近日，来自中国科学院安徽光学精密机械研究所、先进激光技术安徽省实验室、中国科学技术大学、法国滨海大学大气物理化学实验室联合研究团队发表了《基于钕铁硼环形磁体阵列的双中红外波长法拉第旋转光谱NO_x传感器》论文。

Recently, the joint research team from Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Advanced Laser Technology Laboratory of Anhui Province, University of Science and Technology of China, Laboratoire de Physicochimie de l′ Atmosph`ere, Universit´e du Littoral Cˆote d′ Opale, published an academic papers Dual mid-infrared wavelength Faraday rotation spectroscopy NO_x sensor based on NdFeB ring magnet array.

氮氧化物（NO_x，包括二氧化氮（NO₂）和一氧化氮（NO））是对流层臭氧的重要前体，同时也影响羟基和过氧基自由基的浓度。大多数气态化合物在被氧化和从空气中去除或转化成其他化学物质时，都会直接或间接接触到NO_x。在典型的羟基自由基水平下，NO_x的寿命取决于季节和光化学反应速率，通常为几小时。根据IPCC第六次评估报告，NO_x的排放导致净正向变暖，因为它既形成短期臭氧（变暖），又破坏环境甲烷（冷却）。此外，NO_x还导致酸沉降以及化学烟雾和气溶胶的形成。NO和NO₂在大气光化学反应中起着核心作用，针对它们的检测有助于理解这两种气体的来源和去向，以及研究陆地生态系统与大气之间的NO_x交换通量。

Nitrogen oxides (NO_x, the sum of nitrogen dioxide (NO2) and nitric oxide (NO)) are important precursors of tropospheric ozone, and they also influence the concentration of hydroxyl and peroxyl radicals. Most of the compounds that are oxidized and removed from the air or converted to other chemical species are in direct or indirect contact with NO_x. At typical hydroxyl radical levels, the life time of NO_x depends on the season and the photochemical reaction rate, which is typically a few hours. According to the IPCC sixth assessment report, the emissions of NO_x result in net-positive warming from the formation of short-term ozone (warming) and the destruction of ambient methane (cooling). Additionally, NO_x contributes to acid deposition and the formation of chemical smog and aerosols. Since NO and NO₂ play a central role in atmospheric photochemical reactions, their simultaneous detection helps to understand the sources and sinks of these two gases, in addition to studying the NO_x exchange fluxes between terrestrial ecosystems and the atmosphere.

化学发光检测（NO + O₃ → NO₂ + O₂ + hν）是测量NO_x的传统方法。在通过化学发光反应（Mo + 3NO₂ → MoO₃ + 3NO）测量之前，NO₂首先需要在高温（~325°C）下转化为NO。虽然这种方法被广泛使用，但其他氧化氮化合物，如过乙酰亚硝酸酯（PAN）和硝酸（HNO₃），可能会在测量NO_x浓度时引起交叉干扰。同时，这种方法不能区分NO和NO₂。红外吸收法也可用于测量NO和NO₂。在这种方法中，通常需要通过转化器将NO₂还原为NO。由于NO和NO₂是顺磁分子，法拉第旋转光谱（FRS）可以用作实现其高度敏感和选择性检测的潜在方法。FRS通过检测气态介质在纵向磁场中引起的光偏振状态的变化，实现对物种浓度的高灵敏度检测。该方法通过测量光学色散实现气体浓度的检测，因此其动态测量范围比基于比尔-兰伯定律的吸收光谱（动态范围上限≤10%）更大。FRS的另一个重要优势是它对于抗磁性分子（如水和二氧化碳）具有较强的抗干扰能力，从而使其具有高样品特异性。

Chemiluminescence detection (NO+O₃→NO₂+O₂+hν) is the conventional method for measuring NO_x. NO₂ first needs to be converted to NO at high temperature (~325 ^◦C) before it can be measured by chemiluminescence reaction (Mo+3NO₂→MoO₃+3NO). Although this method is more widely used, other oxidized nitrogen compounds, such as peroxyacetyl nitrate (PAN) and nitric acid (HNO₃), can cause cross-interference in the measurement of NO_x concentrations. Simultaneously, this method is non-selective in discriminating between NO and NO₂. The infrared absorption method can also be used for NO and NO₂ measurements. In this method, NO₂ usually needs to be reduced to NO by the converter. As NO and NO₂ are paramagnetic molecules, Faraday rotation spectroscopy (FRS) can be used as a potential method to achieve their highly sensitive and selective detection. FRS enables highly sensitive detection of species concentrations by detecting changes in the polarization state of light induced by a gaseous medium immersed in a longitudinal magnetic field. This method realizes the detection of gas concentration by measuring optical dispersion, so it has a higher dynamic measurement range than absorption spectroscopy (dynamic range upper limit ≤10%) based on Beer-Lambert law. Another significant advantage of FRS is that it is reasonably immune to diamagnetic species (e.g., water and carbon dio_xide), which allows it to exhibit high sample specificity.

