Class-specific depletion of lipid ion signals in tissues upon formalin fixation

Formalin fixation (FF) is widely used in pathology labs due to its ability to prevent tissue degradation and better preserve tissue morphology, thereby enabling long-term storage especially when combined with paraffin embedding. The detection of lipids and metabolites has recently been demonstrated from formalin-fixed paraffin-embedded (FFPE) tissues with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). However, the resulting lipid profiles differ markedly from those typically generated from fresh frozen tissue. To shed light onto the effects of formalin-induced modifications on lipids in tissues 24 h-formalin-fixed and untreated mouse and rat kidney tissues were compared using time-of-flight matrix-assisted laser/desorption-mass spectrometry imaging (MALDI-MSI) and Fourier transform MALDI-MSI combined with tandem mass spectrometry. These analyses revealed distinct and class-specific depletion of ion signals of certain lipid species upon formalin fixation, in particular for those containing amine functionalities. As MSI moves closer towards clinical application and necessitates increased analysis of formalin fixed tissues our results provide a route to rationalize the observed tissue-dependent lipid compositions observed from such tissues. (C) 2019 Elsevier B.V. All rights reserved