New Ionization and Detection Technologies for Mass Spectrometry Imaging. From Small Molecules to Intact Proteins

Abstract

There is a constantly growing interest in biomedical research to visualize changes in the location of various biomolecules in tissue sections as a result of complex diseases. Mass spectrometry imaging is one of the techniques that enable the mapping of molecules on a 2D surface. However, the technique has several limitations that should be addressed by developments in ion generation and ion detection. This thesis presents work on instrument development in the field of mass spectrometry imaging. The improvements presented include both the ion source and the ion detection part of the mass spectrometer. Chapters 3-5 concentrate on developments in secondary ion mass spectrometry and the application of SIMS for biomedical research. Chapter 3 and 4 discusses the implementation of an active pixel detector, the Timepix detector for microscope mode SIMS imaging which detector promises increased speed, spatial resolution and sensitivity for mass spectrometry imaging. Chapter 5 presents an example for the application of high spatial resolution SIMS imaging in biomedical research. SIMS imaging is used to differentiate the different regions of a heart tissue section after myocardial infarction. Also, the changes in lipid metabolism due to heart attack are studied both in positive and negative ion mode. The increased diacylglycerol ion intensities in positive ion mode and the increased fatty acid abundance in negative ion mode indirectly suggest the overexpression of various phospholipase enzymes. These enzymes play important role in inflammatory processes and ventricular remodeling after the heart attack. Thus, SIMS imaging gives a new insight into the molecular changes following myocardial infarction. Chapter 6 focuses on a different aspect of increasing the information content of a mass spectrometry imaging experiment. The chapter presents a recently developed ambient pressure ion source, the Laser Ablation Electrospray Ionization source. A commercial version of this ion source is modified to interface it with an FT-ICR mass spectrometer. Thus, both the advantages of an ambient pressure ion source and the superior mass accuracy and mass resolving power of an FT-ICR mass spectrometer are employed to enable the top-down imaging of multiply charged intact proteins. The systems capabilities are demonstrated with protein solutions that are measured in combination with IRMPD and ECD fragmentation directly from a liquid droplet. Additionally, intact proteins are imaged directly from a frozen lung tissue section and are identified by CID and IRMPD imaging experiments. This work also presents the first example of imaging the distribution of a post-translational modification of a protein. The new developments in ion detection and ion production for mass spectrometry imaging presented in this thesis have the potential to significantly increase the speed, spatial resolution and information content of this technique. Thus, these new developments have the potential to open up new application areas for mass spectrometry imaging.