Dynamic Fabry–Perot microcavity interferometer for VOC–water adulteration detection

Abstract Monitoring chemical adulteration in liquids is essential across many applications, as even slight variations in adulterants can influence chemical behavior, toxicity, and biological compatibility. The present study examines whether volatile organic compound (VOC) water adulteration can be detected by tracking the evolution of microdroplets formed at the tip of a flat optical fiber functioning as a Fabry–Perot interferometer. Here, a visible-range optical fiber interrogator setup that includes a spectrometer records the Fabry–Perot interference reflection spectra generated at the microdroplets’ refractive-index interfaces. By analyzing the dynamics of a pure water microdroplet, it was possible to link spectral interference to droplet geometry and composition. This relation was achieved by combining a Fourier-domain analysis with a physical evaporation model, which allowed estimation of the droplet height (even below 5 µm), evaporation dynamics, and liquid flow rates of approximately 3.634 pL s −1 . Moreover, when mixtures of VOCs (acetone, isopropyl alcohol, and methanol) and water were analyzed, the technique provides adulteration detection sensitivities within the 0%–25% v/v range of 76.5 pm/%water for acetone, 55.4 pm/%water for isopropyl alcohol, and 23.8 pm/%water for methanol. The experimental results were statistically supported by analysis of variance analyses, which confirm the method’s reproducibility and reliability, achieving in all cases extremely low Prob > F. Considering other VOC-water adulteration detection methods that require bulky instrumentation, chemical functionalization, or complex fabrication steps, it is essential to stress that the proposed method in this work provides VOC–water adulteration detection without requiring functional layers or dedicated fabrication processes, thereby enabling low-cost, real-time monitoring.