Laser Post-Ionization Combined with Matrix-Free Laser Desorption/Ionization Enhances the Mass Spectrometry Imaging of Plant Metabolites

While the benefits of mass spectrometry imaging (MSI) coupled with recently described laser post-ionization (LPI) techniques (e.g., MALDI-2) have been well explored for the study of mammalian systems, their benefits for spatial metabolomics of plants have not. Here, we demonstrate that matrix-free laser desorption/ionization (LDI) coupled with LPI can significantly increase the number of plant metabolites detectable in an MSI experiment, compared to LDI alone, including for many flavonoids. Moreover, while many aromatic compounds are detected as their radical cations, a result of the photoionization processes accessible using LPI, many compounds (e.g., non-UV active compounds) also experience a significant increase in the abundance of their protonated ions. This suggests that endogenous UV active compounds, such as flavonoids can act as a MALDI-like matrix in promoting charge transfer upon excitation by the laser pulse used for LPI. MSI datasets using LDI-LPI acquired from Azolla filiculoides reveal rich spectra, containing several thousand peaks, with very few background-related signals, including many poly glycosylated flavonoids. This work provides an avenue to significantly enhance the capabilities for studying region-specific flavonoid metabolism within plants.