Molecular-level characterization of lignosulfonates: high-resolution mass spectrometry approach

Lignosulfonates (LSs), a large-scale by-product of sulfite pulping, are currently the most widely produced and utilized lignin derivative. Like other lignins, LSs are characterized by an extremely complex irregular structure, and their distinctive feature is the presence of sulfo groups in the side chains of the phenylpropane structural units. The present study, for the first time, proposes a mass spectrometric (Orbitrap) methodology for characterizing the chemical composition of LSs based on the use of negative ion mode atmospheric pressure ionization techniques and chemometric approaches to the data treatment. The LS ionization efficiency increases in the series ESI<APCI<APPI with a substantial change in the selectivity toward sulfonated or nonsulfonated species which can be detected mainly in ESI and APCI ionization modes, respectively. 1,4-Dioxane or ammonia-doped APPI-HRMS enables the reliable detection of >1000 LS oligomers of both CHO- and CHOS-classes in the widest molecular weight range (up to 1.2 kDa). For complex MS and MS/MS (broadband collision induced dissociation) data processing, the filtering methods based on a modified Kendrick mass defect analysis and van Krevelen elemental ratios visualization were introduced. These allow rapid distinguishing between typical lignin structures, sulfonated oligomers with different sulfonation and unsaturation degree, and impurity components. The developed approach was successfully tested on the both reagent-grade and real technical LS samples and can be further used to trace the processes of chemical modification of lignin in sulfite pulping and rapid characterization of LS preparations for various industrial applications.

Graphical Abstract