Time resolved simultaneous detection of 14NO and 15NO via mid-infrared cavity leak-out spectroscopy

We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near λ?=?5 μm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R?=?99.988 %), we observed a noise-equivalent absorption coefficient of 3?×?10^?10 cm^?1Hz^?1/2. This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for ¹⁴NO and 40 ppt for ¹⁵NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The δ¹⁵N value was obtained with a precision of ±1.2‰ in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of ¹⁵NO as a tracer molecule for endogenous biomedical processes.     

Infrared laser-spectroscopic analysis of <Superscript>14</Superscript>NO and <Superscript>15</Superscript>NO in human breath

We report on monitoring of nitric oxide (NO) traces in human breath via infrared cavity leak-out spectroscopy. Using a CO sideband laser near 5 μm wavelength and an optical cavity with two high-reflectivity mirrors (R=99.98%), the minimum detectable absorption is 2×10⁻¹⁰ cm⁻¹ Hz^1/2. This allows for spectroscopic analysis of rare NO isotopologues with unprecedented sensitivity. Application to simultaneous online detection of ¹⁴NO and ¹⁵NO in breath samples collected in the nasal cavity is described for the first time. We achieved a noise-equivalent detection limit of 7 parts per trillion for nasal ¹⁵NO (integration time: 70 s).     

Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy

We report on spectroscopic real-time detection of (13)CH(4) in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near lambda = 3 mum. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with R = 99.98% mirrors, we achieved a detection limit of 290 parts in 10(12) (ppt) (13)CH(4) in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 x 10(-9)/cm.     

NO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy and external cavity quantum cascade laser

A gas sensor based on quartz-enhanced photoacoustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric oxide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm⁻¹. Signal and noise dependence on gas pressure were studied to optimize sensor performance. The NO concentration resulting in a noise-equivalent signal was found to be 15 parts per billion by volume, with 100 mW optical excitation power and a data acquisition time of 5 s.     

Application of a broadband blue laser diode to trace NO2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450?nm. A detection limit of 18?ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7?mW at a 1?s integration time, which corresponds to a 1?σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1)?W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.     

Simultaneous detection of 14NO and 15NO using Faraday modulation spectroscopy

We describe a technique of simultaneous detection of ¹⁴NO and ¹⁵NO by means of Faraday Modulation Spectroscopy (FAMOS) based on a cw distributed feedback quantum cascade laser (QCL) operating near 5.4 μm. FAMOS is a spectroscopic method for selective, sensitive, and time-resolved detection of free radical molecules such as NO, in the mid-infrared spectral region. The selected spectral lines are the Q (1.5) for ¹⁵NO located at 1842.76 cm^?1 and the P (9.5) for ¹⁴NO located at 1842.93 cm^?1. The detection limit (1σ) of 6 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁵NO and 62 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁴NO has been achieved. The simultaneous detection was performed using a fast laser frequency switching between the two isotopologues with a time resolution of 2 s. The isotope ratio (δ ¹⁵N) has been determined with a precision (1σ) of 0.52‰ at 800-s averaging time for 100 ppm NO-gas with a time resolution of 2 s. δ ¹⁵N is determined after NO release from nitrite by chemical reduction with potassium iodine.     

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents(CWAs) are recognized as serious threats of terrorist acts against the civilian population.Minimizing the impact of these threats requires early detection of the presence of CWAs.Cavity ring-down spectroscopy(CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species.In this letter,the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate(DMMP).A limit of DMMP detection of approximately 77 ppb is achieved.The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2×10-7cm 1.     

Mid-infrared cavity leak-out spectroscopy for ultrasensitive detection of carbonyl sulfide

We present a ringdown absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ringdown cavity (length, 0.5 m) with R > = 99.99% mirrors, we observed a noise-equivalent absorption coefficient of 7 x 10(-11) cm(-1) Hz(-1/2). This is 2 orders of magnitude improved compared with previous values. With this setup we studied the spectroscopic detection of carbonyl sulfide (here abbreviated OCS) traces in ambient air and in exhaled breath. We achieved a detection limit of 7 parts in 10(12) (parts per trillion) OCS in ambient air, which is unprecedented and shows great promise for environmental and biomedical applications.     

Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10^-13 cm³ molecule^-1 s^-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×10⁹ molecule cm^-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10^-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the ^RR(15) absorption of the b¹Σ_g ⁺-X³Σ_g ^- (1,0) band of molecular oxygen at =14553.947 cm^-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10^-5, 6.9×10^-7 and 1.9×10^-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz. Received: 24 June 2002 / Revised version: 12 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: vlk@physchem.ox.ac.uk     

Continuous-wave 1.55 microm diode-pumped surface emitting semiconductor laser for broadband multiplex spectroscopy

A room-temperature-operating vertical external cavity surface emitting laser is applied around 1550 nm to intracavity laser absorption spectroscopy analyzed by time-resolved Fourier-transform interferometry. At an equivalent path length of 15 km, the high-resolution spectrum of the semiconductor disk laser emission covers 17 nm simultaneously. A noise-equivalent absorption coefficient at 1 s averaging equal to 1.5 x 10(-10) cm(-1)Hz(-1/2) per spectral element is reported for 65 km, the longest path length employed.     

QCL-based TDLAS sensor for detection of NO toward emission measurements from ovarian cancer cells

The development of a sensitive sensor for detecting nitric oxide (NO) emissions from biological samples is reported. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) using a continuous wave, thermoelectrically cooled quantum cascade laser (QCL) and a 100-m astigmatic Herriot cell. A 2f-wavelength modulation spectroscopy technique was used to obtain QCL-based TDLAS NO emission measurements with an optimum signal-to-noise ratio. An absorption line at 1,900.076 cm^?1 was targeted to measure NO with a minimum detection limit of 124 ppt. Positive control measurements with the NO donor DETA NONOate were performed to determine and optimize the sensor performance for measurements of biological samples. Our measurements with NO donor show the potential suitability of the sensor for monitoring NO emission from cancer cells for biological investigations.     

Frequency-agile,rapid scanning spectroscopy: absorption sensitivity of 2 × 10−12 cm−1 Hz−1/2 with a tunable diode laser

We present ultrasensitive measurements of molecular absorption using frequency-agile rapid scanning, cavity ring-down spectroscopy with an external-cavity diode laser. A microwave source that drives an electro-optic phase modulator with a bandwidth of 20 GHz generates pairs of sidebands on the probe laser. The optical cavity provides for high sensitivity and filters the carrier and all but a single, selected sideband. Absorption spectra were acquired by stepping the tunable sideband from mode-to-mode of the ring-down cavity at a rate that was limited only by the cavity decay time. This approach allows for scanning rates of 8 kHz per cavity resonance, a minimum detectable absorption coefficient of 1.7 × 10^?11 cm^?1 after only 20 ms of averaging, and a noise-equivalent absorption coefficient of 1.7 × 10^?12 cm^?1 Hz^?1/2. By comparison with cavity-enhanced laser absorption spectrometers reported in the literature, the present system is, to the best of our knowledge, among the most sensitive and has by far the highest spectrum scanning rate.     

Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν₁+ν₃ water vapor combination band near ˜ν=7181.156 cm^-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N₂, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10^-10 cm^-1 Hz^-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10^-9 for an absorption spectrum of 10 minutes duration. PACS 33.20.-t; 33.70.Jg; 33.70.Fd; 42.62.Fi     

Rapid formaldehyde monitoring in ambient air by means of mid-infrared cavity leak-out spectroscopy

We report the spectroscopic detection of formaldehyde in ambient air using cavity leak-out spectroscopy, a cw variant of cavity ring-down spectroscopy. This technique proved to be suitable for a real-time quantitative analysis of polluted air without any preprocessing of the air sample. Using a tunable CO-overtone sideband laser for the λ=3 μm spectral region and a ring-down cell with R=99.95% mirrors, we achieved a detection limit of 2 parts per billion formaldehyde in ambient air, corresponding to a minimum detectable absorption coefficient of 7×10^-9/cm (sampling time: 2 s). Calibration problems arising from the polarity of the molecule and due to HITRAN database uncertainties are discussed. Received: 28 March 2002 / Revised version: 7 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-211/811-3121, E-mail: muertz@uni-duesseldorf.de     

Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (¹²CH₄, ¹³CH₄) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9×10^-9 /cm. Rapid determination of the ¹³C/¹²Cisotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11%o . Received: 14 July 2000 / Revised version: 25 October 2000 / Published online: 6 December 2000     

