute gas during the calibration experiment process is proposed. The method is based on the appropriate transformation of the spectrum, and its validity is verified by the third group of experiments. Through the above experimental analysis, it concludes that the proposed analyzer is applicable to the trace gas monitoring of the cabin atmosphere in the manned spacecraft.

Nomenclature c = Concentration of Gas α = Absorption Strength of the Gas p0 = Pressure of Mixed Gas Fcell = Constant of Photoacoustic Cell G = Sensitivity of the Acoustic Transducer VPA = Amplitude of Photoacoustic Signal Vnoise = Amplitude of Total Noise Cmin = Minimum Detection Concentration VOA = Volatile Organic Analyzer ANITA = Analyzing Interferometer for Ambient Air VCAM = Vehicle Cabin Atmosphere Monitor

1 Engineer, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 2 Engineer, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 3 Engineer, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 4 Engineer, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 5 Engineer, Wuhan Second Ship Design and Research Institute, Wuhan, 430205, China 6 Principle Engineer, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 7 Director, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. 8 Department Chairman, P.O.Box 9200-74-11, Fengtai, Beijing, 100076, China. FTIR = Fourier Transform Infrared GC/MS = Gas Chromatograph/Mass Spectrometry GC/IMS = Gas Chromatograph/Ion Mobility Spectrometry PAS = Photoacoustic Spectroscopy SMAC = Spacecraft Maximum Allowed Concentration ISS = International Space Station QCL = Quantum Cascade Lasers ICL = Interband Cascade Lasers DFB = Distribute Feedback CMOS = Complementary Metal Oxide Semiconductor CCD = Charge Coupled Device PLS = Partial Least Square BRIT = Beijing Research Institute of Telemetry PNNL = Pacific Northwest National Laboratory

I. Introduction or a few decades ago, air monitoring technology for aerospace application has been developed in Europe, Russia, F and USA. Fourier transform infrared spectrometer (FTIR) and the gas chromatograph (GC) / ion-mobility spectrometer (IMS) systems are two highest scoring systems, since it best fulfilled the requirements on simultaneous multi-compound detection and continuous air quality analyses1. More than six types of sensors were employed for gas detection in International Space Station (ISS) 2.Vehicle Cabin Atmosphere Monitor (VCAM), which identifies trace gases harmful to the crew's health, was flown to the ISS on shuttle mission STS-131 and commenced operations on 6/10/10. It provides measurements of ppb-to-ppm levels of the volatile trace-gas constituents, and of the atmospheric major constituents (nitrogen, oxygen, argon, and carbon dioxide) in a space vehicle or station3, 4, 5. Air quality monitor ANITA (Analyzing Interferometer for Ambient Air) has been successfully put into operation in US lab Destiny6, 7. The second generation of ANITA (ANITA 2) for the ISS has also been reported8, 9. Based on the conclusion from ISS, taking mass and volume into account, photoacoustic spectroscopy (PAS) is a suitable choice to analyze the enclosed air quality for manned space missions. The photoacoustic signal is directly relative to the concentration of the target gas, which is the distinguished characteristic in comparison with the current FTIR technology. There was moving parts in the system that is hard to make reliable alignment under harsh conditions. The GC/IMS has good detection limit, but its weight and power consumption were too high for limited payload. Moreover, periodically calibration on orbit is very difficult. A real-time multi-trace gas monitoring instrument10, based on the photoacoustic spectroscopy technology, was developed by BRIT in 2014. To assess the applicability of the instrument for space missions, a series of experiments have been carried out in this paper.

