# Terahertz absorption spectroscopy instrument for the detection of VOCs emitted by soil microorganisms

Daria Gudz, Françoise Binet, Valérie Gouesbet, Ludovic Frein, Cyril Hamel,

Anthony Carré, François Bondu

► To cite this version:

Daria Gudz, Françoise Binet, Valérie Gouesbet, Ludovic Frein, Cyril Hamel, et al.. Terahertz absorp-
tion spectroscopy instrument for the detection of VOCs emitted by soil microorganisms. OPTIQUE
Normandie 2024, Jul 2024, Rouen (France), France. hal-04674524

HAL Id: hal-04674524

https://hal.science/hal-04674524v1

Submitted on 21 Aug 2024

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# Terahertz absorption spectroscopy instrument for the detection of VOCs emitted by soil microorganisms

Daria Gudz<fnref n="1" />, Françoise Binet<fnref n="2" />, Valérie Gouesbet<fnref n="2" />, Ludovic Frein<fnref n="1" />,
Cyril Hamel<fnref n="1" />, Anthony Carré<fnref n="1" />, François Bondu<fnref n="1" />

## Scientific context

<figure><img src="image_1.png" /></figure>

Soil microorganisms release microbial volatile organic compounds (mVOCs) as intermediate or end products of their metabolism, that are also known as **metabolites**. Studying mVOCs emitted by soil into the air is thus a relevant way to assess its fertility and health. Because most of mVOCs are polar, **rotational spectroscopy** is a promising tool to fill the gaps in the analytical analysis of microbial activity.

## Rotational spectroscopy

<figure><img src="image_2.png" /></figure>

https://commons.wikimedia.org

The rigid rotor solutions of the Schrodinger equation

$$ \hat{H}Y_J^m(\theta, \phi) = E_J Y_J^m(\theta, \phi) $$

$$ E_J = \frac{\hbar^2}{2I} J(J+1) \equiv BJ(J+1) $$

where $B = \hbar/2I$ is defined as the rotational constant for a given system. It is related to the moment of inertia $I$ of a molecule.

<figure><img src="image_3.png" /></figure>

Can the rotational spectroscopy line up with other analytical techniques for detecting and analyzing mVOCs?

## Rotational spectroscopy for mVOC detection

<table><thead><tr><th>Rotational spectroscopy</th><th>Proton-transfer-reaction mass spectrometry (PTR-MS)</th></tr></thead><tbody><tr><td>✓ no mass limit</td><td>✓ In situ</td></tr><tr><td>✓ frequency resolution 10<sup>2</sup> kHz</td><td>✓ 21-151 amu</td></tr><tr><td>✓ molecular integrity</td><td>✓ fast scan: 15 min</td></tr><tr><td>✓ non-ionizing radiation</td><td>✓ transportable</td></tr><tr><td>X slow scan: several hours long</td><td>X fragmentation</td></tr><tr><td>X Ex situ</td><td>X metabolites > 151 amu</td></tr><tr><td>X polar molecules</td><td>X proton affinity > water</td></tr><tr><td>✓ - advantage</td><td>X - limitation</td></tr></tbody></table>

Rotational spectroscopy can be used to detect heavy polar metabolites

<figure><img src="image_4.png" /></figure>

https://doi.org/10.1111/pce.12340ia.org

## Detection system design

<figure><img src="image_5.png" /></figure>

## Detection system performance

Spectral density of the demodulated signal at 100 GHz
Lock-in filter time constant is 1s

<figure><img src="image_6.png" /></figure>

### Vacuum system installation

Ultimate vacuum level:
6 * 10⁻³ mbar
Water outgassing rate:
9.8 * 10⁻³ mbar/h

<figure><img src="image_7.png" /></figure>

Dynamic range of the demodulated signal
Lock-in filter time constant is 1s

<figure><img src="image_8.png" /></figure>

## Frequency verification

<figure><img src="image_9.png" /></figure>

Delay - difference in the optical propagation path between transmission and reception

$$ \Delta L \sim \lambda $$

<table><thead><tr><th>Set frequency</th><th>Estimated frequency</th></tr></thead><tbody><tr><td>100 GHz</td><td>102 ± 3 GHz</td></tr><tr><td>150 GHz</td><td>154 ± 6 GHz</td></tr><tr><td>220 GHz</td><td>227 ± 9 GHz</td></tr></tbody></table>

The uncertainty will be reduced by adding measurements over a larger delay interval. Frequency calibration will be achieved by measuring the absorption spectrum of a known molecule.

## Future prospects

* Study the detection limit of the system using acetonitrile ($C_2H_3N$) as a model for soil mVOCs.

* Demonstrate heavy polar metabolites detection:
  Camphor $C_{10}H_{16}O$ (152 amu)
  Benzophenone $C_{13}H_{10}O$ (182 amu)
  Methyl salicylate $C_8H_8O_3$ (152 amu)

* Define a procedure for collecting mVOCs from a soil sample

<footnote n="1">¹Institut FOTON - UMR CNRS 6082, Univ Rennes, CNRS, Rennes, France</footnote>

<footnote n="2">²ECOBIO - UMR CNRS 6553 Univ Rennes, CNRS, Rennes, France</footnote>