[Zurück]

S. Freitag, M. Baer, L. Buntzoll, G. Ramer, A. Schwaighofer, B. Schmauss, B. Lendl:
"Polarimetric Balanced Detection: Enhanced Attenuated Total Reflection Mid-IR Laser Spectroscopy for Sensing in Liquids";
Poster: SciX 2020, online; 12.10.2020 - 15.10.2020.

We present polarimetric balanced detection for long-term stable and background free evanescent field mid infrared laser spectroscopy in liquid.

Classic ATR IR absorption spectroscopy approaches e.g. in process monitoring, utilize a background spectrum usually recorded prior to sample measurements, hence introducing a temporal mismatch between background and sample spectra. This difference in time often leads to artefacts in the resulting absorbance spectra and impair successful analysis by diverse incidents occurring overtime (e.g. drifts of environmental parameters). A widely tunable monolithic Vernier quantum cascade laser (QCL-XT), a multi-bounce zinc sulfide ATR element and a thermoelectrically cooled balanced detection module consisting of two mercury cadmium telluride elements were combined for analysis of liquids. The setup was placed in the facility of a water treatment plant exposed to a process analytical setting. Fully-automated sample injection into a custom built ATR flow-cell was performed via sequential injection analysis. The device performance was evaluated by on-site measurements of ethanol in water. Polarimetric balanced detection as a new attenuated total reflection (ATR) mid infrared (IR) laser based sensing scheme, exploits unequal effective thicknesses achieved with laser light of different polarization, hence allowing for simultaneous recording of background and sample spectra. Compared to classic absorbance measurements, the root mean square noise could be improved by a factor of 10 demonstrated in a long-term experiment. In addition the noise suppression capabilities of polarimetric balanced detection lead to an improved limit of detection for ethanol in water. The QCL-XT proofed to be a useable alternative to external cavity QCLs commonly employed for liquid phase investigations. By using polarimtetric balanced detection we demonstrate the potential for new polarimetric laser-based ATR mid IR sensing schemes for process monitoring or in-field applications prone to a multitude of interference.

https://publik.tuwien.ac.at/files/publik_293107.pdf