Getting Closer to Absolute Molar Masses of Technical Lignins

Abstract

Determination of molecular weight parameters of native and, in particular, technical lignins are based on size exclusion chromatography (SEC) approaches. However, no matter which approach is used, either conventional SEC with a refractive index detector and calibration with standards or multi‐angle light scattering (MALS) detection at 488 nm, 633 nm, 658 nm, or 690 nm, all variants can be severely erroneous. The lack of calibration standards with high structural similarity to lignin impairs the quality of the molar masses determined by conventional SEC, and the typical fluorescence of (technical) lignins renders the corresponding MALS data rather questionable. Application of MALS detection at 785 nm by using an infrared laser largely overcomes those problems and allows for a reliable and reproducible determination of the molar mass distributions of all types of lignins, which has been demonstrated in this study for various and structurally different analytes, such as kraft lignins, milled‐wood lignin, lignosulfonates, and biorefinery lignins. The topics of calibration, lignin fluorescence, and lignin UV absorption in connection with MALS detection are critically discussed in detail, and a reliable protocol is presented. Correction factors based on MALS measurements have been determined for commercially available calibration standards, such as pullulan and polystyrene sulfonate, so that now more reliable mass data can be obtained also if no MALS system is available and these conventional calibration standards have to be resorted to.

Lignin-Rich PHWE Hemicellulose Extracts Responsible for Extended Emulsion Stabilization

“…For phenolic fractions containing mainly lignin, the estimated molar masses were lower than those of the starting materials, for GGM-phe significantly lower 2,800 Da and for GX-phe 2,500 Da. Although present knowledge indicates that analysis by SEC gives underestimated molar masses for lignins (Zinovyev et al, 2018), the results show that GGM-phe and GX-phe have similar molar masses but that their molar masses are different from those of the starting hemicelluloses and fractions of purified hemicelluloses. Furthermore, the polydispersity, M w /M n , for all GGM samples was in the range of 7.5-8.0, although for GX samples the value was higher for GX-phe (7.6) and lower for starting GX (4.9) and GX-pur (4.7).…”

Linkage Abundance and Molecular Weight Characteristics of Technical Lignins by Attenuated Total Reflection‐FTIR Spectroscopy Combined with Multivariate Analysis

“…We, and others, have previously found that the results of lignin GPC measurements are very sensitive to the exact method and equipment used for their determination. Additionally, GPC results using multi‐angle light scattering detection at 785 nm have recently shown that standard detection methods (e.g., refractive index) in combination with sulfonated polystyrene standards are subject to substantial (non‐linear) errors in the molecular‐weight results . While the conditions used for our GPC analysis (UV detection, polystyrene standards, acetylated lignin) are not identical to those used in the referenced study, the same type of errors will undoubtedly be present.…”

“…Additionally,G PC resultsu sing multi-anglel ight scattering detection at 785 nm have recently shown that standard detection methods (e.g.,r efractive index)i nc ombination with sulfonated polystyrene standards are subjectt os ubstantial (nonlinear) errors in the molecular-weightr esults. [44] While the conditions usedf or our GPC analysis (UV detection, polystyrene standards, acetylated lignin) are not identicalt ot hose used in the referenced study,t he same type of errors will undoubtedly be present. As for the linkage abundance accuracy,i ti sa lso important to appreciate that HSQC NMR quantification is only semi-quantitative, relyingo ni nternal lignin aromatic signals (G 2 ,S 2/6 ,H 2/6 )f or reference.…”

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight.

“…We believe this further demonstrates the limitations of the M w determination of lignin using GPC as suggested in the recent literature. [16] Despite these likely sources of error, it was decided to test the calibration curve, and hence the overall utility of the proposed DOSY approach, on additional bulk samples of lignins ( Figure 4). Therefore, 4 different lignins were prepared and subjected to DOSY NMR analysis.…”

“…[15] Recently, a multiangle laser light scattering (MALLS) method was developed that circumvented issues arising from lignin's florescence when more conventional analyses of this type were performed. [16] The most popular method used to analyse lignin MW is gel permeation chromatography (GPC). [17] This excellent technique separates lignin as a function of MW by size exclusion chromatography and the resulting elution profile is calibrated using polymeric standards (e. g. polystyrene).…”