II. Instrument Development The photoacoustic Analyzer consists of four main components: light sources, Data Process and Signal photoacoustic Cell, ultra-sensitive acoustic Control detection unit (transducer), and data process Laser Driver & unit. The light sources are combination of TEC Control different types of light source, such as Signal Condition Distribution Feedback (DFB) Laser, Black Circuit Laser 3 Laser 2 Radiation Source, Quantum Cascade (QC) Laser, and Interband Cascade (IC) Laser 10 and so on . The photoacoustic cell is Transducer shown in Figure 1. The cell is made up of a Mirror nonresonant cylindrical tube. The laser sources are coupled into the cylinder cavity Optical Window using right angle mirror. A tilted optical window is installed in each end of the cylinder cell to reduce the noise caused by Laser 1 optical windows. The ultra-sensitive Figure 1. Schematic of the photoacoustic anlyzer. acoustic transducer is an optical 2 International Conference on Environmental Systems

microphone, which is an ultra-thin cantilever fixed on interferometer. The readout circuit and gas exchange unit are controlled by digital signal process unit. The analyzer is designed to analyze 32 gases, including water and carbon dioxide. Table 1. Target compound list for PA spectrometer. No. Name Measurement Range (ppm) Detection limit(ppm) 1 methanol 0.2-100 0.03 2 ethanol 1-100 0.05 3 acetaldehyde 0.1-10 0.1 4 toluene 1-100 0.05 5 benzene 0.01-10 0.01 6 freon11 2-500 0.01 7 freon12 2-500 0.005 8 meta-xylene 2-100 0.1 9 ortho-xylene 2-100 0.05 10 para-xylene 2-100 0.05 11 ammonia 0.1-100 0.01 12 methane 10-1000 0.1 13 carbon monoxide 1-100 0.2 14 carbon dioxide 100-10000 100 15 nitrogen dioxide 0.1-100 0.04 16 glycol 10-500 0.05 17 hydrogen sulfide 0.1-100 0.1 18 acetone 1-100 0.05 19 formaldehyde 0.05-10 0.05 20 2-propanol 3-100 0.06 21 dichloroethane 0.03-10 0.03 22 perfluoro propane 0.1-100 0.1 23 acetic acid 0.5-100 0.04 24 n-butanol 1-100 0.2 25 ethyl benzene 1-100 0.05 26 furan 0.01-10 0.01 27 acrolein 0.01-10 0.01 28 methylamine 1-100 0.3 29 vinyl chloride 0.5-100 0.5 30 freon 218 1-100 0.005 31 chloroform 0.05-10 0.05 32 water 100-50000 10

A. Theory The amplitude of the photoacoustic (PA) signal VPA can be calculated according to Beer-Lambert law and wave equation, and it will be proportional to the absorption length and the light power, but inversely proportional to the modulation frequency and the cell volume11.

VPA  c  Fcell W0  p0 G (1)

Where α, W0 , p0, c, G, and Fcell are the absorption strength of the gas, the optical power of the laser, total pressure of the mixed gas, concentration of absorption gas, sensitivity of the acoustic transducer, cell constant of the cell, respectively. The detection limit of the gas cmin can be deduced as follow:

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cmin Vnoise /  Fcell W0  p0 G (2)

Where Vnoise is the minimum detectable transducer signal determined experimentally. Vnoise is the larger one between the background signal and the noise. The value of Vnoise depends on several factors. In addition to photon noise, gas flow noise, acoustic and electronic noise from the system, the background generated by the windows and scattered light plays an important role. The External-Cavity Quantum Cascade Laser was used as the main laser source for the analyzer, with typical wavelength covered from 1000cm-1~1250cm-1. Another two DFB lasers (center wavelength: 1.57μm for H2S, 2.3μm for CO) and IC-QCL laser (center wavelength: 3.4μm for NO) are combined with EC-QC laser to cover all the gases listed in Table 1. In order to save the analysis time, modulation frequencies were set to 70Hz, 90Hz, and 40Hz for the EC-QCL, DFB and IC-QCL lasers, respectively. Compared with Quartz enhanced photoacoustic analyzer12, Figure 2. The PA spectrometer. that had high noise reduction ability in resonant working conditions, optical coherent optical thermal displacement technology was used. The condenser microphone and the electret microphone are widely used to sense the PA signal. The detection limit of the microphone is dB levels, and can’t meet the application requirements. Interferometer is used to measure the ultra- weak acoustic signal. Michelson interferometer is installed in the middle of the cell to detect the acoustic signal. Thin cantilever is used to increase the sensitivity of the transducer. The displacement detection limit for the interferometer is ppm level13, 14. The schematic measurement is shown in the paper10. The photoacoustic cell was working in non-resonant condition, which would not be influenced by the gas concentration and diversity. The low working frequency will have high acoustic response to compensate the low quality factor. Like resonant photoacoustic cell, acoustic buffer chamber was added in the gas inlet and outlet to reduce the unnecessary noise.

