Size Exclusion Chromatography (SEC) of lignin poses many challenges. In numerous studies chromatograms of lignin show a bimodal molar mass distribution. Is this a true characteristic of lignin, is it caused by molecular associations or aggregations, or could it could be an artefact from using column combinations with porosities that do not match properly? To improve resolution and enable separation over a larger molar mass range, multiple columns with different porosities are often connected in series. If the porosities do not match properly, the result appears as a shoulder or bimodality in the chromatogram. To understand whether the bimodal distribution is a sample characteristic or an analyze artefact, we have used different columns, column combination and samples to see when the results is a bimodal distribution and when only one peak is formed. Results show that the bimodality of lignin can be an artifact originating from column mismatch. Using single porosity columns with a low molar mass cut-off should be avoided since it can cause false bimodality.

This paper describes improvements in the methodology for NMR spectroscopic analysis of dissolved kraft lignins and black liquors, for structural analysis as well as for reaction monitoring. The described methodologies are variations and applications of non-uniform sampling (NUS) in 2D spectra and diffusion ordered spectroscopy (DOSY), with reduced experiment time and/or increased resolution and novel ways to track reactions through monitoring of diffusion of a reactant mixture.

The use of NMR spectroscopy at high magnetic fields is a common tool in the analysis of lignin samples. In the presented study, NMR measurements on a group of softwood and hardwood kraft lignins at low field (1.0 T) with a benchtop NMR spectrometer, containing a permanent magnet, are investigated and evaluated. NMR based diffusion measurements were performed, for which the results were found to be fast, repeatable, and in good agreement numbers to measurements at high field. Measurements were also performed on a sample in alkaline solution, as a model for black liquor analysis, with promising initial results.

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).

RISE Bioeconomy, FPInnovations and VTT Technical Research Centre of Finland Ltd are working on a draft for an official ISO method for the determination of lignin molar mass. The draft will include two methods that can be used by different laboratories as reference methods, which are based on organic and aqueous size exclusion chromatography (SEC). Four samples of kraft lignin of different origin were analysed by three laboratories using the current in-house methods as well as a harmonised analytical setup. It was clear that the results differed largely between the different methods, for example, the weight average molar mass (Mw) for the LignoForce softwood lignin (LF-SW) ranged from 3,300 t0 11,100g/mol using the different methods. Harmonisation of aqueous SEC was successful. The method comprises SEC using 0l.1M sodium hydroxide as the mobile phase, a mobile phase flow rate of 0.5mL/min, Waters columns MCX 1,000A and 100,000A in series and calibration/calculation using polystyrene sulphonate standards using a linear fit. However, organic SEC was more complex and would require further investigations. Although the aqueous SEC analysis was repeatable, it was concluded that a strategy for data evaluation should be developed in order to achieve a method applicable for quality assessments.

Knowledge of the molecular properties, e.g. the molar mass, of lignin are of central importance for industrial applications, since thermal and mechanical characteristics are governed by these. Here, a method for absolute determination of weight-average molar mass of kraft lignin, based on diffusion constants gained from PFG-NMR calibrated with lignin fractions characterized by MALDI-TOF-MS, is presented. The NMR method is compared to polystyrene calibration, and size exclusion chromatography with tetrahydrofuran as mobile phase.

The lack of official standardised methods for lignin has meant that the results from analysis of lignin properties are largely dependent on which method or laboratory that is used. Therefore, relevant and harmonised analytical methods are essential to allow producers of biorefinery products to prepare specifications for their products and to improve the possibilities for systematic development of process and product. The current status of the most important methods for lignin characterisation has been evaluated together with method development or method adaption in order to meet the requirements for high-value lignin products.

Heat treatment is a promising pre-processing method in the conversion of kraft lignins to carbon fibers. In this study, a kraft lignin was subjected to heat treatment at different lengths of time and at different temperatures. The molecular structure of the untreated and four heat treated lignins were investigated mainly by a variety of nuclear magnetic resonance spectroscopy (NMR) methods. The results were compared to structural information obtained by pyrolysis-gas chromatography-/mass spectrometry (py-GC/MS) and Fourier transform infrared spectroscopy (FTIR). It was observed that during the thermal treatment, the amount of β-O-4 ether moieties decreased, and an intramolecular conversion of secoisolariciresinol residues to cyclic ethers occurred. The effects on shape and size of the lignins were investigated by size exclusion chromatography (SEC) and NMR-diffusion measurements, by which the hydrodynamic radii were found to increase as a result of thermal treatment. It is discussed whether this depends on reduced flexibility and/or increased molar mass. In addition, the trends from the SEC and NMR analyses were compared to changes in glass transition temperature as determined by differential scanning calorimetry (DSC).