The general molecular properties and in particular, the molar mass of lignin are of central importance for industrial applications, as these data govern important thermal and mechanical characteristics. The focus of the present paper is pulsed field gradient-nuclear magnetic resonance (PFG-NMR), which is suitable for determination of lignins’ weight-average molar mass, based on diffusion constants. The method is calibrated by lignin fractions characterized by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). It could be demonstrated on a set of softwood kraft lignins that the PFG-NMR approach gives results in very good agreement with those obtained using conventional size exclusion chromatography (SEC).
Benchtop nuclear magnetic resonance (NMR) spectroscopy is an emerging field with an appealing profile for industrial applications. The instrumentation offers the possibility to measure NMR spectra in situations where high-field NMR spectroscopy is considered too expensive or complicated. In this study, we investigated the scope and limitations of 1H NMR measurements on kraft lignins and black liquors at low magnetic field strengths (1.0 and 1.5 T). The ability to quantify different classes of compounds was investigated and found to be promising. NMR-based diffusion measurements were performed, with the aim of gaining insight into the molar mass of the lignins at hand. These measurements were fast, repeatable and in good agreement with established methods.