In Situ Water Quantification in Natural Deep Eutectic Solvents Using Portable Raman Spectroscopy

Raman spectroscopy is a cost-effective, label free and non-destructive analytical tool [1] that delivers specific molecular information to enable the determination of the chemical composition of samples within seconds, with minimum or no requirements for sample preparation. The technique is based on the illumination of samples with a monochromatic laser source, resulting in a fraction of the light being inelastically scattered (the Raman effect) which can be collected as spectral signatures [2]. Raman spectroscopy is considered as an approach well suited for the monitoring and control of chemical and pharmaceutical processes [3,4], end point prediction of chemical synthesis reactions [5] or monitoring of polymorphic transformation in crystallisation processes [6,7]. In the microscopic mode, Raman spectroscopy can access molecular information at the micron level [8,9], and notable applications have been extensively covered in the literature, e.g., the mapping of biological tissues and cells for diagnosis or drug interactions [10–15]. Raman systems designed for research are generally equipped with multiple laser sources and offer a multitude of customizable parameters to optimise the data collection, such as adjustable pinhole, high magnification objectives and interchangeable gratings for different spectral resolutions