Analysis of lignin monomers and oligomers in technical lignins using chromatography and mass spectrometry

Lignin is the most abundant aromatic biopolymer on earth and has the potential to play an important role in the transition from an oil-based refinery to a biorefinery-based industry. The isolation of lignin from the biomass can be achieved by several different technical processes and isolated lignins are referred to as technical lignins in literature. During the last decades, the conversion of technical lignins into economically valuable aromatic chemicals became a growing research interest.
For the understanding of the chemical nature of technical lignins and the understanding of the chemical reactions occurring during conversion processes, selective and sensitive analytical methods are crucial. Therefore, analytical methods using liquid chromatography (LC) and supercritical fluid chromatography (SFC) coupled to electrospray ionisation–mass spectrometry (ESI–MS) were developed in this thesis work. Main emphases were set on the optimisation of the ESI of lignin monomers (LMs), the identification of unknown LMs and lignin oligomers (LOs) in complex technical lignin samples and on the separation of LMs and LOs using SFC.
Several ESI parameters were studied for the ionisation of LMs to identify which ESI parameters have significant influences. First, the ESI of a wide range of LMs were studied using SFC/ESI–MS, then the influence of specific ESI parameters on different groups of LMs was analysed. Significant ESI parameters, such as the concentration of the makeup solvent additive or the desolvation gas temperature, were identified as significant parameters for the ESI of a wide range of LMs. Furthermore, it has been shown that for the different groups of LMs, different ESI parameters are of importance. For instance, compounds with two methoxy groups seem to need more desolvation energy compared to compounds with one or no methoxy groups.
The identification of unknown LMs and LOs in technical lignin samples using MS is very challenging due to the very complex sample mixtures and the lack of commercially available reference standards. Therefore, a non-targeted analysis strategy using SFC/ESI–high-resolution multiple stage tandem MS combined with Kendrick mass defect-based principal component analysis–quadratic discriminant analysis classification models was developed. The developed method assures an identification confidence of level 3 without the need of reference standards and without the study of MS fragmentation patterns. Furthermore, it was demonstrated that a higher identification confidence level and tentative chemical structures can be obtained with multiple stage tandem MS.
For the characterisation of unknown compounds in complex samples, clean mass spectra of the investigated compound are useful. In this work, the application of SFC for the separation of LMs and LOs has been investigated using a stationary phase screening approach. It was found that a SFC stationary phase with both hydrogen bonding and π-π-interaction chemistries offers the highest overall resolution power combined with a selective separation of LMs and lignin dimers.