(Sub-)millimeter spectroscopy instrument for the soil metabolite detection
Résumé
We present a spectrometer, working in the (sub-)millimeter frequency range for the detection of volatile molecules. Our main objective is to introduce a new analytical detection method dedicated to the soil metabolism. The photomixing frequency synthesizer emits continuous waves with frequencies ranging from 25 to 742 GHz. Thanks to laser frequency stabilizations onto an optical cavity, we achieve 40 kHz linewidth of the synthesized frequency. A thermal stabilization of the cavity ensures frequency repeatability with a relative frequency precision of 2·10−6. With an original method of phase difference reduction, we achieve coherent homodyne detection independent of synthesized frequencies. The instrument performance is illustrated by the detection of rotational transitions of acetonitrile vapor in the 164.5 – 166.5 GHz range at partial pressures around 1 mbar. We establish a new instrument sensitivity analysis to define the detection limit from the detector noise characteristics. This work paves the way for the detection of rotational spectroscopic signatures of new environmental molecules, which will help the completion of the existing microbial volatile organic compound database, mVOC 4.0.
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