Photoionization using acetone as a dopant is distinguished by a significantly higher intensity of signals and the lowest sensitivity to contaminants present in the lignin preparation. This ionization method can be considered as preferred for studying the dioxane lignin preparations of woody plants. Copyright © 2016 John Wiley & Sons, Ltd.

In this study we compared the efficiencies of six crystalline matrices and their mixtures for the MALDI of lignin and studied the effect of sample application technique and matrix to analyte ratio on the quality of the mass spectrum. It was found that the best results can be obtained when α cyano 4 hydroxycin namic and 2,5 dihydroxybenzoic acids, and also 2,4,6 trihydroxyacetophenone are used as matrices, taken in 10 to 100 fold excesses with respect to lignin and sequentially applied onto the target in the order analytematrix-analyte. The use of ionic liquids as martices for obtaining lignin MALDI mass spectra was proposed for the first time. It was shown that the N tert butyl N isopropyl N methylammonium α cyano 4 hydroxy cinnamate ionic liquid, which forms a homogeneous solution with lignin, gives substantially better results compared to crystalline matrices in the intensity and reproducibility of the mass spectra obtained. The struc ture of MALDI mass spectra of spruce dioxane lignin related to the predominance of units derived from α guaiacylpropanone in the macromolecule is analyzed.

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the study of lignin is still extremely limited due to its low ionization efficiency. We have developed an approach for obtaining high-intensity MALDI mass spectra of lignin, based on the use of ionic liquids as matrices. Thirty-two ionic liquids consisting of large nitrogen-containing cations and anions of aromatic acids, traditionally used as crystalline matrices, were tested. It was established that ionic liquids based on N,N-diisopropyl-N-ethylammonium, N-isopropyl-N-methyl-N-tert-butylammonium, 3-aminoquinolinium, pyridinium, and 1-methylimidazolium cations and anions of ferulic, α-cyanohydroxycinnamic, and 2,5-dihydroxybenzoic acids as MALDI matrices provided high efficiency of lignin desorption/ionization with generation of singly charged protonated molecules of its oligomers. The use of such matrices in combination with the MALDI quadrupole ion trap-time-of-flight technique allows high-intensity mass spectra of lignin to be obtained without interferences from the matrix in the molecular weight range up to 3 kDa, adequately reflecting the molecular mass characteristics of lignin preparations. Using ionic liquid matrices, MS and MS MALDI mass spectra of lignins for various precursor ions were first obtained, including in the region of large (> 2 kDa) molecular weights. Differences in tandem mass spectra of coniferous and deciduous lignins, reflecting the structural features of corresponding oligomers were demonstrated. Graphical abstract ᅟ.

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