大多数这些报道的FRS传感器使用螺线管提供外部纵向磁场，从而导致能耗高和产生过多焦耳热。同时产生目标磁场所需的高电流交流电路会产生不受控制的电磁干扰（EMI），通常会降低FRS传感器的长期稳定性。此外，当前报道的FRS传感器只能在吸收池中进行单组分测量，不能满足复杂环境中同时进行多组分测量的需求。

Most of these reported FRS sensors use solenoid coils to provide an external longitudinal magnetic field, which makes them suffer from high power consumption and excessive Joule heat generation. The high-current alternating current circuit required to generate the target magnetic field produces uncontrolled electromagnetic interference (EMI), which usually deteriorates the long-term stability of the FRS sensors. In addition, the currently reported FRS sensors are only capable of single-component measurements in the absorption cell and cannot meet the demand for simultaneous multi-component measurements in complex environments.

在本研究中，提出了一种新型的低能耗FRS传感器，基于钕铁硼（NdFeB）环形磁体阵列，实现在单个吸收池中同时检测NO和NO₂。分析了同轴双波长赫里奥特池（DWHC）的环形磁体阵列的磁场分布特性。使用两台室温连续波中红外量子级联激光器（QCL），波长分别为5.33 µm（1875.81 cm⁻¹）和6.2 µm（1613.25 cm−1），同时探测DWHC内的磁光效应。通过对激光波长进行高频调制，有效抑制了1/f噪声。优化了双波长FRS NO_x传感器的性能，包括调制幅度、调制频率、样品气压和分析器偏置角。本研究提出的FRS传感器为现场可部署的微量气体检测设备提供了理想解决方案。宁波海尔欣光电科技有限公司为此研究提供了HPPD-M-B 前置放大制冷一体型碲镉汞（MCT）光电探测器，用以分别检测2个激光束。

In the present work, a novel low-power FRS sensor based on a neodymium-iron-boron (NdFeB) ring magnet array was proposed to achieve simultaneous detection of NO and NO₂ in a single absorption cell. The magnetic field distribution characteristics of a ring magnet array coaxial to a dual-wavelength Herriott cell (DWHC) were analyzed. Two room-temperature continuous wave mid-infrared quantum cascade lasers (QCL) with wavelengths of 5.33 µm (1875.81 cm⁻¹) and 6.2 µm (1613.25 cm⁻¹), respectively, were used simultaneously to probe magneto-optical effects within the DWHC. The 1/f noise was effectively suppressed by high-frequency modulation of the laser wavelength. The performance of the dual-wavelength FRS NO_x sensor was optimized with respect to modulation amplitude, modulation frequency, sample gas pressure, and analyzer offset angle. The FRS sensor proposed in this work provides a preferable solution for field deployable trace gas detection equipment. The laser detected by two TEC-cooled mid-infrared thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K).

(a) Schematic diagram of the dual mid-infrared wavelength FRS NO_x sensor based on a NdFeB ring magnet array;

(b) Optical layout of the FRS NO_x sensor.

thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K),

结论

本研究开发了一种基于NdFeB环形磁铁阵列的双中红外波长FRS传感器，用于同时检测NO₂和NO。在光学路径长度为23.7米，积分时间为100秒的条件下，NO₂和NO的检测限分别为0.58 ppb和0.95 ppb。高频激光波长调制与外部静态磁场相结合，最大程度地减小了低频噪声对FRS信号的影响。基于有限元方法分析了使用的永磁体阵列的磁场分布特性，帮助确定与其耦合的吸收池长度。采用双波长赫里奥特池放大两种不同偏振光波长与氮氧化物分子之间的相互作用，从而实现了在单个吸收池内对两种顺磁分子的高灵敏度检测。本文提出的FRS NO_x传感器在大气环境监测或生态系统NO_x通量观测等领域，具有进一步发展成为便携式、可在实地使用的仪器的巨大潜力。

Conclusion

In this work, a dual mid-infrared wavelength FRS sensor based on a NdFeB ring magnet array was developed for the simultaneous detection of NO₂ and NO. The detection limits for NO₂ and NO were 0.58 ppb and 0.95 ppb, respectively, at an optical path length of 23.7 m and an integration time of 100 s. High frequency laser wavelength modulation was combined with an external static magnetic field to minimize the effect of low frequency noise on the FRS signal. The magnetic field distribution characteristics of the used permanent magnet array were analyzed based on the finite element method, which helped to determine the length of the absorption cell coupled to it. A dual-wavelength Herriott cell was used to amplify the interaction between two different wavelengths of linearly polarized light and nitrogen oxide molecules, thus achieving highly sensitive detection of two paramagnetic molecules within a single absorption cell. The FRS NO_x sensor presented in this work shows great potential for further development into a portable, field-deployable instrument with applications in atmospheric environmental monitoring or ecosystem NO_x flu_x observation.

Reference:

Yuan Cao, Kun Liu, Ruifeng Wang, Guishi Wang, _xiaoming Gao, Weidong Chen，Dual mid-infrared wavelength Faraday rotation spectroscopy NO_x sensor based on NdFeB ring magnet array, Sensors & Actuators: B. Chemical 388 (2023) 133805

https://doi.org/10.1016/j.snb.2023.133805