Polarimetric Balanced Detection: Background-Free Mid-IR Evanescent Field Laser Spectroscopy for Low-Noise, Long-term Stable Chemical Sensing.

Stephan Freitag,Bernhard Lendl,Andreas Schwaighofer,Laura Buntzoll,Georg Ramer,Bernhard Schmauss,Matthias Baer

doi:10.1021/acssensors.0c01342

Stephan Freitag, Bernhard Lendl + Show 5 more

Open Access

https://doi.org/10.1021/acssensors.0c01342

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Journal: ACS Sensors	Publication Date: Dec 29, 2020
Citations: 17	License type: cc-by

Affiliation: TU Wien, University of Erlangen-Nuremberg

Abstract

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.

Highlights

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
In conventional ATR IR absorption spectroscopy approaches, e.g., in process monitoring, a background spectrum needs to be recorded at a certain point in time, usually prior to the sample measurements, which are recorded over a longer period of time
To the best of our knowledge, this is the first report of polarimetric-based balanced detection in combination with quantum cascade lasers (QCLs)-XT for liquid sensing using ATR IR spectroscopy

Summary

Introduction

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. The use of the mixed-polarized beam with a lower effective thickness (see eq 2) as a reference and the p-polarized beam with a higher effective thickness (see eq 3) for sampling leads to enhanced long-term signal stability because background and sample spectra are recorded simultaneously (see Figure 4C).

Results

Conclusion