The reactivity and physicochemical properties of lignins are partly governed by their molar mass distribution. The development of reliable standard methods for determination of the molar mass distribution is not only relevant for designing technical lignins for specific applications, but also for monitoring and elucidating delignification and pulping processes. Size-exclusion chromatography (SEC) offers many advantages, such as wide availability, short analysis time, low sample demand, and determination of molar mass distribution over a wide range. A collaborative study has been undertaken within the “Eurolignin” European thematic network to standardise SEC analysis of technical lignins. The high-molar-mass fraction of polydisperse lignins was shown to be the main source of intra- and interlaboratory variations, depending on the gel type, elution solvent, detection mode, and calculation strategy. The reliability of two widespread systems have been tested: one based on alkali and a hydrophilic gel (e.g., TSK Toyopearl gel) and the other based on THF as solvent and polystyrene-based gels (e.g., Styragel). A set of practical recommendations has been deduced.

Lignin is a highly branched polymer consisting of phenylpropane units, and it is one of the ingredients of the supporting matrix in plant cell walls. The morphology of several lignins extracted from plant cell walls using different methods was studied by small-angle and ultra-small-angle X-ray scattering. A power-law type intensity was observed for the dry lignins, but on the basis of the power-law exponent the fractal approach often applied to lignins is not fully justified. However, the intensity of kraft lignin did show a power law with surface fractal dimension D(s) = 2.7 +/- 0.1. The specific surface area of the lignins ranged from about 0.5 to 60 m(2)/g with 20% relative accuracy. The radius of gyration was determined from small-angle X-ray scattering data for aqueous solutions of kraft lignin. The shape of the particles in NaCl and NaOH solutions was found to be elongated. The particles were about 1-3 nm thick, while the length (5-9 nm) depended on the solvent and on the lignin concentration. The size of these primary particles was approximately the same as the size of the pores in the fractal aggregates of the dry kraft lignin. Their size was determined to be about 3.5 nm.

Modern multidimensional NMR spectroscopic methods were applied to investigate the effects of kraft pulping and oxygen delignification on lignin side-chain structures. In addition to the two-dimensional HSQC measurements, the three-dimensional HSQC-TOCSY technique was utilized to elucidate the (1)H-(1)H and (1)H-(13)C correlations of individual spin systems and thus indicate a certain lignin side-chain structure. Unlike earlier, nonlabeled samples were used for 3D measurements. According to 2D and 3D NMR spectra, most of the structures identified in milled wood lignin (MWL) are still present in technical lignins after kraft pulping and oxygen delignification. Although the main reaction during kraft pulping is the cleavage of beta-O-4 linkages, these structures are still left in spent liquor lignin as well as in residual lignin. The amount of coniferyl alcohol and dihydroconiferyl alcohol end groups, as well as some unidentified saturated end groups, is higher in technical lignins than in MWL. Contrary to our earlier observations, no diphenylmethane structures were observed in any technical lignins. Vinyl aryl ether structures could not be detected in technical lignins either.

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Summary A major problem in the development of new bleaching technologies for pulp is the difficulty of analysing the changes induced by bleaching chemicals in the structures of residual lignin. Separation and isolation of the residual lignin before the analysis may modify the constituents of interest, whereas if the pulp is analysed directly, the responses from cellulose and hemicelluloses will overlap the signal from the residual lignin. In this study, UV resonance Raman spectroscopy, a powerful technique for detecting trace components in complex mixtures, was applied to detect the changes in the residual lignin content after bleaching stages. The resonance Raman technique was found to be highly sensitive and selective for lignin structures allowing the detection of trace amounts of lignin after the final bleaching stages. Furthermore, it enabled rapid and easy determination of hexenuronic acid content. UV resonance Raman spectroscopy would appear to be a technique of great potential for pulping and bleaching research.

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