This review is devoted to the application of MS using soft ionization methods with a special emphasis on electrospray ionization, atmospheric pressure photoionization and matrix-assisted laser desorption/ionization MS and tandem MS (MS/MS) for the elucidation of the chemical structure of native and modified lignins. We describe and critically evaluate how these soft ionization methods have contributed to the present-day knowledge of the structure of lignins. Herein, we will introduce new nomenclature concerning the chemical state of lignins, namely, virgin released lignins (VRLs) and processed modified lignins (PML). VRLs are obtained by liberation of lignins through degradation of vegetable matter by either chemical hydrolysis and/or enzymatic hydrolysis. PMLs are produced by subjecting the VRL to a series of further chemical transformations and purifications that are likely to alter their original chemical structures. We are proposing that native lignin polymers, present in the lignocellulosic biomass, are not made of macromolecules linked to cellulose fibres as has been frequently reported. Instead, we propose that the lignins are composed of vast series of linear related oligomers, having different lengths that are covalently linked in a criss-cross pattern to cellulose and hemicellulose fibres forming the network of vegetal matter. Consequently, structural elucidation of VRLs, which presumably have not been purified and processed by any other type of additional chemical treatment and purification, may reflect the structure of the native lignin. In this review, we present an introduction to a MS/MS top-down concept of lignin sequencing and how this technique may be used to address the challenge of characterizing the structure of VRLs. Finally, we offer the case that although lignins have been reported to have very high or high molecular weights, they might not exist on the basis that such polymers have never been identified by the mild ionizing techniques used in modern MS.

Wheat straw lignin was extracted by the CIMV process using organic acid media at pilot plant scale. The product was analyzed by gel permeation chromatography (GPC), 1H and 13C NMR spectroscopy, infrared attenuated total reflectance‐Fourier transform infrared analysis (ATR‐FTIR), and gas chromatography (GC) to clarify its structure and functionality. In most cases, lignin was esterified before analysis. Control of the esterification was conducted via ATR‐FTIR and NMR. GC analysis was used to quantify total hydroxyl group of lignin by saponification of propionylated lignin and was also used to quantify phenolic hydroxyl groups of lignin by aminolysis of propionylated lignin. Acetylated lignin was analyzed by GPC. Carboxylic group of lignin was determined by pH metric titration. Lignin extracted from the CIMV process was observed as a low molecular weight polymer with a low polydispersity index and high free hydroxyl content. The potential of lignin as a natural polyphenol was confirmed by the analytical results obtained. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Wheat straw Biolignin TM was used as a substitute of bisphenol-A in epoxy resin. Synthesis was carried out in alkaline aqueous media using polyethyleneglycol diglycidyl ether (PEGDGE) as epoxide agent. Structural study of Biolignin TM and PEGDGE was performed by solid-state 13 C NMR and gel permeation chromatography, respectively, before epoxy resin synthesis. Biolignin TM based epoxy resins were obtained with different ratios of Biolignin TM : PEGDGE and their structures were analyzed by solid-state 13 C NMR. The crosslinking of PEGDGE with Biolignin TM was highlighted in this study. Properties of Biolignin TM based epoxy resins were analyzed by differential scanning calorimetry and dynamic load thermomechanical analysis as well as compared with those of a bisphenol-A epoxy-amine resin. Depending on the epoxy resin formulation, results confirmed the high potential of Biolignin TM as a biosourced polyphenol used in epoxy resin applications.

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