Mid-IR Dispersion Spectroscopy – A Powerful Tool for Liquid-Phase Chemical Analysis

Abstract

Mid-infrared (mid-IR) dispersion spectroscopy is an attractive, novel approach to liquid phase analysis that overcomes the limitations of conventional mid-IR absorption spectroscopy. This technique detects inherent refractive index fluctuations (phase shifts) caused by IR absorption, rather than measuring changes in intensity. It delivers quantitative and qualitative information about the sample equivalent to absorption spectroscopy with the advantages of immunity to source intensity fluctuations, constant sensitivity, baseline-free detection, 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 custom-made dual-channel transmission flow cell, placed between the interferometric arms, which is filled with a reference solution (solvent) and a sample solution (solvent + analyte) prior analysis. 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, which can be of interest for many applications across various industrial areas.