Non-invasive monitoring by terahertz spectroscopy of pollutants

Abstract The detection and monitoring of Volatile Organic Compounds (VOCs) are critical for environmental protection and public health due to their role in air pollution and atmospheric chemistry. Traditional analytical techniques such as Gas Chromatography Mass Spectrometry (GC-MS) and Fourier Transform Infrared Spectroscopy (FTIR), while effective, present significant limitations in real-time and on-site applications due to long processing times, complex sample preparation, and susceptibility to atmospheric interferences. Terahertz (THz) spectroscopy has emerged as a promising alternative, offering non-destructive, label-free, and real-time detection of VOCs with high molecular specificity. However, despite its potential, its application in VOCs analysis remains underexplored, particularly in the gas-phase and condensed-phase (liquid and solid) environments. This thesis investigates the feasibility of THz spectroscopy for VOCs detection by employing two complementary approaches: Quantum Cascade Lasers (QCLs) and Continuous Wave Frequency-Domain Spectroscopy (CW FDS). The first part of the study focuses on the analysis of condensed-phase samples, employing Ultra-Wideband (UWB) THz-QCLs in combination with Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), for liquids compounds, and Anisotropic Terahertz Microscopy-Fourier Transform Infrared (ATM-FTIR) for solid sucrose. Investigating liquid samples, including water, isopropanol, and their mixtures, reveals distinct spectroscopic signatures related to molecular interactions in the THz range. Furthermore, in the solid-phase experiments, the sucrose monolayer exhibits polarization-dependent absorption, allowing for a deeper investigation of anisotropic optical properties in THz spectroscopy. In the second part, I participated in the studies of gas-phase VOCs detection by using the THz-FDS system, analyzing the absorption spectra of various alcohols and binary mixtures over the 0.06 to 1.2 THz range. The results demonstrate the ability of THz-FDS to resolve molecular rotational transitions with high accuracy, distinguishing between different alcohol isomers and analyzing their interactions in mixed gas-phase environments. The findings highlight the strengths of THz spectroscopy for VOCs monitoring, particularly its capability for real-time, non-invasive detection without extensive sample preparation. Despite challenges such as strong water vapor absorption and limited power of THz sources, this research demonstrates that THz spectroscopy can be effectively applied to both gas-phase and condensed-phase environmental analysis. The methodologies and results presented in this thesis contribute to the ongoing development of THz-based analytical techniques and lay the foundation for future advancements in portable, field-deployable THz detection systems for industrial and environmental monitoring.