宽带腔增强吸收光谱技术应用于大气NO3自由基的测量

NO3自由基是夜间大气化学中最重要的氧化剂,控制着多种痕量气体成分的氧化及去除,了解NO3自由基的化学过程对研究灰霾等大气污染过程意义重大.NO3自由基浓度低、活性强,实现大气NO3自由基的高灵敏度准确测量相对困难.本文介绍了大气NO3自由基的宽带腔增强吸收光谱定量方法,采用红光LED作为宽带腔增强吸收光谱系统光源,设计低损耗且适合国内高颗粒物环境的采样气路,并通过LED光源测试确定最佳工作电流和温度;通过采用白天的大气谱作为背景光谱参与NO3自由基的光谱拟合过程,减少水汽对NO3自由基光谱反演的干扰;通过对镜片反射率和有效腔长进行标定,对系统性能进行Allan方差分析,该宽带腔增强吸收光谱系统在光谱采集时间为10 s的情况下,NO3自由基极限探测灵敏度为0.75 pptv,总测量误差约为16%.在合肥开展了实际大气NO3自由基观测,观测期间NO3自由基的浓度范围从低于探测限到23.4 pptv,NO3自由基浓度呈现夜间高、白天低的特征,符合NO3变化规律,表明该宽带腔增强吸收光谱系统能够用于实际大气NO3自由基的高灵敏度测量.     

Cavity ring-down measurements of seeded NO in premixed atmospheric-pressure H2/air and CH4/air flames

Quantitative aspects of using cavity ring-down absorption spectroscopy near 226 nm for measurements of NO mole fractions in premixed atmospheric-pressure flames are discussed. Measurements in methane–air flames showed strong broadband absorption near 226 nm by hot CO₂ molecules, precluding using the cavity ring-down method in these flames at atmospheric pressure. In hydrogen–air flames, the broadband absorption at this wavelength was substantially lower. Absorption cross sections derived from non-seeded cavity ring-down spectra suggest that absorption by water is the major contribution to the background in these flames. The detectability limit for NO by cavity ring-down measurements in hydrogen–air flames using the current setup is estimated to be 10 ppm. Effects of the cold boundary layer on the measured NO mole fraction were accounted for by measuring the radial distributions of temperature and NO mole fraction using coherent anti-Stokes Raman scattering and laser-induced fluorescence (LIF), respectively. Measurements performed in seeded stoichiometric and lean hydrogen–air flames showed no reburning at temperatures above 1750 K, demonstrating the adequacy of using these flames for calibration of LIF measurements. At lower temperatures, the mole fraction of NO in the hot gases was up to 30% lower than that expected from the degree of seeding in the cold gases. PACS 42.62.Fi; 42.68.Ca; 82.33.Vx     

Quantum-noise-limited cavity ring-down spectroscopy

We demonstrate a heterodyne-detected cavity ring-down spectroscopy (CRDS) method that allows for a noise-equivalent absorption coefficient of 6 × 10^?14 cm^?1 Hz^?1/2, the lowest which has been reported in a CRDS measurement. It is shown that heterodyne-detected CRDS also reaches the quantum noise limit at reasonable optical powers. In addition to offering ultra-high sensitivity, this technique provides high frequency agility over a range of 2 THz in the near-infrared, which allows entire absorption bands to be recorded in minutes. As a demonstration experiment, high resolution spectra of a near-infrared carbon dioxide band have been recorded.     

高灵敏的连续激光光腔衰荡光谱仪及其应用

基于连续可调谐的钛宝石激光光源,建立了光腔衰荡光谱(CRDS)装置,实验表明,其不但具有10-4cm-1的光谱分辨率,测靖灵敏度也好于10-10·cm-1.通过对C2H2气体在12 696.4 cm-1附近的吸收光谱测量,验证了该装置的定量测量能力,并通过对混有痕量C2H2气体的氮气样品的光谱测量,表明该装置对C2H2...     

NO2分子高灵敏度痕量探测技术研究

 搭建了一台基于蓝光LED的非相干宽带腔增强吸收光谱系统,并将其应用于NO2分子的高灵敏度痕量探测研究。在3 s采样时间下, 系统探测灵敏度为3.2×10-9 cm-1（1 σ）,对应NO2的探测极限约为187 pmol/mol。利用Allan方差对系统最佳采样时间及系统稳定性进行分析,当采样时间延长至30 s时,系统的探测极限可提高至44 pmol/mol。将该系统应用于实际大气中NO2的连续测量,其测量结果与商业化NOx分析仪（Thermo 42i）进行了比对测试。