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: 3 International Conference on Environmental Systems 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.