Where is the metabolite? Submicron lateral resolution meets high resolution mass spectrometry imaging

Mass spectrometry imaging (MSI) is an advanced analytical technique that aims to determine metabolite localization within a biological tissue. Therefore, ionization is performed in a spatially resolved manner and hence a mass spectral fingerprint of each pixel is obtained. From this information images of mass distributions of each metabolite can be reconstructed.

My unit operates two mass spectrometry imaging techniques, MALDI (Matrix assisted laser desorption ionization) mass spectrometry and Secondary ion mass spectrometry (SIMS).

In today’s lecture I will highlight recent developments in my lab that address the key challenges in mass spectrometry imaging; (1) highest lateral resolution MSI to resolve cellular or even subcellular features and determine chemical composition in biological samples, presenting a recently published study in which we were able to image organic crystals in zebrafish eyes in highest lateral resolution and furthermore determine the relationship between chemical composition and crystal shape [1] and (2) use of high-resolution mass spectrometry in combination with Ar-ion beam induced fragmentation for high confidence metabolite annotation in MSI experiments. Analyzing tomato imaging data, we explored the possibility to identify characteristic fragment ions of secondary metabolites directly from the Orbitrap-SIMS imaging run. In contrast to MALDI, SIMS using the Argon gas cluster ion beam allows the parallel detection of molecular ions and adducts and characteristic fragments. We analyzed the obtained images using correlation analysis of ion distributions and manual identification of co-localized masses as fragments of steroidal glycoalkaloid and saponin molecular ions. Thereby we were able to distinguish between differentially localized molecular ions and assign their characteristic fragments accordingly. This allowed the assignment and confident identification of a glycoalkaloid and a saponin in the outer cell layers of the developing plant within the seed and showed that these compounds in contrast to other steroidal glycoalkaloids are not detoxified during ripening and hence present a specific defense strategy to protect the embryonic plant.