QCL-based Multi-Pathlength Mid-Infrared Transmission Spectroscopy for Enhanced Analytical Capabilities

Abstract

Mid-infrared spectroscopy, with its fixed transmission pathlength design, has long been a well-established method for the analysis of proteins and polypeptides in solution. Research shows that the optimum transmission of a solvent should be approximately 1/𝑒, resulting in an optimal absorbance of ~0.4 (or log10𝑒). [1] Achieving this value for a strong absorbing matrix proves challenging due to the varied background absorbance across different wavelengths. Proteins with their characteristic amide I (1700 – 1600 cm-1) and amide II (1600 – 1500 cm-1) bands are especially difficult to distinguish with spectroscopic analysis in aqueous solutions due to the strong water absorption in this region. In this work, we demonstrate a system that measures samples simultaneously at different pathlengths to determine the most optimal pathlength for each characteristic band of the solute. To achieve this, we employ an external-cavity quantum cascade laser (EC-QCL) that serves as a powerful source of IR radiation. The laser beam is subsequently expanded to a line and directed through the wedge-shaped transmission cell hosting pathlengths from ~10 μm to 400 μm. The transmitted radiation is then detected by a pyroelectric linear array with 256 elements. The hereby proposed configuration combines the advantages of quantum cascade lasers over conventional thermal emitters while also providing the capability to cover a wider dynamic range of absorbance values. We anticipate our setup to increase the signal-to-noise ratio for measurements of proteins in aqueous solutions and provide greater insight into sample and measurement results.

References: [1] P. R. Griffiths, J. A. de Haseth, Fourier Transform Infrared Spectroscopy 2007, p. 256.