Time-of-flight secondary ion mass spectrometry investigation of the influence of ambient air exposure on the migration of brain lipids in a matrix commonly used in matrix assisted laser desorption ionization mass spectrometry imaging

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging are established techniques for the imaging of biological samples. MALDI-MSI uses organic matrices to enhance desorption and ionization of biomolecules. Before analysis, the sample of interest must be coated with a matrix and the analytes must migrate into the matrix in order for molecular ion signals to be observed. The mechanisms involved in this migration and the sample handling procedures that influence this migration are not well understood. This leads to problems with reproducibility and accuracy of the images. In this study, ToF-SIMS was used to study the effects of exposure to mild ambient environmental conditions on migration of analytes in an α-Cyano-4-hydroxycinnamic acid (CHCA) matrix layer. A mouse brain section was coated with CHCA using an in-house built vapor deposition system and transferred to the ToF-SIMS instrument without breaking vacuum. The brain section was analyzed with ToF-SIMS immediately after vacuum transfer to the instrument, after 24 h storage in vacuum, and following a series of exposures to environmental conditions commonly observed in ambient laboratory air. The redistribution of lipids was observed to be dependent on the laboratory air humidity with minimal migration of most lipids below 50% relative humidity. Different migration behaviors were observed for different lipids as well as for different tissue types. The data show a complex multicomponent process of interdiffusion of the matrix and the brain lipids.