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。     

Sub-part-per-billion monitoring of nitric oxide by use of wavelength modulation spectroscopy in combination with a thermoelectrically cooled, continuous-wave quantum cascade laser

We used a thermoelectrically cooled, continuous-wave, quantum cascade laser operating between 1847 and 1854 cm(-1) in combination with wavelength modulation spectroscopy for the detection of nitric oxide (NO) at the sub-part-per-billion by volume (ppbv) level. The laser emission overlaps the P7.5 doublet of NO centered around 1850.18 cm(-1). Using an astigmatic multiple-pass absorption cell with an optical path length of 76 m, we achieved a detection limit of 0.2 ppbv at 10 kPa, with a total acquisition time of 30 s. The corresponding minimal detectable absorption is 8.8 x 10(-9) cm(-1) Hz(-1/2).     

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 lambda = 5 microm. 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 x 10(-10) cm(-1)Hz(-1/2). This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for (14)NO and 40 ppt for (15)NO 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 delta(15)N value was obtained with a precision of +/-1.2 per thousand in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of (15)NO as a tracer molecule for endogenous biomedical processes.     

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.     

基于光腔衰荡光谱的便携式呼气异戊二烯分析仪性能评估

呼气异戊二烯是一种内源性代谢产物,其含量与人体血液中的胆固醇水平存在关联。但人体呼气影响因素众多,寻找其与胆固醇水平诊断参数的定量相关性,需要对选取的特定人群进行有效的呼吸气体分析（实时、在线、高灵敏度、高选择性、高精度的大量呼气数据获取）。光腔衰荡光谱（CRDS）是一种具有极高灵敏度、稳定性和选择性的光谱技术。采用目前市场在售的单波长紧凑型半导体紫外激光器,搭建了一套基于CRDS的呼气异戊二烯分析仪,该分析仪主要由激光系统、真空腔体、光电探测模块以及数据采集模块构成。线性拟合的结果显示所获得的衰荡信号接近单指数衰减（R2=0.998 39）,符合朗伯-比尔定律。探究了不同信号平均次数对衰荡信号稳定性的影响,综合考虑衰荡信号的稳定性和分析仪的响应时间,采用128次作为实验过程中的信号平均次数。对呼气异戊二烯分析仪的性能进行了测试,为了表征分析仪的稳定性,持续测量了分析仪16 min的真空衰荡时间。使用氮气、空气和呼吸样本,测量了呼气异戊二烯分析仪的重复性和响应速度。为了测试分析仪的线性度,测量了不同粒子数密度的异戊二烯标准气体（10×10-9,30×10-9,50×10-9,100×10-9,200×10-9）的衰荡时间。最后分析了在224 nm测量异戊二烯存在的光谱干扰问题（NO,N₂O和丙酮）。实验表明：分析仪具有高的灵敏度（检测极限为0.49×10-9）、良好的重复性、稳定性（0.48%）、近实时的响应速度（1秒测量一个数据）和良好的线性度（R2=0.993 13）,将检测极限提高至现有水平的1/1 000。研究证明基于CRDS的便携式呼气异戊二烯分析仪可实现对人体呼气异戊二烯的有效分析。     

Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection

We present and characterize a two-dimensional (2D) imaging spectrometer based on a virtually imaged phased array (VIPA) disperser for rapid, high-resolution molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8?μm. We demonstrate detection of CH₄ at 3.1?μm with >3750 resolution elements spanning >80 nm with ～600 MHz resolution in a <10 μs acquisition time. In addition to broadband detection, we also demonstrate rapid, time-resolved single-image detection by capturing dynamic concentration changes of CH₄ at a rate of ～375 frames per second. Changes in absorption above the noise floor of 5×10^-4 are readily detected on the millisecond time scale, leading to important future applications such as real-time monitoring of trace gas concentrations and detection of reactive intermediates.     

