Advancing Inline Sensing: Mid‑Infrared Photothermal Spectroscopy for Industrial Liquids

Real-time monitoring of industrial liquids is becoming increasingly important in chemical, pharmaceutical, and food production. Accurate, non-invasive sensing allows manufacturers to maintain consistent product quality, optimize processes, and meet regulatory and sustainability requirements. Traditional inline spectroscopy methods, like Raman or ATR‑FTIR, have been widely used but face limitations with certain compounds, turbid liquids, or highly absorbing media. New research introduced mid-infrared photothermal spectroscopy as a new approach for sensitive, in-line monitoring of complex liquids directly within production streams.

This technique combines mid-IR excitation with photothermal detection, capturing thermal responses rather than direct absorption. By measuring subtle heat-induced optical effects, it allows precise monitoring without interfering with the process or requiring extensive sample preparation. This development provides manufacturers with real-time insights that improve process control and efficiency while supporting environmental objectives.

Challenges in Inline Liquid Monitoring

Achieving reliable inline sensing in industrial processes requires overcoming several obstacles. Process liquids often vary in chemical composition and optical properties, production environments can be harsh, and conventional probes may lack sensitivity or struggle with scattering and absorption. Our work addressed these challenges by designing a photothermal spectroscopy system capable of detecting thermal effects induced by mid-IR absorption, enabling sensitive, adaptable monitoring even in complex liquids.

At the same time, the method brings clear benefits for industrial implementation:

• Sensitive, non-invasive monitoring: Detects chemical and physical properties without interfering with the process.

• Improved process reliability: Provides accurate, real-time data to reduce deviations and off-spec production.

• Sustainability advantages: Supports more efficient material use and reduced waste, contributing to EU climate-neutrality targets.

By integrating these capabilities, the system provides actionable insights, helping manufacturers maintain high-quality output and optimize production in real time.

How Photothermal Spectroscopy Works

The system uses a laser to excite molecular vibrations in the liquid sample. Absorption of mid-IR light generates local heat, which causes thermoelastic effects and refractive index changes. These effects are measured using a visible probe beam through techniques such as photothermal mirror (PTM) and photothermal deflection (PTD). PTM tracks surface displacement caused by local heating, while PTD detects beam deflection due to thermal gradients. Together, these approaches allow indirect, highly sensitive detection of molecular changes that would be difficult to capture with traditional transmission measurements.

This sensing approach makes mid-IR photothermal spectroscopy especially valuable in industrial settings where liquids may be turbid, highly absorbing, or flowing at high speeds, enabling continuous, non-destructive monitoring.

Industrial Relevance and Benefits

By providing accurate real-time data on liquid composition and process conditions, our work enables manufacturers to:

• Reduce off-spec production and material waste

• Maintain consistent product quality

• Optimize energy and resource use

These improvements are particularly relevant in chemical, pharmaceutical, and food manufacturing, where precise control over liquid composition directly affects product performance, efficiency, and sustainability.

Project Partners

The work was carried out by Institute of Chemical Technologies and Analytics, TU Wien ↗, Department of Physics, Universidade Estadual de Maringá ↗, and Department of Physics, Universidade Tecnológica Federal Do Paraná ↗, and Competence Center CHASE GmbH ↗.

The paper was published in Analytical and Bioanalytical Chemistry, 2025, the authors are Dominik Kau‑Wacht, Nelson G. C. Astrath, Gustavo V. B. Lukasievicz, Leopold Lindenbauer, Alicja Dabrowska, Karin Wieland, Bernhard Lendl: Laser-based mid-IT photothermal spectroscopy of liquids: a new avenue for in-line sensing in process analytical technology - CHASE PDF ↗