Laser-based mid-IR photothermal spectroscopy of liquids: a new avenue for in-line sensing in process analytical technology.

Access to real-time chemical and physical information is of fundamental importance in modern producing industries, as it is needed for process monitoring and process control. It also enables process optimization, meeting regulatory requirements. This need motivates new developments in process analytical technologies. Optical in-line probes have emerged as powerful tools for non-invasive monitoring using a range of different spectroscopic techniques. In this regard, mid-infrared spectroscopy is of special interest as it can be used to retrieve both qualitative and quantitative information in a non-destructive and label-free manner. Recently, photothermal methods were also developed in the mid-infrared range, providing a high sensitivity and minimal sample preparation, making them ideal for detecting molecular and structural properties of gases, liquids, and in imaging applications. This study explores the application of reflection-based photothermal beam deflection (PTD) and photothermal mirror (PTM) spectroscopy in comparison with established fiber-optic-based attenuated total reflection spectroscopy (ATR) for real-time analysis of solutes in the mid-infrared range. Both techniques use the same ZnS window, incorporated in a flow cell for experimental simplicity and acting as the sensing interface. Furthermore, the presented PTM and PTD techniques also use the same excitation and probe lasers for ease of comparison. The results demonstrate the effectiveness of these techniques in detecting different concentrations of caffeine in chloroform with similar detection limits to previously presented approaches as well as a state-of-the-art commercial fiber-optic-based ATR process spectrometer. The investigated photothermal techniques hold promise for incorporation in a compact probe design void of any mid-IR fibers. This will pave the way for a new generation of rugged, sensitive, and long-term stable mid-IR in-line probes for use in demanding process analytical technology (PAT) applications.