Comparison of cw and pulsed operation with a TE-cooled quantum cascade infrared laser for detection of nitric oxide at 1900 cm-1

A quantum cascade laser operating near room temperature with thermoelectric (TE) cooling has been used in both continuous-wave (cw) mode (-9 °C) and pulsed mode (+45 °C) to detect atmospheric nitric oxide using spectral lines at 1900.07 cm^-1 (5.3 μm). The totally non-cryogenic spectrometer integrates the laser with a 69-m astigmatic multi-pass cell and a TE-cooled infrared detector to enable operation for extended time periods without operator attention. The pattern of reflections on the astigmatic cell mirrors has been designed to minimize optical interference fringes, which are substantially greater with cw mode than with pulsed operation. The detection method uses direct absorption with rapid- scan sweep integration to achieve sub-second time response. Detection precision for NO in air of 0.5 parts in 10⁹ Hz^-1/2 (1σ) is obtained in pulsed mode with an Allan variance minimum corresponding to 0.1 parts in 10⁹ after 30-s averaging time. The precision in cw mode improves to 0.1 parts in 10⁹ Hz^-1/2 and 0.03 parts in 10⁹ after 30-s averaging, corresponding to an absorbance per unit path length of 2×10^-10 cm^-1. The advantages and disadvantages of cw compared to pulsed operation are discussed. PACS 07.88.+y; 42.62.Fi; 82.80.Gk; 92.60.Sz     

Demonstration of soft x-ray laser of Ne-like Ar at 69.8 nm pumped by capillary discharge

The demonstration of a 69.8?nm laser on 3p 3P2 - 3s 1P1 (J=2-1) transition of Ne-like Ar pumped by capillary discharge is reported in this Letter. A main current of 12?kA with rise time of 43?ns was chosen to generate the plasma in a 35?cm long capillary filling with pressure of Ar as low as 11?Pa, resulting in a gain coefficient of 0.34?cm(-1) and gain-length product of 11 at 69.8?nm. Also observed is a weak laser on 3p 3D2 - 3s 3P1 (J=2-1) transition of Ne-like Ar at 72.6?nm in the same condition. In addition, multiwavelength lasing at 46.9, 69.8, and 72.6?nm was simultaneously realized in a capillary discharge plasma column, conditioning the pressure of Ar to 13?Pa.     

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.     

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     

High-Resolution Laser Spectroscopy of YbCl: The A(2)Pi-X(2)Sigma(+) Transition

The A(2)Pi-X(2)Sigma(+) transition of (174)Yb(35)Cl and (172)Yb(35)Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0-0 and 1-0 bands with a measurement accuracy of approximately 0.0035 cm(-1). Resolved fluorescence was used to record the 0-1, 0-2, and 0-3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A(2)Pi state are A(e) = 1491.494(2) cm(-1) and R(e) = 2.4433(1) ?, and for the X(2)Sigma(+) state, R(e) = 2.4883(2) ? and gamma(e) = 4.59(2) x 10(-3) cm(-1). The interaction between the X(2)Sigma(+) and A(2)Pi states has been investigated and is shown to be the main contributor to the spin-rotation splitting in the ground state. Copyright 2000 Academic Press.     

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.     

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).     

Mid-infrared laser absorption spectrometers based upon all-diode laser difference frequency generation and a room temperature quantum cascade laser for the detection of CO, N2O and NO

We describe the performance of two mid-infrared laser spectrometers for carbon monoxide, nitrous oxide and nitric oxide detection. The first spectrometer for CO and N₂O detection around 2203 cm^-1 is based upon all-diode laser difference frequency generation (DFG) in a quasi-phase matched periodically-poled lithium niobate (PPLN) crystal using two continuous-wave room-temperature distributed feedback diode lasers at 859 and 1059 nm. We also report on the performance of a mid-infrared spectrometer for NO detection at ∼ 1900 cm^-1 based upon a thermoelectrically-cooled continuous-wave distributed feedback quantum cascade laser (QCL). Both spectrometers had a single-pass optical cell and a thermoelectrically cooled HgCdZnTe photovoltaic detector. Typical minimum detection limits of 2.8 ppmv for CO, 0.6 ppmv for N₂O and 2.7 ppmv for NO have been demonstrated for a 100 averaged spectra acquired within 1.25 s and a cell base length of 21 cm at ∼ 100 Torr. Noise-equivalent absorptions of 10^-5 and 10^-4 Hz^-1/2 are typically demonstrated for the QCL and the DFG based spectrometers, respectively. PACS  42.55.Px; 42.62.Fi; 42.65.-k; 42.72.Ai; 42.68.Ca     

