MALDI mass spectrometry imaging of bioactive lipids in mouse brain with a Synapt G2-S mass spectrometer operated at elevated pressure: improving the analytical sensitivity and the lateral resolution to ten micrometers.

Mass spectrometers from the Synapt-G1/G2 family (Waters) are widely employed for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). A lateral resolution of about 50 μm is typically achieved with these instruments, that is, however, below the often desired cellular resolution. Here, we show the first MALDI-MSI examples demonstrating a lateral resolution of about ten micrometers obtained with a Synapt G2-S HDMS mass spectrometer without oversampling. This improvement became possible by laser beam shaping using a 4:1 beam expander and a circular aperture for spatial mode filtering and by replacement of the default focusing lens. We used dithranol as an effective matrix for imaging of acidic lipids such as sulfatides, gangliosides, and phosphatidylinositols in the negative ion mode. At the same time, the matrix enables MS imaging of more basic lipids in the positive ion mode. Uniform matrix coatings with crystals having average dimensions between 0.5 and 3 μm were obtained upon spraying a chloroform/methanol matrix solution. Increasing the cooling gas pressure in the MALDI ion source after adding an additional gas line was furthermore found to increase the ion abundances of labile lipids such as gangliosides. The combined characteristics are demonstrated with the MALDI-MSI analysis of fine structures in coronal mouse brain slices.