III. Experiments The PA spectrometer is shown in Figure 2. When calibrations were carried out, the laser sources were working in step scan mode with the speed of 3.3 wavelengths per second. The gas pressure was adjusted to 1200mbar, and the gas flow rate is 1.3 l/min. The cell temperature was controlled at 45 °C , while the pressure of cell was kept on 930mBar. The PA cell is 35ml, so the gas exchange time can be finished in 18 seconds.

A. Background Subtraction

As mentioned in previous work9, background signal was calculated through purified nitrogen. The actual background was slightly different at different places, due to environmental vibration, acoustic noise and background compounds. So firstly background must be removed before further calculation. In the paper, a modified partial least square (PLS) was used to subtract the background from the paper.

B. Water Vapor and Carbon Dioxide Test Compared with the target trace gases, the concentration of water vapor and carbon dioxide in the air is tremendously high. In order to make the quantitative analysis of their effects on the detection of other trace Figure 3 . Water vapor test apparatus. gases, the spectrum of water vapor and 4 International Conference on Environmental Systems

carbon dioxide must be calibrated accurately. Before analyzing water vapor and carbon dioxide, the system background must be calibrated to set the proper baseline for the instrument firstly. Water Vapor Analysis: As Shown in Figure 3, standardized water vapor is obtained by passing the purified nitrogen into the purified distilled water. The concentration of saturated water vapor is then calculated through its temperature and pressure. Controlling the temperature of water vapor, different concentrations of water vapor are generated. As shown in Figure 4 the three spectrums, with the concentration of 8061ppm, 12135ppm and 24647ppm, are calculated as the variation of eight successive measurement results respectively. The very dry air with the concentration about 100ppm was obtained through purified nitrogen.

H O 8061ppm 2 0.6 H O 12135ppm 2 H O 24647ppm 2 0.5

0.4

0.3

PASignal,a.u.

0.2

0.1

0 1000 1050 1100 1150 1200 1250 Wavenumer,cm-1 Figure 4 Experimental calibration data for water vapor in the concentrations of 8061ppm，12135ppm， 24647ppm

In order to reduce the influence of background，the residual spectrums, are analyzed . The concentrations of residual spectrums are 4074ppm, 12512ppm, 16586ppm. As shown in Figure 5, the linear analysis is performed, at the wavelength of 1136cm-1, 1175 cm-1, 1213 cm-1, where the absorption peaks exist.

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0.45 Wavenumber 1136 cm-1 -1 0.40 Wavenumber 1213 cm Wavenumber 1175 cm-1 0.35

0.30

0.25

0.20

PA Signal,aru.PA 0.15

0.10

0.05

4000 6000 8000 10000 12000 14000 16000 18000 Concentration,ppm

Figure 5 the linear analysis for the residual spectrums at the wavelength of 1136cm-1, 1175 cm-1, 1213 cm-1 The results show good linearity at all selected wavelength, so the influence of water vapor could be eliminated by linear correction. Carbon Dioxide Analysis:

Gas Box Dew Point Transducer

Vent Vent

Dynamic Plug Vacuum Pump Dilution Ball Valve System Pressure Bleed Gauge Valve Swtich Stop Valve 1 Stop Valve 2 Buffer Tube Four-Pass T-Connector Ball Valve 2

Vent Outlet Inlet

PAS Analyzer Figure 6.Calibration cylinder test apparatus with dew point transducer. As Shown in Figure 6, PA spectrometer is calibrated with a standard carbon dioxide gas. The carbon dioxide is supplied through gas box in customized gas supply system, and then mixed in the dynamic dilution system. Different concentrations of carbon dioxide can be generated through the dynamic dilution system.

