S. Freitag, M. Baer, L. Buntzoll, G. Ramer, A. Schwaighofer, B. Schmauss, B. Lendl:
"Polarimetric Balanced Detection: Background Free Mid-IR Evanescent Field Laser Spectroscopy for Low Noise, Long-term Stable Chemical Sensing";
ACS Sensors, 6 (2021), S. 35 - 42.

Kurzfassung englisch:
In this work, we introduce polarimetric balanced
detection as a new attenuated total reflection (ATR) infrared (IR)
sensing scheme, leveraging unequal effective thicknesses achieved
with laser light of different polarizations. We combined a
monolithic widely tunable Vernier quantum cascade laser (QCLXT)
and a multibounce ATR IR spectroscopy setup for analysis of
liquids in a process analytical setting. Polarimetric balanced
detection enables simultaneous recording of background and
sample spectra, significantly reducing long-term drifts. The rootmean-
square noise could be improved by a factor of 10 in a longterm
experiment, compared to conventional absorbance measurements
obtained via the single-ended optical channel. The sensing
performance of the device was further evaluated by on-site measurements of ethanol in water, leading to an improved limit of
detection (LOD) achieved with polarimetric balanced detection. Sequential injection analysis was employed for automated injection
of samples into a custom-built ATR flow cell mounted above a zinc sulfide multibounce ATR element. The QCL-XT posed to be
suitable for mid-IR-based sensing in liquids due to its wide tunability. Polarimetric balanced detection proved to enhance the
robustness and long-term stability of the sensing device, along with improving the LOD by a factor of 5. This demonstrates the
potential for new polarimetric QCL-based ATR mid-IR sensing schemes for in-field measurements or process monitoring usually
prone to a multitude of interferences.

mid-infrared spectroscopy, attenuated total reflection, quantum cascade lasers, balanced detection, process analytical technology

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