MALDI-TOF/TOF tandem mass spectrometry imaging reveals non-uniform distribution of disaccharide isomers in plant tissues.

Mass spectrometry imaging (MSI) is a powerful technique for investigating the biomolecular locations within tissues. However, the isomeric compounds are rarely distinguished due to inability of MSI to differentiate isomers in the probing area. Coupling tandem mass spectrometry with MSI can facilitate differentiating isomeric compounds. Here MALDI-TOF/TOF tandem mass spectrometry imaging approach was applied to probing the spatial distributions of isomeric disaccharides in plant tissues. First, MS/MS imaging analysis of disaccharide-matrix droplet spots demonstrated the feasibility of distinguishing isomeric species in tissues, by measuring the relative intensity of specific fragments. Then, tandem MS imaging of disaccharides in onion bulb tissues indicated that sucrose and other unknown non-sucrose disaccharides exhibit heterogeneous locations throughout the tissues. This method enables us to image disaccharide isomers differentially in biological tissues, and to discover new saccharide species in plant. This work also emphasizes the necessity of considering isobaric compounds when interpreting MSI results.