Laser Desorption/Ionization of Low-Molecular-Weight Lignin Oligomers

The study compares the efficiency of laser desorption/ionization (LDI) of six dimeric compounds, corresponding to the most important types of bonds between monomeric structural fragments of lignin macromolecules, on three types of different stainless steel target plates. It was found that ground steel target plates demonstrate the best results because of the uniform crystallization of analytes and good surface coverage. The most important ionization mechanisms for lignin oligomers with conjugated structures and increased acidity, along with protonation (cationization), involve the formation of molecular ions due to direct photoionization. For all of the studied compounds, partial in-source decay was observed, the contribution of which became significant at high laser fluence. The LDI method was successfully used to obtain mass spectra of low-molecular-weight products of the depolymerization of technical lignins. It was shown that the use of hybrid mass spectrometers with a quadrupole ion trap leads to an increase in the percentage of signals of protonated molecules in the mass spectra of lignin depolymerization products because of the occurrence of side ion-molecular reactions in the plume and ion trap.