Caffeine is the most widely consumed stimulant and is the subject of significant ongoing research and discussions due to its impact on human health. The industry’s need to comply with country-specific food and beverage regulations underscores the importance of monitoring caffeine levels in commercial products. In this study, we propose an alternative technique for caffeine analysis that relies on mid-infrared laser-based photothermal spectroscopy (PTS). PTS exploits the high-power output of the quantum cascade laser (QCL) sources to enhance the sensitivity of the mid-IR measurement. The laser-induced thermal gradient in the sample scales with the analytes’ absorption coefficient and concentration, thus allowing for both qualitative and quantitative assessment. We evaluated the performance of our experimental PTS spectrometer, incorporating a tunable QCL and a Mach–Zehnder interferometer, for detecting caffeine in coffee, black tea, and an energy drink. We calibrated the setup with caffeine standards (0.1–2.5 mg mL⁻¹) and we benchmarked the setup’s capabilities against gas chromatography (GC) and Fourier-transform infrared (FTIR) spectroscopy. Quantitative results aligned with GC analysis, and limits of detection matched the research-grade FTIR spectrometer, indicating an excellent performance of our custom-made instrument. This method offers an alternative to established techniques, providing a platform for fast, sensitive, and non-destructive analysis without consumables as well as with high potential for miniaturization.

Protein polarimetry has been evaluated as a simple and straightforward technique to detect the cryptic denaturation of exemplary proteins. The general rules of rotation vs. amino acid and structural composition and the respective knowledge gaps were reviewed, and the specific rotation of cystine was determined in 4 M NaCl solution as [α]_D²⁰ = –302.5°. The specific rotations at 589 nm and 436 nm and the ratio were measured for several model proteins, some purified plasma-derived proteins and for three monoclonal antibodies. The immunoglobulin G concentrates all showed a narrow ratio range likely characteristic for this protein class. Heat denaturation experiments were conducted at temperatures between 50 and 85 °C both for short-time (10 min) and for prolonged periods of heat exposure (up to 210 min). Denaturation by heat resulted not only in the known levorotatory shift, but also in a shift in the specific rotation ratio. The stabilizing effect of fatty acids in bovine serum could be demonstrated by this parameter. Polarimetry thus appears to be a particularly sensitive and simple method for the characterization of the identity and the thermal stability of proteins and should therefore be added again as a complimentary method to the toolbox of protein chemistry.

Mid-infrared spectroscopy is a powerful technique for probing molecular structure and composition, yet its application to aqueous systems remains limited by strong solvent absorption and the dynamic range of mid-IR detectors, which restrict the usable optical pathlength. Here, we present a quantum cascade laser-based spectroscopic setup that enables simultaneous acquisition of absorption spectra measured in transmission across a continuum of optical pathlengths using a custom-made wedge-shaped flow cell and a pyroelectric detector array. This design eliminates the need for mechanical adjustments or sample dilution, allowing direct analysis of complex liquid samples. We introduce the pathlength-to-noise ratio as a novel metric for determination of the optimal pathlength region for every wavenumber. By weighing detector signals based on this ratio, we achieve robust integration of multi-pathlength data into a single coherent mid-IR absorption spectrum. The system was validated using aqueous protein solutions of bovine serum albumin and β-lactoglobulin in the range of 1 to 25 mg mL ^-1 . Characteristic amide I and II bands were successfully resolved, and secondary structure differences were detected. This work establishes a flexible approach to mid-IR absorption spectroscopy, aiming to eliminate the limitations of detectors with a small dynamic range and to get a better understanding of the optimal pathlength for complex samples.