Lignin is an abundant renewable resource with interesting properties, e.g., its natural antioxidant activity. As a complex crosslinked copolymer, it is difficult to fully understand the relationship between its structure and properties. Comprehensive knowledge of its structure though is essential for future applications such as being an additive for food packaging materials.
A frequently used strategy to characterize lignin’s antioxidant activity is by bulk-type assays. Those assays include, e.g., DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), FRAP (ferric ion reducing antioxidant potential), and FC (Folin-Ciocalteu).
In the case of the 3D biopolymer lignin (mixture of different molecular mass fractions, different functional group contents, different monomer compositions) bulk-type assays only give a single average value for the investigated property. As different fractions may influence each other in an additive, synergistic, or inhibitory way it is important to know the individual contribution of those fractions.
In this study, both size-exclusion chromatography (SEC) and reversed-phase liquid chromatography (RP-LC) were used in a two-dimensional setup combined with post-column ABTS assay to qualitatively map and assess the antioxidant profile of softwood Kraft lignin. The results show that each of the chromatographic techniques alone are unable to sufficiently resolve the sample’s complexity presented by lignin. Two-dimensional chromatography reveals that both polymeric lignin and oligomeric lignin have antioxidant activity, though smaller fractions show higher specific antioxidant activity.