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, MS² and MS³ 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.

Keywords: Ionic liquids; Lignin; MALDI mass spectrometry; Matrix; QIT-TOF.