Title: Polarimetric Balanced Detection: Background-Free Mid-IR Evanescent Field Laser Spectroscopy for Low-Noise, Long-term Stable Chemical Sensing
Authors: Freitag, Stephan  
Baer, Matthias 
Buntzoll, Laura 
Ramer, Georg  
Schwaighofer, Andreas  
Schmauss, Bernhard  
Lendl, Bernhard  
Category: Original Research Article
Issue Date: 29-Dec-2020
ISSN: 2379-3694
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 (QCL-XT) 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 root-mean-square noise could be improved by a factor of 10 in a long-term 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.
Keywords: attenuated total reflection; balanced detection; mid-infrared spectroscopy; process analytical technology; quantum cascade lasers
DOI: 10.1021/acssensors.0c01342
Organisation: E164-02-1 - Forschungsgruppe Prozessanalytik 
License: CC BY 4.0 CC BY 4.0
Publication Type: Article
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