Nano-DESI Mass spectrometry imaging of eicosanoids in mouse kidney tissue using selected ion monitoring. Nano-DESI mass spectrometry imaging (MSI) is a technique for label-free spatial and molecular characterization of surfaces and biological samples. Eicosanoids are lipid mediators derived from eicosapolyenoic acid- products of arachidonic acid oxidation. Eicosanoids have been of interest to the medical field for many years. Major focus on this lipid class came from the development of nonsteroidal anti-inflammatory drugs (NSAIDs), some of these including aspirin, naproxen, ibuprofen, and acetaminophen work by blocking either the formation or the effects of eicosanoids. These lipids also play important roles in various body functions (cardiovascular, renal, gastrointestinal, neuronal) and as mediators of inflammation, asthma, fever, pain, hypertension, and stroke. Typically, eicosanoids occur in subnanomolar concentrations, despite their high level of bioactivity, which makes them significantly more difficult to analyze via direct mass spectrometry. Here, selected ion monitoring (SIM) is used to increase the signal-to-noise of the identified eicosanoids compared to a broadband full scan mode.

Ambient ionization techniques, such as nanoDESI and nanoESI, allow for the direct analysis of complex samples under atmospheric pressure with no sample pretreatment. These ionization techniques are utilized for a variety of applications, including lipidomics, online reactions and imaging of small molecules. Nanoelectrospray ionization (nanoESI) is an ionization technique that is similar to electrospray ionization (ESI) but uses smaller sample volumes. NanoESI can be used for complex biological sample analysis and when coupled with online photochemical reactions, such as the Paternò-Büchi (PB) reaction, structural information about lipids can be determined. Likewise, nanoDESI is another ambient ionization technique that employs the ESI mechanism but incorporates online liquid extraction of analytes. This technique is easily incorporated to mass spectrometry imaging (MSI) to provide spatial localization of biomolecules in tissues. Additionally, nanoDESI allows for tunable solvent extraction and online derivatization reactions. These techniques were used to determine structural information of neutral lipids, to image lipids from different developmental stages of lung tissue, and to image and quantify small molecule drugs and metabolites in tissue.

<p>   </p> <p>Ambient Mass Spectrometry Imaging (MSI) is a powerful analytical tool in biomedical research that enables simultaneous label-free spatial mapping of hundreds of molecules in biological samples under native conditions. Nanospray desorption electrospray ionization (nano-DESI) is an emergent ambient MSI technique developed in 2010 that uses localized liquid extraction of molecules directly from surfaces. Like other liquid-extraction based techniques, nano-DESI relies on gentle removal of molecules from surfaces and soft ionization. High sensitivity and spatial resolution, versatility of the solvent composition, which may be used to tailor the extraction and ionization of selected molecules, quantification capabilities at the single-pixel level as well as compensation for matrix effects by adding a known standard to the solvent, and online derivatization are key features of nano-DESI MSI that position it as a unique analytical tool for studying biological systems. </p> <p>Despite the advantages that nano-DESI provides, there are still challenges associated with the structural characterization, extraction, and detection of certain molecular classes. Therefore, my dissertation research has focused on addressing these analytical challenges by developing innovative approaches that substantially enhance the performance of the nano-DESI technique in the study of complex biological systems. </p> <p>In this thesis, a systematic study of the solvent composition is carried out to aid in the detection of neutral lipids such as triglycerides thereby expanding the molecular coverage of nano-DESI experiments. Taking advantage of the versatility of the solvent composition, I developed an approach for the online derivatization of unsaturated lipids into lipid hydroperoxides using the reaction of singlet oxygen with C=C bonds. This method further expands the specificity of nano-DESI MSI by enabling the detection and imaging of positional lipid isomers. To aid in the analysis of complex mixtures and provide additional structural information in the form of collision cross sections, coupling of nano-DESI with a drift-tube ion mobility spectrometry is also reported along with examples of the powerful capabilities of this platform. Lastly, nano-DESI MSI is used to address the complexity in the analysis of individual skeletal muscle fibers. This collaborative project involves the development of a robust image registration approach of immunofluorescence imaging and high-spatial resolution nano-DESI MSI to obtain accurate chemical maps specific to each fiber type. The developments described in this thesis are key to understanding the dynamic metabolic processes on a molecular level with an unprecedented specificity and sensitivity.</p> <p>  </p>