Are Phosphatidic Acids Ubiquitous in Mammalian Tissues or Overemphasized in Mass Spectrometry Imaging Applications?

Mass spectrometry imaging (MSI) is an invaluable tool for the spatial visualization of molecules in vivo. However, the question of whether observed annotations are endogenous or artificial (i. e., from in-source fragmentation) is critical and has been largely unexplored in multimodal MSI. In matrix-assisted laser desorption/ionization (MALDI)-MSI datasets from researchers worldwide, PAs were found to represent up to 18 % of annotations in rat brain. Rat brain was additionally imaged here using nanospray desorption electrospray ionization (nano-DESI), a softer ionization strategy. No PAs observed with MALDI were present in the nano-DESI dataset. Further investigation strongly indicated lipid fragmentation to PAs for MALDI-MSI, but not with nano-DESI-MSI. We finally extend this observation to the MALDI-MSI analyses of human tissues, showing that PA annotations comprised up to 16 % of annotations. Therefore, this study shows that MSI annotations should be carefully interrogated, as in-source fragmentation or modification of lipids may contribute substantially to false annotations and incorrect biological interpretations.