Ultra-sensitive carbon monoxide detection by using EC-QCL based quartz-enhanced photoacoustic spectroscopy

A quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide detection at ppbv levels was developed with a 4.65???m external-cavity quantum cascade laser operating both in continuous wave (cw) and pulsed modes. A?23-fold enhancement of the measured CO signal amplitude was obtained when water vapor, acting as a catalyst for vibrational energy transfer, was added to the targeted analyte mixture. In the cw mode, a noise-equivalent sensitivity (NES, 1??) of 2 ppbv was achieved at a gas pressure of 100?Torr, for 1-s lock-in amplifier (LIA) time constant (TC), which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of $1.48\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ . In the pulsed mode, the determined NES and NNEA were 46?ppbv and $1.07\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ , respectively, for a 3-ms LIA TC at atmospheric pressure with a laser scan rate of 18?cm^-1/s and a 50?% duty cycle. An intercomparison between cw and pulsed QEPAS-based CO detection is also reported.     

藏红T荧光熄灭法测定痕量亚硝酸根

研究了盐酸介质中亚硝酸根与藏红T发生的重氮化反应 ,基于藏红T的荧光强度随亚硝酸根的加入量增加而明显降低的现象 ,建立了荧光熄灭法测定痕量亚硝酸根的新方法。方法的激发波长为 52 5 0nm ,发射波长为 556 0nm。在盐酸浓度为 0 0 96mol·L- 1 ,反应时间为 1 0min的条件下 ,藏红T的荧光强度与亚硝酸根的加入量在 4～ 2 0 0× 1 0 - 9g·mL- 1 NO- 2 间存在线性关系 ,方法的检出限为 4× 1 0 - 9g·mL- 1 NO- 2 。方法操作简便、反应迅速、灵敏度高 ,已用于水样中痕量亚硝酸根的测定 ,并对反应的机理进行了讨论。     

Near-infrared methane sensor based on a distributed feedback laser

In this article, a near-infrared methane detection system using tunable diode laser absorption spectroscopy technology was designed and implemented. The distributed feedback laser was driven by a self-developed temperature and current controller to allow scanning the selected absorption wavelength at 1654?nm. Laser temperature fluctuation was lower than 0.01%, and the output emission wavelength was linear and stable. The emitted beam passed a reflective gas chamber and was received by the Indium Gallium Arsenide photodiode detector. Through a data acquisition card, a digital lock-in amplifier was developed to extract the second harmonics with real-time monitoring and adjustment. Based on Allan deviation analysis, the limit of detection was about 48?ppm with a path length of 30?cm, at an integration time of 6?s. The experimental results revealed a maximum detection error of less than 3% at a gas concentration higher than 100?ppm. The fluctuations rates in long-term (9?hr) stability measurements for 1?×?10³?ppm and 1?×?10⁴?ppm methane samples were 0.8% and 0.48%, respectively, indicating good stability for the sensor. In the control module design, compared with previous reports on methane detection systems, the current system uses a self-developed temperature controller, a current driver and a signal processor, to allow real-time display and adjustments. The potential for adjustable wavelength scanning is available for multi-gas detection based on a single detection system.     

开放光路非相干宽带腔增强吸收光谱技术测量大气NO2

采用蓝色发光二极管(LED)作为非相干宽带腔增强吸收光谱技术(IBBCEAS)系统光源,测量了436～470nm波段内NO2样气的吸收,验证IBBCEAS的高探测灵敏度。通过氮气和氦气两者瑞利散射截面的差异标定了镜片在430～490nm波段内的反射率,并利用纯氧中氧气二聚体(O2-O2)在477nm处的吸收验证了镜片反射率标定的准确性。镜片反射率在461nm处最大且为0.99937,光学腔长度为73.5cm时的最大有效光程为1.17km。当光谱采集时间为20s时,NO2的探测灵敏度(1σ)达到了0.25×10-9。进行了开放光路下环境大气中NO2和O2-O2在454～486nm波段内的吸收测量,结果表明大气中气溶胶等颗粒物的Mie散射消光降低了IBBCEAS仪器的探测灵敏度(1.04×10-9)。大气中O2-O2的测量为IBBCEAS吸收光程的在线标定提供了一种可行的途径。     

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.