External Cavity Quantum Cascade Laser based Mid-IR Dispersion Spectroscopy for Qualitative and Quantitative Analysis of Liquid-Phase Samples

Abstract

Acquisition of classical absorption spectra of liquids in the mid-IR range with quantum cascade lasers (QCLs) is often limited in sensitivity by noise from the laser source. Alternatively, measure-ment of molecular dispersion (i.e. refractive index) spectra poses an experimental approach that is immune to intensity fluctuations and further offers a direct relationship between the recorded signal and the sample concentration. In this work, we present an EC-QCL based Mach-Zehnder interfer-ometer setup to determine dispersion spectra of liquid samples. We present two approaches for ac-quisition of refractive index spectra and compare the qualitative experimental results. Furthermore, the performance for quantitative analysis is evaluated. Finally, multivariate analysis of a spectrally complex mixture comprising three different sugars is performed. The obtained figures of merit by partial least squares (PLS) regression modelling compare well with standard absorption spectros-copy, demonstrating the potential of the introduced dispersion spectroscopic method for quantita-tive chemical analysis.