Novel NO₂ Sensor Proposed Based on Static Magnetic Field Faraday Rotation Spectroscopy

Low-power FRS nitrogen dioxide sensor based on ring array permanent was proposed by a research team led by Prof. GAO Xiaoming from Hefei Institutes of Physical Science of Chinese Academy of Sciences, according to a paper published in Analytical Chemistry.

Faraday rotation spectroscopy (FRS) enables the detection of paramagnetic molecules by detecting changes in the polarization state of the linearly polarized light caused by a gaseous medium immersed in an external longitudinal magnetic field.

The advantage is that FRS is not disturbed by diamagnetic molecules such as CO₂ and H₂O, so it exhibits a high species specificity. In addition, the FRS has a very high detection sensitivity due to the use of a pair of nearly-crossed polarizers that greatly suppress the laser intensity noise.

The current FRS signal is mainly generated by modulating the Zeeman splitting of the sample absorption lines by an alternating magnetic field which is generated by a solenoid coil. The drawback is that when exciting the magneto-optical effect, this sinusoidal electromagnetic field suffers from high power consumption, generation of large amounts of Joule heat, electromagnetic interference and other defects.

To solve these problems, the research team proposed a static magnetic field FRS sensing device based on rare-earth permanent magnets.

Based on the magnetic field distribution characteristics of neodymium-iron-boron (NdFeB) permanent magnet rings, 14 identical NdFeB magnet rings were combined in a non-equidistant form, resulting in a static magnetic field with an average magnetic field strength of 346 Gauss over a length of 380 mm. By fitting the Herriott cell coaxially to the permanent magnet array, the interaction between the linearly polarized light and the sample was thus greatly enhanced.

A Q-branch spectral feature in the ν3 fundamental band of NO₂ at 1613.25 cm^-1 was probed using a mid-infrared quantum cascade laser. The NO₂ detection limit of 0.4 ppb was achieved at an optical length of 23.7 m.

"We expect it to be developed into a robust field-deployable environment monitoring system," said CAO Yuan, first author of the paper.

Reference: Cao Y, Liu K, Wang R, et al. NO2 sensor based on faraday rotation spectroscopy using ring array permanent magnets. Anal Chem. 2023;95(2):1680-1685. doi: 10.1021/acs.analchem.2c04821

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