Mass Spectrometry Imaging of Plant Metabolites Enhanced by Laser Postionization Combined with Matrix-Free Ultraviolet Laser Desorption/Ionization

While the benefits of mass spectrometry imaging (MSI) coupled with recently described laser postionization (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. Herein, it is demonstrated that matrix-free ultraviolet 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, including many polyglycosylated flavonoids, but with very few background-related signals. This work provides an avenue to significantly enhance the capabilities for studying region-specific flavonoid metabolism within plants.