Metal Oxide Laser Ionization Mass Spectrometry Imaging of Fatty Acids and Their Double Bond Positional Isomers

We present a novel combination of a metal oxide laser ionization mass spectrometry imaging (MOLI MSI) technique with off-line lipid derivatization by ozone for the detection of fatty acids (FA) and their carbon-carbon double bond (C═C) positional isomers in biological tissues. MOLI MSI experiments were realized with CeO₂ and TiO₂ nanopowders using a vacuum matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectrometer in the negative mode. The catalytic properties of these metal oxides allow FA cleavage from phospholipids under UV laser irradiation. At the same time, fragile ozonides produced at the sites of unsaturation decomposed, yielding four diagnostic ions specific for the C═C positions. Advantageously, two MOLI MSI runs from a single tissue sprayed with the metal oxide suspension were performed. The first run prior to ozone derivatization revealed the distribution of FAs, while the second run after the reaction with ozone offered additional information about FA C═C isomers. The developed procedure was demonstrated on MSI of a normal mouse brain and human colorectal cancer tissues uncovering the differential distribution of FAs down to the isomer level. Compared to the histological analysis, MOLI MSI showed the distinct distribution of specific FAs in different functional parts of the brain and in healthy and cancer tissues pointing toward its biological relevance. The developed technique can be directly adopted by laboratories with MALDI TOF analyzers and help in the understanding of the local FA metabolism in tissues.