Inherent limitations in the ionisation processes of Matrix-Assisted Laser Desorption/Ionisation limit the overall sensitivity and molecular coverage that can be achieved during mass spectrometry imaging (MSI) experiments. In recent years post-ionisation techniques, most notably MALDI-2, have been developed to enhance ionisation efficiency, with increases of up to 100-fold reported for several analyte classes. In this presentation, the development of a timsTOF Pro mass spectrometer (Bruker Daltonics, Bremen, Germany) equipped with a plasma post-ionisation and atmospheric pressure MALDI ion source will be described. This instrument yields a similar performance to MALDI-2 and enables high-performance MSI of lipids from tissues and single cells. Most notably, signals for many glycosphingolipids and phospholipids are observed to increase drastically compared to conventional MALDI approaches.
Owing to the high sensitivity of the initial system, it was recently rebuilt into a transmission-mode implementation whereby the MALDI laser is introduced from the back of the samples through a 50x microscope objective, with the resulting ions captured in a heated inlet capillary, ionised by the cold plasma source (SICRIT, Plasmion GmbH, Augsburg, Germany) and then mass analysed. This approach enables laser spot sizes as low as 1 µm to be achieved. Applied to mouse brain imaging high high-contrast images of various lipid species could be recorded, revealing spatial distributions of lipids within tissue at cellular and sub-cellular resolution, including throughout different regions of the cerebellum. Moreover, the system is well-suited for imaging of individual cells and has been deployed for imaging sub-cellular localisations of lipids in iPSC-derived astrocytes and neurons, for example enabling unique lipid composition of the cell body and cellular processes to be resolved.
This innovative instrument thus provides an exciting avenue to pursue lipid imaging studies at the sub-cellular level with a spatial resolution comparable to optical microscopy.