Mass spectrometry imaging is of growing interest for chemical mapping of lipids at the surface of tissue sections. Many efforts have been devoted to optimize matrix choice and deposition technique for positive ion mode analyses. The identification of lipid species desorbed from tissue sections in the negative mode can be significantly improved by using 9-aminoacridine together with a robust deposition method, yielding a superior signal-to-noise ratio and thus a better contrast for the ion images in comparison to classical matrices such as α-cyano-4-hydroxycinnamic acid, 2,5-dihydroxybenzoic acid, or 2,4,6-trihydroxyacetophenone. Twenty-eight different lipid species (phosphatidic acids, phosphatidylethanolamines, phosphatidylserines, phosphatidylglycerols, phosphatidylinositols, phosphatidylinositol-phosphates, and sulfatides) were scrutinized on rat brain tissue sections, and systematic MS/MS studies were conducted. It was possible to identify isobaric species differing by their fatty acid chains thanks to the improved sensitivity.

Mass spectrometry imaging is becoming a more and more widely used method for chemical mapping of organic and inorganic compounds from various surfaces, especially tissue sections. Two main different techniques are now available: matrix-assisted laser desorption/ionizaton, where the sample, preliminary coated by an organic matrix, is analyzed by a UV laser beam; and secondary ion mass spectrometry, for which the target is directly submitted to a focused ion beam. Both techniques revealed excellent performances for lipid mapping of tissue surfaces. This article will discuss similarities, differences, and specificities of ion images generated by these two techniques in terms of sample preparation, sensitivity, ultimate spatial resolution, and structural analysis.

We describe here the detection and identification of 13 primary metabolites (AMP, ADP, ATP, UDP-GlcNAc, ...) directly from rat brain sections by chemical mass spectrometry imaging. Matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS/MS) was combined with 9-aminoacridine as a powerful matrix. We also demonstrate that a new robotic sprayer allows us to homogeneously coat the surface with the matrix, enabling the acquisition of chemical images at a 50 microm resolution, leading us to precisely and simultaneously localize each metabolite over the tissue surface. These experiments open a new field of investigation for chemical mass spectrometry imaging and are of great interest for both chemists and biologists.