Mid-Infrared Dispersion Spectroscopy – A New Avenue for Liquid Phase Analysis (IR-12.1)

Abstract

Mid-infrared (mid-IR) dispersion spectroscopy is an attractive, novel approach to liquid phase analysis that overcomes the limitations encountered in conventional mid-IR absorption spectroscopy. The technique detects phase shifts inherent with IR absorption, rather than measuring changes in intensity. It delivers quantitative and qualitative information about the sample equivalent to absorption spectroscopy while offering numerous advantages: immunity to source intensity fluctuations, constant sensitivity, baseline-free detection, extension of optimum pathlengths, and high dynamic range beyond the capabilities of the Beer-Lambert’s law. [1] In this work, we discuss the theoretical principles of the technique and experimentally demonstrate the advantages of dispersion spectroscopy over conventional absorption spectroscopy. Moreover, we present the latest configuration of the developed spectroscopic instrument for dispersion sensing in liquids. In brief, it consists of a Mach-Zehnder interferometer illuminated by a tunable quantum cascade laser. The sample is introduced to an instrument via a dual-channel transmission flow cell, placed between the interferometric arms, which is filled with a reference solution (solvent) and a sample solution (solvent + analyte) for the measurement. IR absorption in the sample solution introduces phase shifts between the interferometric arms proportional to the sample’s refractive index allowing the dispersion spectrum to be recorded and analyzed. Our example applications demonstrate the power of our technique and the developed setup for analysis of various analytes (i.e., proteins, carbohydrates), complex mixtures, and chemical reaction monitoring. [2-3] In summary, the presented work illuminates the potential of dispersion spectroscopy as an upcoming robust and sensitive way of recording IR spectra of liquid samples that can be harnessed to construct miniaturized, more reliable, and accessible sensors