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. Ideally, a solvent’s transmission should be around 1/e, correlating to an optimal absorbance of ~0.4 (or log10e). However, when dealing with strong absorbing matrix like water, the varied background absorbance across different wavenumber regions proves to be a challenge. Proteins, characterized by their unique 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 overlapping bending vibration of water. Our research introduces a system that measures samples at multiple pathlengths simultaneously 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 guided through the wedge-shaped transmission cell with pathlengths ranging from ~10 µm to 400 µm. A pyroelectric linear array, consisting of 256 elements, detects then the transmitted radiation. We aim to employ caffeine in water as a model sample system to illustrate the variability in optimal pathlengths depending on the absorbance of the solvent. The hereby proposed configuration combines the advantages of QCLs over conventional thermal emitters while also providing the capability to cover a wider dynamic range of absorbance values. Consequently, it is possible to increase the signal-to-noise ratio for measurements of proteins in aqueous solutions and provide greater insight into the investigated sample.