In-flight measurements of ambient methane, nitrous oxide and water using a quantum cascade laser based spectrometer

In-flight measurements of ambient methane, nitrous oxide and water have been made using frequency down-chirped radiation from a compact, pulsed, quantum-cascade laser spectrometer. In three flights from Oxford airport in October 2006 the variations of the concentration of these three trace gases could be measured and related to possible sources in the flight path. PACS 33.20.EA; 42.62.Fi; 42.68.Ca; 85.35.Be     

Sensitive detection of methane and nitrous oxide isotopomers using a continuous wave quantum cascade laser

A continuous wave quantum cascade laser (QCL), operating near 8.1 μm, was used for wavelength modulation spectroscopy of methane (CH₄) and nitrous oxide (N₂O) stable isotopes. Several rotational transitions of ¹⁴N₂ ¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N₂ ¹⁸O, ¹⁴N₂ ¹⁷O, ¹³CH₄ and ¹²CH₄ fundamental bands were detected. The noise-equivalent absorbance was measured to be less than 10^-5 in a 1-Hz bandwidth. A characterization of the laser source was also performed. The use of a QCL spectrometer for high-precision isotope ratio measurements is discussed. Received 14 March 2002     

Development of a compact quantum cascade laser spectrometer for field measurements of CO2 isotopes

We report the development of a field-deployable, pulsed quantum cascade laser spectrometer. The instrument is designed to measure the ¹³C/¹²C isotopic ratio in the CO₂ released from volcanic vents. Specific ¹²CO₂ and ¹³CO₂ absorption lines were selected around 4.3 m, where the P-branch of ¹²CO₂ overlaps the R-branch of ¹³CO₂ of the 00⁰1–00⁰0 vibrational transition. This particular selection makes the instrument insensitive to temperature variations. A dual-channel cell balances the two absorption signals. We provide details of the instrument design and a preliminary demonstration of its performance based on laboratory measurements of ¹⁶O¹²C¹⁶O and ¹⁶O¹²C¹⁸O. PACS 42.55.Px; 42.62.Fi; 07.88.+y     

Quantum cascade laser photoacoustic detection of nitrous oxide released from soils for biofuel production

In order to investigate the generation of greenhouse gases in sugarcane ethanol production chain, a comparative study of N₂O emission in artificially fertilized soils and soils free from fertilizers was carried out. Photoacoustic spectroscopy using quantum cascade laser with an emission ranging from 7.71 to 7.88 µm and differential photoacoustic cell were applied to detect nitrous oxide (N₂O), an important greenhouse gas emitted from soils cultivated with sugar cane. Owing to calibrate the experimental setup, an initial N₂O concentration was diluted with pure nitrogen and detection limit of 50 ppbv was achieved. The proposed methodology was selective and sensitive enough to detect N₂O from no fertilized and artificially fertilized soils. The measured N₂O concentration ranged from ppmv to ppbv.     

A compact quantum-cascade laser based spectrometer for monitoring the concentrations of methane and nitrous oxide in the troposphere

A spectrometer, based on the frequency down-chirped radiation from a pulsed quantum-cascade laser, capable of detecting fluctuations in local ground level methane and nitrous oxide concentrations over a time scale of minutes, is presented. The applicability of this instrument to low-level atmospheric detection is demonstrated through sampling gases in two locations. PACS 33.20.Ea; 42.62.Fi; 42.68.Ca; 85.35.Be     

基于7.6μm量子级联激光的光声光谱探测N_2O气体

近年来,气候变化对地球的生态环境产生严重影响,而大气温室气体在气候变化中具有重要的作用.一氧化二氮(N_2O)作为一种重要的温室气体,其浓度变化对大气环境产生重要影响,因此对其浓度的探测在大气环境研究中具有重要意义.本文开展了基于中国自主研发的7.6μm中红外量子级联激光的共振型光声光谱探测N_2O的研究,建立了N_2O光声光谱传感实验系统.此系统在传统的光声光谱探测的基础上优化改进,采用双光束增强的方式,增加了有效光功率,进一步提高了系统的探测灵敏度.探测系统以1307.66 cm~(-1)处的N_2O吸收谱线作为探测对象,结合波长调制技术对N_2O气体进行探测研究.通过对一定浓度的N_2O气体在不同调制频率和调制振幅的光声信号的探测,确定了系统的最佳调制频率和调制振幅分别为800 Hz和90 mV.在最优实验条件下对不同浓度的N_2O气体进行了测量,获得了系统的信号浓度定标曲线.实验表明,在锁相积分时间为30 ms时,系统的浓度探测极限为150×10~(-9).通过100次平均后,系统噪声进一步降低,实现了大气N_2O的探测,浓度探测极限达到了37×10~(-9).     

