Low-field benchtop versus high-field NMR for routine 31P analysis of lignin, a comparative study

Abstract

Over the past decade, advances in lignin characterization methods have led to a better understanding of lignin structure and reactivity. Novel chromatographic and spectrometric techniques, especially NMR, are now essential and routine for lignin analysis. Among these methods, quantitative 31P NMR spectroscopy proved itself as a powerful technique for the qualitative and quantitative analyses of diverse types of hydroxyl groups in lignin. Nevertheless, 31P NMR spectroscopy is typically accomplished using high-field NMR equipment that necessitates substantial investment and operational cost, limiting its ability to be employed as a routine tool for analysis and quality control in a production environment. In this work 31P NMR experiments were performed on several lignins, including technical, depolymerized, and modified lignins, using both an 80 MHz benchtop NMR and a 400 MHz high-field conventional NMR. Both NMR spectrometers showed similar results for both nonfunctionalized and modified lignins. This work demonstrates that low-field benchtop NMR is an excellent alternative to the conventional high-field NMR equipment to analyze the hydroxyl content of lignins.