Dual-Beam Photothermal Spectroscopy Employing a Mach–Zehnder Interferometer and an External Cavity Quantum Cascade Laser for Detection of Water Traces in Organic Solvents

Abstract

We report on a mid-infrared (mid-IR) photothermal spectrometer for liquid-phase samples for the detection of water in organic solvents, such as ethanol or chloroform, and in complex mixtures, such as jet fuel. The spectrometer is based on a Mach−Zehnder interferometer (MZI) employing a He-Ne laser, a mini-flow cell with two embedded channels placed in the interferometer's arms, and a tunable external cavity quantum cascade laser (EC-QCL) for selective analyte excitation in a collinear arrangement. In this study, the bending vibration of water in the spectral range 1565−1725 cm −1 is targeted. The interferometer is locked to its quadrature point (QP) for most stable and automated operation. It provides a linear response with respect to the water content in the studied solvents and photothermal analyte spectra, which are in good agreement with FTIR absorbance spectra. The method is calibrated and validated against coulometric Karl Fischer (KF) titration, showing comparable performance and sensitivity. Limits of detection (LODs) for water detection in the single-digit ppm range were obtained for chloroform and jet fuel due to their low background absorption, whereas lower sensitivity has been observed for water detection in ethanol due to pronounced background absorption from the solvent. In contrast to KF titration, which requires toxic reagents and produces waste, the developed method works reagent-free. It can be applied in an online format in the chemical industry as well as for fuel quality control, being industrial applications where traces of water need to be accurately determined, preferably in real-time. It thus holds great promise as a green alternative to the offline KF titration method, which is the current standard method for this application.

Mid-Infrared Photothermal Spectroscopy for the Detection of Caffeine in Beverages

“…The spectrometer was first calibrated with the standard solutions, and then the commercial drink samples were measured. For the optimum SNR previously determined in our work [ 29 ], 30 scans were averaged per spectrum, resulting in a total acquisition time of 90 s per sample.…”

“…The schematic and operational principles of our experimental photothermal spectrometer were described in detail in our previous works [ 29 , 30 ]. Nevertheless, a concise overview of the setup is hereby presented to aid the reader in understanding our detection and readout scheme.…”

“…We recently demonstrated a setup employing a tunable QCL and a Mach–Zehnder Interferometer (MZI) for detecting mid-IR absorption in liquid-phase samples. Our previous work demonstrated the setup’s applicability to trace-level water detection in organic solvents and aircraft jet fuel [ 29 ]. In this paper, our goal is to demonstrate the effectiveness of this technique for the measurement of the caffeine content in commonly consumed beverages, such as coffee, tea, and energy drinks.…”

Beyond Karl Fischer titration: a monolithic quantum cascade sensor for monitoring residual water concentration in solvents

“…It is usable in a wide variety of matrices including organic solvents, oils, petrochemical and pharmaceutical products and jet-fuel. [1][2][3][4][5] This method is based on the reaction of residual water with sulfur dioxide and iodine in an alcohol solution containing a base. Quantification in KF titration is typically achieved through coulometric or volumetric techniques.…”

“…These complications, together with the need for typically toxic and expensive consumables, result in offline analysis only, where the time-demanding sampling and the risk of sample contamination during handling, can outweigh the good sensitivity and selectivity of the KF technique. 5,7,8 In comparison, alternative (chemical) analysis methods 9 have advantages for selective criteria like no need for reagents (e.g. electrochemical methods, 10 visual inspection method, drying to constant weight or distillation), the possibility for online monitoring (e.g.…”