基于中红外分布反馈量子级联激光器的光声光谱技术用于痕量甲烷气体检测

由于工业监控和环境检测的需要,甲烷气体检测日益得到人们的关注。研究了基于中红外分布反馈量子级联激光器(DFB-QCL)的光声光谱技术,并应用于痕量甲烷的检测。自主研发的DFB-QCL室温工作时的激射波长在7.6μm附近,覆盖了甲烷的特征吸收谱线1 316.83cm-1。待测甲烷气体充入亥姆霍兹光声谐振腔中,DFB-QCL的工作频率为234Hz、室温脉冲工作时峰值功率为80mW。中红外光经过甲烷吸收后,产生的声波信号经麦克风检测,由锁相放大器对信号进行采集并输入计算机进行处理。按信噪比为1计算,得到甲烷的探测极限为189nmol.mol-1。     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb Hz^-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com     

Photoacoustic detection of nitric oxide with a Helmholtz resonant quantum cascade laser sensor

Nitric oxide detection has been performed with a Helmholtz resonant photoacoustic sensor previously developed in Reims. The cell was coupled with a continuous wave distributed feedback quantum cascade laser working at cryogenic temperature in continuous wave mode in the 5.4 μm region. A detection limit of 20 ppb of nitric oxide in nitrogen with a laser power of 3 mW can be predicted. Possible applications and improvements are discussed.     

Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation

A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2ν₃ band of methane at 1.65 μm. The apparatus was designed for on-line process control in the manufacturing of the novel low-water-peak fibres developed for optical telecommunications. The importance of collisional relaxation processes in the generation of the photoacoustic signal is reported in the particular case of CH₄ detection in dry O₂ and O₂–N₂ mixtures. The negative influence of these effects results in a strongly reduced and phase-shifted photoacoustic signal, induced by a fast resonant coupling between the vibrational states of methane and oxygen, associated with the slow relaxation of the excited oxygen molecules. An unusual parabolic response of the sensor with respect to the methane concentration has been observed and is discussed. Finally, the beneficial effect of several species, including water vapour and helium, acting as a catalyst to hasten the relaxation of the CH₄–O₂ system, is demonstrated. PACS 42.62.Fi; 33.20.Ea; 34.50.Ez     

基于中红外量子级联激光器和石英增强光声光谱的CO超高灵敏度检测研究

采用石英增强光声光谱(QEPAS)技术对CO痕量气体展开检测研究. 为了实现超高灵敏度探测, 采用输出波长为4.6 μm的新颖中红外高功率分布反馈量子级联激光器为光源, 实现了对CO气体基频吸收带的激发与测量. 在优化了调制深度、气体压强和提高了CO分子的振动-转动弛豫速率后, 获得了1.95 ppbv的优异探测极限. 在分析检测结果的过程中, 讨论了能级寿命对信号强度的影响, 并对QEPAS信号强度的表达式进行了修正.     

基于单个量子级联激光器的大气多组分测量方法

利用单个新型中红外量子级联激光器作为激光光源,结合长程光学吸收池技术开展了大气多组分同时测量方法的研究.通过结合基于自适应性Savitzky-Golay滤波的数据处理算法,有效地提高了系统检测灵敏度和光谱分辨率.研究结果表明,在1 s的时间分辨率和1 atm压力条件下,采用二次微分探测技术可实现CO,N_2O和H_2O测量精度分别为8.20 ppb,7.90 ppb和64.00 ppm(1 ppb=10~(-9),1 ppm=10~(-6));通过提高信号平均时间,在最佳的积分时间(85 s)时,系统可实现的最小检测限分别为1.25 ppb(CO),1.15 ppb(N_2O)和35.77 ppm(H_2O).整个系统具有结构紧凑,成本相对较低,通过选择其他波段的量子级联激光器的激光光源,即可实现对其他分子的实时分析.本系统可广泛应用于大气化学等领域的应用研究.     

Photoacoustic detection of ozone using a quantum cascade laser

A 9.5-m pulsed quantum cascade laser (QCL) and a differential photoacoustic (PA) detector were used to measure trace concentrations of 100 ppbv ozone at ambient pressure with high selectivity. The QCL was tuned by temperature variation between -41 °C and 30.6 °C and the corresponding wavelengths were determined by the PA spectrum of CO₂. Good agreement was found between the measured PA spectrum and the simulated HITRAN spectrum of ozone. The PA signal showed a linear dependence on the ozone concentration in the investigated 4300–100 ppbv range. In comparison with recently published results, in which a similar QCL in combination with an optical absorption analysis technique was applied, an improvement in the ozone-detection sensitivity by a factor of about 200 was achieved. PACS 42.62.Fi; 82.80.Kq; 82.80.Gk; 92.60.Sz     