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0.1 CO ,769ppm 2 CO ,2605ppm 2 CO ,20000ppm 0.08 2

0.06

0.04

PASignals,a.u.

0.02

-0.02 1000 1050 1100 1150 1200 1250 Wavenumbers,cm-1 Figure 7 Experimental calibration data for carbon dioxide in the concentration of 769ppm, 2605ppm and 20000ppm

The carbon dioxide spectrums, with the concentration of 769ppm, 2605ppm and 20000ppm, are given in Figure 7. The calibrated results show the value of spectrum is not monotonic to the concentration of carbon dioxide. Carbon dioxide has weak absorbance and wide line width, so it can be easily regarded as background noise. The influence of carbon dioxide must be corrected nonlinearly. Further calibration was needed to correct the background model.

C. Calibration Test for Freon Gases The cross interference between the trace gases with the same molecular functional group is very important to the reliability of the instrument. The second group of experiments is designed for the gases with the absorption spectrum overlapped or closed to each other. 6 Freon-11,10ppm Freon-12,10ppm Freon-22,10ppm 5 Freon-13B1,10ppm Gases Mixture

PASignal, a.u.

0 1000 1050 1100 1150 1200 1250 Wavenumbers, cm-1 Figure 8 PA signal of different types of Freon with the same concentration of 10ppm and their mixture.

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The Freon gases have the similar physical and chemistry nature. Four types of Freon gases, including Trichlorofluoromethane(Freon-11), Dichlorodifluoromethane(Freon-12), Chlorodifluoromethane(Freon-22), Bromotrifluoromethane(Freon-13B1), have been calibrated individually with the same concentration of 10ppm. In order to analyze the concentration more reliable, the mixed gas samples are added to the scanning sample. Figure 8 shows the absorption peak of Freon-11 is 1081cm-1, 1085cm-1,the absorption peaks of Freon-12 are at 1103cm-1,1159cm-1,1161cm-1, ,the absorption peaks of Freon-22 are at 1107cm-1,1118cm-1, 1132cm-1, the absorption peak of Freon-13B1 is 1085cm-1,1091cm-1, 1209cm-1. Most of the peaks can be also find in the spectrum of their mixture, the concentration of individual gases cloud be calculated by the value of their absorption peaks roughly. More accurate concentration analysis of a large number of compounds at controlled precision is possible with statistical methods evaluating many points of a spectral measurement. The results indicate that the selectivity of the instrument can be improved by using multiple characteristic peaks of the absorption spectrum in the algorithm. With increasing the integration time and filtering of spectrum, the noise level obtained a significant improvement. With 10 minutes of integration time, the PA could reach the minimum detectable limit as high as the ppt level, for some sensitive gases, such as Bromotrifluoromethane, Chlorodifluoromethane.

D. Expansion of Spectrum Library With the development of the Fourier transform infrared (FT-IR) spectroscopy15, the high quality infrared spectral reference libraries, such as the NIST16, PNNL17, HITRAN18 databases, have been established. Since both the PA and FT-IR are based on the absorption spectrum of the gas, it’s possible to build an effective transformation between the reference libraries and the experimental libraries. If so, it’s easy to extend the PA libraries on the basis of the libraries existed. It’s also possible to replace the toxic target gas by the non-toxic substitute gas in the process of calibration experiment.

0.015 0.6 Freon-11,1ppm Freon-11,1ppm 0.5

0.01 0.4

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0.005 PASignals,a.u. 0.2

FT-IRSignals,a.u.

