The next generation of mid-IR laser-based refractive index (dispersion) spectroscopy of liquid-phase analytes

Abstract

Mid-infrared dispersion spectroscopy is a novel alternative approach to classical absorption spectroscopy for qualitative and quantitative analysis of liquid-phase samples focused on broadband refractive index variation sensing originating from IR absorption. We present the redesigned and improved version of an external cavity-quantum cascade laser-based MachZehnder interferometer setup dedicated for refractive index sensing of liquids, which outperforms classic absorption spectroscopy. The refined version of the setup features greater compactness, a new dual-channel transmission cell and a hysteresis-free piezo-actuator for phase locked interferometric detection. Moreover, a new routine for fast and almost simultaneous acquisition of real and imaginary part of the complex refractive index (i.e., dispersion and absorption spectra) was introduced for mutual validation of the spectra. Dispersion spectra at sample temperatures ranging from 15 to 90°C can be recorded as the setup shows a stable noise-floor over that temperature range. Introduction of a hysteresis-free piezoactuator to the system enabled fast spectral acquisition at constant sensitivity with speed rates of 100 cm-1 /s, long-term stability and allowed to improve the reproducibility, robustness, and limits of detection of the method. We compare the performance of the refined setup with the previously demonstrated version by comparing the figures of merit for univariate glucose detection. In this context, the dispersion and absorption spectra of glucose were acquired and assessed. The achieved limit of detection for dispersion sensing was 5 times lower when compared to previous version and ~2 times lower than for classic absorption sensing at 5 times shorter spectra acquisition times. In summary, the improvements in the instrumentation for dispersion spectroscopy have improved the sensitivity, reliability, and quality of the method. The achieved results set a basis for further extension of the range of application presented for this technique.