Optical–feedback cavity–enhanced absorption: a compact spectrometer for real–time measurement of atmospheric methane

We report on the application of the new technique of optical–feedback cavity–enhanced absorption spectroscopy to the real–time quantitative measurement of tropospheric methane traces from an airplane using a compact, low cost instrument based on a telecommunication–type diode laser operating close to room temperature. Methane concentration is obtained by fitting the absorption line centered at 1658.96 nm (6026.23 cm^-1) which belongs to the first overtone transition of the CH stretch vibration. The measurement rate is about 30 Hz, but the response time is limited to about 0.3 s by the gas flow in the measurement cell. The instrument provides the absolute ambient methane concentration accurate to ±1% (±20 ppb) without need for a periodic calibration. This is demonstrated by a hands–off comparison with a self–calibrating chromatographic setup during 10 days. The observed measurement stability can be extrapolated to much longer time periods. With respect to the short–term performance (minutes) fast concentration changes at the level of 1 ppb can be detected, and we believe this performance can be extended to the long term. Finally, a laboratory comparison with a lead–salt mid–infrared diode laser multipass spectrometer (operating close to 3028 cm^-1 at liquid nitrogen temperature) demonstrates a similar performance. PACS 07.88.+y; 42.55.Px; 42.62.Fi     

Spectroscopic detection of biological NO with a quantum cascade laser

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near λ=5.2 μm. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO₂ concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed. Received: 1 November 2000 / Revised version: 19 January 2001 / Published online: 20 April 2001     

Pulsed quantum cascade laser instrument with compact design for rapid, high sensitivity measurements of trace gases in air

We have developed a compact instrument for sensitive, rapid and continuous measurement of trace gases in air, with results presented here for methane (CH₄), nitric oxide (NO), nitrous oxide (N₂O) and ammonia (NH₃). This instrument takes advantage of recent technology in quantum cascade (QC) lasers and infrared detectors, which allows high sensitivity without cryogenic liquids, e.g., 0.2 ppb (0.2×10^-9) of NH₃ in air in 1 s. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument operates continuously, requiring little operator attention, and web-based remote access is provided for instrument control, calibration and data retrieval. The instrument design includes a thermoelectrically (TE) cooled pulsed distributed feedback (DFB) QC laser, a low volume (0.5 l) multipass cell offering 76 m absorption path length and a TE cooled detector. Integrated software for laser control and data analysis using direct absorption provides quantitative trace gas measurements without calibration gases. The instrument may be applied to field measurements of gases of environmental concern. PACS  07.57.Ty; 42.62.Fi; 92.70.Cp     

Detection of ammonia released from zeolite by the quantum cascade laser based photoacoustic set-up

We report the first results obtained with a Brazilian sedimentary zeolite (hereafter called the zeolite concentrate, ZC) to reduce ammonia volatilization – loss of the nitrogen nutrient – in tropical farms. Preliminary laboratory tests were made with both ammonium sulfate and urea into ZC. The reported field tests – obtained in rose cultivations – show a significant effect of the use of urea mixed with ZC on the stem growth of the rose buds. In the laboratory tests, the observations of the retention of ammonia by ZC were done using the photoacoustic set-up described in this work. The report data were obtained at temperatures of 25 and 60°C, simulating typical tropical weather temperatures.     

External cavity quantum cascade laser for quartz tuning fork photoacoustic spectroscopy of broad absorption features

We demonstrate mid-infrared spectroscopy of large molecules with broad absorption features using a tunable external cavity quantum cascade laser. Absorption spectra for two different Freons are measured over the range 1130-1185 cm(-1) with 0.2 cm(-1) resolution via laser photoacoustic spectroscopy with quartz tuning forks as acoustic transducers. The measured spectra are in excellent agreement with published reference absorption spectra.     

Frequency-locking and threshold current-lowering effects of a quantum cascade laser and an application in gas detection field

In this paper,the frequency-locking and threshold current-lowering effects of a quantum cascade laser are studied and achieved.Combined with cavity-enhanced absorption spectroscopy,the noninvasive detection of H_2 with a prepared concentration of 500 ppm in multiple dissolved gases is performed and evaluated.The high frequency selectivity of 0.0051 cm~(-1) at an acquisition time of 1 s allows the sensitive detection of the(1-0) S(l) band of H_2 with a high accuracy of(96.53±0.29)%and shows that the detection limit to an absorption line of 4712.9046 cm~(-1) is approximately(17.26±0.63) ppm at an atmospheric pressure and a temperature of 20 ℃.