Fractionation and Absolute Molecular Weight Determination of Organosolv Lignin and Its Fractions: Analysis by a Novel Acetone- Based SEC-MALS Method

Accurate and reliable structural characterization of technical lignins is still challenging and inhibits industrial utilization as only poor understanding of structure-property relationship is available. Especially, molar mass analysis of technical lignins is of paramount interest; however, the usage of conventional size exclusion chromatography (SEC) and synthetic polymer standards as a consequence of inaccessible lignin standards is predominantly found in academia. This leads to a huge discrepancy in molecular weight analyses of lignins between different laboratories and consequently to fragile comparability. In this study, organosolv (OS) lignin from beech wood was exemplarily used to develop and evaluate a new acetone-water-based SEC method with the ability to be coupled to an electrospray ionization-mass spectrometer (ESI-MS) or to a multi-angle light scattering detector (MALLS or short MALS). The eluent system used shows very good solubility for the lignins investigated and hence lowers the probability for molecular aggregation of lignin molecules in solution. Solvent fractionation in acetone-water mixtures was conducted to acquire molecular weight classes. The starting lignin (parent lignin) was initially dissolved in 80 wt % acetone-water. Various fractions of the parent lignin were produced by a stepwise reduction of the acetone content. Molecular weights based on narrow polyethylene glycol (PEG)/polyethylene oxide (PEO) standards and absolute molar masses by coupling SEC with MALS were obtained, and the drawback of using polymer standards is discussed in detail. At a lower acetone content, low-molecular-weight fractions were found. Additionally, the specific refractive index increments (dn/dc) were determined for the parent lignin and its fractions. The impact of dn/dc on the final molecular weight (MW) was evaluated considering the chemical composition obtained by nuclear magnetic resonance spectroscopy (NMR) analysis. Furthermore, light scattering revealed that the absorption behavior for this OS lignin is low and neglectable. This article proposes a new acetone-based analytical method for direct determination of absolute molar masses of OS lignin molecules.