Lignins are complex irregular polyphenolic compounds, obtained in massive quantities as by-products during chemical wood pulping processes. The search of novel lignin applications in addition to those coming from the pulp industry arouses the heightened interest to the amplification of a rapid and effective way of lignin characterization. Sample fractionation with gel permeation chromatography (GPC) or asymmetrical flow field-flow fractionation (AsFlFFF) may provide comprehensive information about primary characteristics of technical lignins, such as molar mass distribution and polydispersity index. Diffusion-ordered NMR spectroscopy (DOSY) may be applied as an alternative source of information about molar weight characteristics of the samples. Lignin analysis is hindered due to inconstancy of the structural and chemical characteristics of lignin fractions. Chemical treatment during sulphite or kraft pulping processes affects the solubility of analytes due to the cleavage of alkyl aryl ether inter-unit linkages and formation of hydrophilic HSO3- or OH- / HS- groups. Additionally, the tendency to aggregate in the solution may obstruct the analysis. The hindering effects may be minimized through the selection of proper mobile phase and sample preparation steps for each individual fraction. Aqueous and organic-based GPC systems were adjusted for characterization of technical lignin samples: milled wood lignins, lignosulfonates and kraft lignins. Mobile phase parameters were optimized by adjusting ionic strength and pH of the solution. Molar mass determination was performed based on the calibration with narrow polystyrene sulfonate standards. AsFlFFF fractionation was performed for water soluble samples. A variety of critical AsFlFFF parameters including laminar and cross-flow rates, sample concentration, injection and focusing time were adjusted towards an appropriate set of parameters for optimal sample separation and recovery. Self-diffusion coefficients obtained by DOSY measurements were related to molar mass values by performing the calibration with polystyrene and dextrane standards. The results obtained for different lignin samples will be discussed; and a critical overview on the fractionation techniques in comparison to DOSY analysis, and calibration parameters are presented.

Comparing different approaches to measure molar mass of lignin: SEC, DOSY and AsFlFFF

249th National Meeting (2015)

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