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

Suha Elderderi,Cécile Enguehard-Gueiffier,Charlotte Leman-Loubière,Emilie Munnier,Hugh J Byrne,Laura Wils,Igor Chourpa,Abdalla A Elbashir,Franck Bonnier,Leslie Boudesocque-Delaye

doi:10.3390/molecules26185488

Abstract

Raman spectroscopy is a label-free, non-destructive, non-invasive analytical tool that provides insight into the molecular composition of samples with minimum or no sample preparation. The increased availability of commercial portable Raman devices presents a potentially easy and convenient analytical solution for day-to-day analysis in laboratories and production lines. However, their performance for highly specific and sensitive analysis applications has not been extensively evaluated. This study performs a direct comparison of such a commercially available, portable Raman system, with a research grade Raman microscope system for the analysis of water content of Natural Deep Eutectic Solvents (NADES). NADES are renewable, biodegradable and easily tunable “green” solvents, outcompeting existing organic solvents for applications in extraction from biomass, biocatalysis, and nanoparticle synthesis. Water content in NADES is, however, a critical parameter, affecting their properties, optimal use and extraction efficiency. In the present study, portable Raman spectroscopy coupled with Partial Least Squares Regression (PLSR) is investigated for rapid determination of water content in NADES samples in situ, i.e., directly in glassware. Three NADES systems, namely Betaine Glycerol (BG), Choline Chloride Glycerol (CCG) and Glucose Glycerol (GG), containing a range of water concentrations between 0% (w/w) and 28.5% (w/w), were studied. The results are directly compared with previously published studies of the same systems, using a research grade Raman microscope. PLSR results demonstrate the reliability of the analysis, surrendering R2 values above 0.99. Root Mean Square Errors Prediction (RMSEP) of 0.6805%, 0.9859% and 1.2907% w/w were found for respectively unknown CCG, BG and GG samples using the portable device compared to 0.4715%, 0.3437% and 0.7409% w/w previously obtained by analysis in quartz cuvettes with a Raman confocal microscope. Despite the relatively higher values of RMSEP observed, the comparison of the percentage of relative errors in the predicted concentration highlights that, overall, the portable device delivers accuracy below 5%. Ultimately, it has been demonstrated that portable Raman spectroscopy enables accurate quantification of water in NADES directly through glass vials without the requirement for sample withdrawal. Such compact instruments provide solvent and consumable free analysis for rapid analysis directly in laboratories and for non-expert users. Portable Raman is a promising approach for high throughput monitoring of water content in NADES that can support the development of new analytical protocols in the field of green chemistry in research and development laboratories but also in the industry as a routine quality control tool.

Highlights

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
These are absent in the spectrum of water recorded inside the vial, and the solution inside the vials can be anacan be analysed without any contribution from glass features, thanks to the 75 mm focal lysed without any contribution from glass features, thanks to the 75 mm focal length of length of the device, enabling focusing of the laser inside the solution
The Root Mean Square Errors Prediction (RMSEP) values with the Raman microscope were respectively 0.4715%, 0.3437% and 0.7409% w/w for Choline Chloride Glycerol (CCG), Betaine Glycerol (BG) and Glucose Glycerol (GG) compared to 0.6805%, 0.9859% and 1.2907% w/w, presently obtained with the portable Raman directly into glass vials

Summary

Introduction

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 analysis can be performed directly through packaging materials for identification of raw pharmaceutical materials in amber glass bottles, opaque polypropylene containers, paper, blue plastic sacking [16] and USP vials [17], enabling, for example, characterisation of artesunate antimalarial drugs through blister bags [18] or discrimination and quantification of chemotherapeutic solutions in glass bottles [19] or perfusion plastic bags [20] In this configuration, the signal is collected in situ (or non-invasively) from a voxel corresponding to a reduced volume into the sample without requirement for samples withdrawal. Technological achievements have greatly improved the transferability of Raman spectroscopy to a wider range of applications with miniaturisation of instruments [21]

Methods

Results

Discussion

Conclusion