Direct imaging of plant metabolites in leaves and petals by desorption electrospray ionization mass spectrometry

• Direct DESI imaging is presented on plant material.
• Direct imaging with and without preceding removal of the cuticle wax layer.
• A new ternary spray solvent mixture shows better penetration through the cuticle.
• Imaging of very long chain fatty acids (VLCFAs) from the cuticle.

Two different approaches to direct imaging of plant material with desorption electrospray ionization (DESI) mass spectrometry are presented and demonstrated on leaves and petals of Hypericum perforatum. The direct imaging approaches are in contrast to previous DESI imaging studies where indirect analysis via imprints were used in order to overcome the morphological barrier presented by the layer of cuticular waxes covering the surface of a leaf or a petal. In order to enable direct imaging of such plant materials, a new ternary solvent system is introduced, providing a higher and more stable signal from soft plant materials than the binary solvent systems typically used in DESI. With this ternary solvent system, it was possible to image a number of very long chain fatty acids (VLCFAs), a significant class of metabolites located in the cuticle layer in leaves and petals, as well as other plant metabolites. In the case of the petals of H. perforatum, all common metabolites could be imaged directly using the ternary solvent, whereas in the case of leaves from the same plant, only some of the metabolites were accessible, even with the ternary solvent system. For these samples, the leaves could be imaged with direct DESI after chloroform had been used to remove most of the cuticle, thus exposing lower layers in the leaf structure. A number of considerations regarding selection of samples and instrumental parameters that must be made in direct DESI imaging of plant materials are discussed.

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