Analyzing an NO2 Sensor Based on Faraday Rotation Spectroscopy

According to a recent study, a low-power faraday rotation spectroscopy (FRS) nitrogen dioxide (NO2) sensor based on a ring array was discoevred by scientists headed by Professor Xiaoming Gao from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences (CAS).

Schematic diagram of static magnetic field FRS NO2 sensor. Image Credit: by CAO Yuan.

The study has been reported in the Analytical Chemistry journal.

FRS allows the detection of paramagnetic molecules by detecting variations in the polarization state of linearly polarized light as a result of a gaseous medium immersed in an external longitudinal magnetic field.

It has not been disturbed by diamagnetic molecules like H2O and CO2, so it displays a high species specificity. It also has very high detection sensitivity as a result of the use of a pair of nearly-crossed polarizers that hugely repress laser intensity noise.

The present FRS signal is primarily produced by the Zeeman splitting of the sample absorption lines controlled by an alternating magnetic field produced by a solenoid coil. But when exciting the magneto-optical effect, this sinusoidal electromagnetic field provides high power consumption, production of huge amounts of electromagnetic interference, Joule heat, etc.

For such problems to be resolved, the scientists suggested a static magnetic field FRS sensing device based on rare-earth permanent magnets.

As per the magnetic field distribution characteristics of neodymium-iron-boron (NdFeB) permanent magnet rings, the researchers integrated 14 identical NdFeb permanent magnet rings in the form of non-equidistance and obtained a static magnetic field with an average magnetic field intensity of 346 gausses over a length of 380 mm.

The interaction happening between linearly polarized light and the sample was considerably improved by fitting the Herriott cell in a coaxial to the permanent magnet array.

A Q-branch spectral feature present in the ν3 fundamental band of NO2 at 1,613.25 cm-1 was probed by a mid-infrared quantum cascade laser. The NO2 detection limit of 0.4 ppb was obtained at an optical length of 23.7 m.

We expect it to be developed into a robust field-deployable environment monitoring system.

Yuan Cao, Study First Author, Hefei Institutes of Physical Science, Chinese Academy of Sciences

This study was financially supported by the Research and Development Project of Scientific Instruments and Equipment of CAS, the National Natural Science Foundation of China, the Open Project of Advanced Laser Technology Laboratory of Anhui Province, etc.

Journal Reference:

Cao, Y., et al. (2022) NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets. Analytical Chemistry.

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