New gel permeation chromatography (GPC) tools for lignocellulosic materials characterization

The evaluation of the molar mass of lignin is an important tool in order to understand its reactivity and phys. properties. Gel Permeation Chromatog. (GPC) has been extensively used for this purpose and numerous publications describe different GPC methods for different types of lignins. Anyways the accurate determination of lignin molar mass remains a major challenge due to the lignin complex and variable structure. The analyses became even more problematic in case of lignocellulosic materials. In 2011, a GPC method for the determination of the mol. weight distribution of the whole lignocellulosic material was investigated. The method was based on the dissolution of the ball-milled samples in the ionic liquid 1-allyl-3-methylimidazolium chloride ([amim]Cl). Under these homogeneous conditions, a derivatization reaction was performed with benzoyl chloride. The benzoylated wood was found to be completely soluble in the THF eluent with marked UV detector sensitivity. An important evolution of this approach was the development on 2012 of a method for detecting the presence and the amount of lignin-carbohydrate complexes (LCCs). The complete acetylation and benzoylation of the milled native substrates were still obtained in ionic liquid media, and the systematic comparison between their GPC-UV chromatograms has revealed a straightforward technique in the detection of LCCs. In fact, benzoylated specimens enabled the UV-detection (240 nm) of all substrate components (cellulose, hemicellulose and lignin), whereas acetylated specimens accounted for the sole contribution of LCCs (and free lignin, 280 nm) due to the lack of chromophores in the unbound acetylated polysaccharide portion. This novel approach was applied in different field, from the evaluation of biorefinery processes to the chem. characterization of archaeol. woods. Finally, on 2021 an approach based on fluorescence labeling was developed to understand the phenols distribution in tech. lignins. Selective derivatization with dansyl chloride of lignin phenolic was quant. achieved. Reference acetylated lignin and labeled lignin were analyzed by GPC coupled to a UV-Vis detector (280 nm) and a fluorescence detector (λex: 390 nm, λem: 550 nm) to discern the dansyl-linked phenol response from the lignin aromatic skeleton input. Valuable information about the phenolic group distribution as a function of mol. weight was gathered.