Insights into the MALDI Process after Matrix Deposition by Sublimation Using 3D ToF-SIMS Imaging

Abstract

Imaging mass spectrometry (IMS) has become a powerful tool to characterize the spatial distribution of biomolecules in thin tissue sections. In the case of matrix-assisted laser desorption ionization (MALDI) IMS, homogeneous matrix deposition is critical to produce high-quality ion images, and sublimation in particular has shown to be an excellent matrix deposition method for the imaging of lipids. Matrix deposition by sublimation is, however, a completely solvent-free system, which ought to prevent the mixing of matrix and analytes thought to be necessary for successful MALDI. Using 3D time-of-flight secondary ion imaging mass spectrometry, we have studied the matrix-tissue interface in 3D with high resolution to understand the MALDI process of lipids after matrix deposition by sublimation. There is a strong indication that diffusion is the process by which lipids migrate from the tissue to the matrix layer. We show that triacylglycerols and phospholipids have a delayed migratory trend as compared to diacylglycerols and monoacylglycerols, which is dependent on time and matrix thickness. Additional experiments show that a pure lipid's capacity to migrate into the matrix is dependent on its fluidity at room temperature. Furthermore, it is shown that cholesterol can only migrate in the presence of a (fluid) lipid and appears to fluidize lipids, which could explain its colocalization with the diacylglycerols and monoacylglycerols in the matrix.

Immersion by rotation‐based application of the matrix for fast and reproducible sample preparations and robust results in mass spectrometry imaging

“…The underlying composition and physiology of the tissues differ and, accordingly, also the interaction between the matrix and the ionized compounds. Our results indicate that the type of the solvent used for the matrix solution has a crucial role not only on the ionization processes but also on the matrix‐tissue surface morphology . This observation can be explained by a solvent‐dependent rearrangement of the matrix molecules during the solvent evaporation and then a different interaction with the analytes .…”

“…Our results indicate that the type of the solvent used for the matrix solution has a crucial role not only on the ionization processes 25 but also on the matrix-tissue surface morphology. 26 This observation can be explained by a solvent-dependent rearrangement of the matrix molecules during the solvent evaporation 27,28 and then a different interaction with the analytes. 29 In general, there are evidences that the morphology of co-crystal analyte-matrix influence the ionization efficiency precluding a quantitative analysis in MALDI-MS. 30,31 For this reason, in order to influence the morphology of the matrix layer, several techniques were developed such as fast evaporation, 32 seeded layer, 33 sandwich, 34 electrospray deposition, 35 and sublimation.…”

Silver spray deposition for AgLDI imaging MS of cholesterol and other olefins on thin tissue sections

“…The plateau observed signifies that enough AgNO 3 has been deposited onto the sample to allow for maximal desorption and ionization for the amount of CHO present in the sample and that an increase in the amount of silver salt does not improve the total CHO signal. Surprisingly, the AgNO 3 layer is more than double that of conventional MALDI matrices indicated in the literature to be around 0.2 mg/cm 2 in thickness . It is important to note that the amount of AgNO 3 deposited on the tissue may vary based on the thickness of the tissue section and its histology including the amount of CHO present in the sample.…”

Novel approach to enhance sensitivity in surface‐assisted laser desorption/ionization mass spectrometry imaging using deposited organic‐inorganic hybrid matrices

“…In contrast, the MALDI method is sensitive to phospholipids. Phospholipids are suggested factors involved in the suppression of ionization and are also reportedly incorporated into MALDI crystals by vapor deposition . Given that Zr oxide is known to adsorb phospholipids, use of Zr as a metal nanostructure was expected to be effective.…”