Real-Time Mid-IR Spectroscopy Inside Reactors with a Modular Endoscopic Probe

<i>In operando</i> monitoring of chemical reactions is crucial for understanding reaction kinetics and mechanisms. Conventional attenuated total reflection (ATR)-IR probes offer real-time measurement capabilities but are limited by shallow penetration depths of the evanescent wave, restricting detection mainly to high-concentration species and surface-localized events. To overcome these limitations, we built an endoscopic Fourier-transform infrared spectroscopy (FT-IR) probe for in operando chemical analysis and direct observation of chemical reactions proceeding inside reaction vessels. An endoscopic probe constructed with hollow-core optical fibers enables transmittance measurements inside reactors and real-time acquisition of mid-IR absorption spectra from the initiation to the endpoint continuously. A key feature of this endoscopic probe is its adjustable optical path length, which allows absorbance optimization across a wide dynamic range, including under dilute conditions. The probe's performance was validated by measuring the C ═ O bond absorbance in acetone solutions across varying pathlengths and concentrations, demonstrating compliance with the Beer-Lambert law. To demonstrate its applicability for reaction monitoring, the probe was applied to monitor the Schiff base formation reaction between acetone and benzylamine in real time, capturing the transformation from C ═ O to C ═ N bonds and the appearance of the O-H stretching modes associated with water formation. Reaction time-resolved IR spectra were further analyzed using two-dimensional correlation spectroscopy (2D-COS), which revealed that water formation precedes imine stabilization, providing mechanistic insight into the condensation process. This compact and modular probe demonstrates high spectral accuracy and full compatibility with conventional FT-IR spectrometers. It offers a promising approach for real-time quantitative monitoring of chemical reactions, particularly for detecting low-concentration intermediates and elucidating reaction kinetics and mechanisms.