Comparison of CID and UVPD Activation to Achieve the Structural Characterization of Some Pyrolysis Bio-Oil Components

Compounds produced by pyrolysis or hydrothermal liquefaction of lignocellulosic biomass have the potential to serve as sustainable alternatives to those derived from fossil resources. Indeed, many chemicals present in the resulting bio-oils can be utilized in the production of fuels or chemical building blocks. However, the heavy and highly oxygenated components of bio-oils require upgrading via deoxygenation or catalytic cracking. A comprehensive structural elucidation and characterization of the functional groups present in bio-oil components is essential to define the most efficient upgrading treatment, which is of importance, particularly for lignin derivatives, as some of them are resistant to upgrading processes.

Tandem mass spectrometry is a valuable tool for structural analysis. However, traditional collisional activation (CID) does not always provide sufficient information due to possible preferential fragmentation sites and patterns. Alternative activation techniques, such as ultraviolet photodissociation (UVPD), may offer additional data. The partial complementarity of UVPD and CID-MS/MS for the structural analysis of lignin-derived compounds was investigated.

In this study, six lignin model compounds that efficiently absorb UV photons were investigated: 2,5-dihydroxybenzoic acid and vanillin, known to be components of pyrolysis bio-oils; coniferyl and sinapyl alcohols, two fundamental units of lignin; and two lignin dimers, pinoresinol, and guaiacylglycerol-β-guaiacylether (GGE). Collision-induced dissociation (CID) of the deprotonated standards produced by electrospray ionization, yielded characteristic fragment ions. UV activation generated additional fragment ions, some of which were valuable for determining the nature of the aromatic ring substitution.

Although only a small selection of compounds was examined, UVPD proved to be effective in providing additional structural information on some bio-oil components generated by lignocellulosic biomass pyrolysis. The results obtained on a Fourier transform ion cyclotron resonance mass spectrometer pave the way for two-dimensional Fourier transform mass spectrometry experiments to systematically elucidate the structures of lignin-derived compounds in bio-oils by UVPD.