Ionic liquid matrices for MALDI mass spectrometry of lignin

Abstract

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.

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analysis for characterization of lignin oligomers using cationization techniques and 2,5‐dihydroxyacetophenone (DHAP) matrix

“…Appropriate selection of a MALDI matrix with or without modifiers will ultimately resolve previously reported poor ionization and concomitant low signal intensities. Recently, Kosyakov et al reported on the use of ionic liquids as novel MALDI matrices for lignin analysis . Although the MALDI spectra of their ionic liquid matrices show intense matrix‐related background ions, their approach shows promise for the analysis of intact kraft and extracted lignins.…”

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

“…ILMs were successfully used to a variety of LMW molecules such as carbohydrates, [44] oligosaccharides [45] and lignin. [46] Binding SOMs to a supplementary structure can increase the molecular weight of the matrix, and in some cases even enable noise-free measurements in the LMW region. For instance, CHCA was covalently bound to different aromatic compounds via a Knoevenagel reaction to deliberately shift the molecular weight.…”

“…Furthermore, some of the ILMs had dual polarity mode ability: e. g., DHB/3‐AP allowed the detection of lipids in both positive and negative mode. ILMs were successfully used to a variety of LMW molecules such as carbohydrates, [44] oligosaccharides [45] and lignin [46] …”

Atmospheric pressure ionization mass spectrometry as a tool for structural characterization of lignin

“…MALDI also results in the predominant formation of singly charged species, which leads to a less preferable ionization of high MW species and a more predominant formation of ions featuring low m/z values 12,13 . Improved ionization of high MW species was, however, achieved when using ionic liquids as crystalline matrices 8,16 . Nevertheless, the analyte MW is typically limited to the operating range of the mass analyzer 17 .…”