Mid-IR dispersion spectroscopy - A new avenue for liquid phase analysis.

Mid-IR dispersion spectroscopy is an attractive, novel approach to liquid phase analysis that extends the possibilities of traditional methods based on the detection of absorption via intensity attenuation. This technique detects inherent refractive index changes (phase shifts) induced by IR light interaction with absorbing matter. In contrast to classic absorption spectroscopy, it provides extended dynamic range, baseline-free detection, constant sensitivity, and inherent immunity to power fluctuation. In this paper, we provide a detailed experimental and theoretical characterization and verification of this method with special focus on broadband liquid sample analysis. For this purpose, we develop a compact benchtop dispersion spectroscopy setup based on an EC-QCL coupled to a Mach-Zehnder interferometer. Phase-locked interferometric detection enables to fully harness the advantages of the technique. By instrument operation in the quadrature point combined with balanced detection, the full immunity towards laser power fluctuations and the environmental noise can be achieved. On the example of ethanol (0.5-50% v/v) dissolved in water, it is experimentally demonstrated that changes of the refractive index function are linearly related to concentration also for strongly absorbing, highly concentrated samples beyond the validity of the Beer-Lambert law. Characterization of the sensitivity and noise behavior indicates that the optimum applicable pathlength for liquid analysis can be extended beyond the ones for absorption spectroscopy. Experimental demonstration of the advantages over classical absorption spectroscopy illuminates the potential of dispersion spectroscopy as upcoming robust and sensitive way of recording IR spectra of liquid samples.