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0 0 1000 1050 1100 1150 1200 1250 1000 1050 1100 1150 1200 1250 -1 Wavenumbers,cm Wavenumbers,cm-1

0.015 0.6 Freon-12,1ppm Freon-12,1ppm

0.5

0.01 0.4

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0.005 PASignals,a.u. 0.2

FT-IRSignals,a.u.

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0 0 1000 1050 1100 1150 1200 1250 1000 1050 1100 1150 1200 1250 -1 Wavenumbers,cm-1 Wavenumbers,cm

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0.015 0.6 Freon-22,1ppm Freon-22,1ppm

0.5

0.01 0.4

0.3

0.005 PASignals,a.u. 0.2

FT-IRSignals,a.u.

0.1

0 0 1000 1050 1100 1150 1200 1250 1000 1050 1100 1150 1200 1250 Wavenumbers,cm-1 Wavenumbers,cm-1

0.015 0.6 Freon-13B1,1ppm Freon-13B1,1ppm 0.5

0.01 0.4

0.3

0.005 PASignals,a.u. 0.2

FT-IRSignals,a.u.

0.1

0 0 1000 1050 1100 1150 1200 1250 1000 1050 1100 1150 1200 1250 -1 Wavenumbers,cm Wavenumbers,cm-1 Figure 9 Comparison between the PA spectrums and FT-IR spectrums Figure 9 shows the comparison between the PA spectrums and PNNL spectrums. The left pictures show the PNNL spectrum (1 ppm-meter at 296K), the right pictures are the same material’s PA spectrum obtained (1ppm at the experiments before). Transformed model with spectral parameters which are based on the background, the line width, the laser power, and the window functions was built. Calibration spectrum can be obtained using the PNNL spectrum combined with the fitting parameter taken into account the line shape, mode and power, and temperature effect to the wavelength. Use the custom PA spectrum could manifest reduce the calibration time. -3 x 10 3.5 Freon-12B2,1ppm 3

2.5

1.5

FT-IRSignals,a.u. 1

0.5

0 1000 1050 1100 1150 1200 1250 -1 Wavenumbers,cm Figure 10 Comparison of the Freon-12B2’s spectrums, between the one calibrated by PA and the one calculated based on the PNNL spectrum Figure 10 shows the difference of the Freon-12B2’s spectrums, between the calibrated PA spectrum and the off- the-shelf PNNL spectrum. The calculated spectrum is 2.4% higher than the one calibrated near the absorption peak at 1094 cm-1 and is 1.2% higher near the absorption peak at 1153 cm-1. If the calculated spectrum is used in the calculation of mixture spectrum, the calculation error will significantly increase when the concentration of Freon- 9 International Conference on Environmental Systems

12B2 is high. When the concentration is very low, it’s useful to extend the libraries of the PA Spectrum. It’s also very helpful to qualitative analyze the trace gases not suitable for routine laboratory, such as the toxic target gas, the unstable gases.

IV. Conclusion Further experiments were carried out to validate the algorithm that made the instrument more reliable. In order to reduce the complexity of the calibration and cost of the time, calibration spectrums were proposed based on the known absorption spectrum. Corresponding experiments show that the proposed model can be used to calibrate the instrument. Water vapor and carbon dioxide were needed to subtract from the spectrum before trace gases quantitative analysis after baseline correction. The concentration of water is linear in its normal concentration range, while the carbon diode has nonlinear behavior. Spectrum overlapping can be solved using several finger pattern zones through partial least square regression analysis. The final weight of the PA analyzer was 15 kg, and the peak power consumption was 25 W. The minimum detectable concentration is 0.005 ppm for Freon. A full analysis could be done in 45 minutes with the accuracy of 10%. In the future, the full test will be performed to investigate the overall cross interference for more gases, and validate the reliability for space mission.

V. Acknowledgements This work was carried out at Beijing Research Institute of Telemetry and supported with Department of Manned Spaceship, Chinese Academy of Space Technology.

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