Mass Spectrometry-Based Metabolite Identification

For the MS profiling and MS/MS data analysis, Bruker FlexAnalysis 3.4 (Bruker Daltonics) was used for the preliminary viewing and processing of the mass spectra. Once the monoisotopic peak list was generated and exported, two metabolome databases (METLIN and HMDB) (Tautenhahn et al., 2012 (link); Wishart et al., 2022 (link)) were used for the search of the detected m/z values of precursor ions and CID fragment ions against potential metabolite identities within an acceptable mass error of ±5 ppm. Three ion adduct forms (i.e., [M + H]⁺, [M + Na]⁺, and [M + K]⁺) were considered for the database search. For MALDI tissue imaging, Bruker FlexImaging 4.1 software was used for the reconstitution of the ion maps of the detected flavonoids (Figure 1E). For the generation of the ion images using FlexImaging, the mass filter width was set at 5 ppm.