Development of Low-Flow High-Resolution Desorption Electrospray Ionization Mass Spectrometry Imaging

Desorption electrospray ionization mass spectrometry (DESI-MS) imaging is a well-established technique for molecular analysis of biological samples, although its spatial resolution has been limited when compared to other MS imaging techniques. Here, we describe the development and optimization of a low-flow DESI-MS method that allows for sub-10 μm spatial resolution tissue imaging using a commercial DESI sprayer. Key technical modifications that enabled low-flow high-resolution DESI-MS imaging include reduced solvent flow rates below 350 nL/min, increased solvent pump back-pressure for spray stability, and optimized sprayer design and geometry. We applied low-flow DESI to image porcine liver and rat brain tissue sections at a spatial resolution of 5-10 μm, and the resulting ion images showed high spatial fidelity and detailed tissue histologic features. Building on the nondestructive nature of DESI-MS, we demonstrate that a tissue section can be first imaged with low-flow DESI at lower resolution (100-200 μm pixel size), followed by high-resolution (5-10 μm pixel size) imaging of selected regions of interest in the same tissue section. Lastly, we applied low-flow DESI to image and classify human thyroid cancer tissue sections and fine-needle aspiration (FNA) biopsies at 10 μm spatial resolution, achieving accurate identification of cancer cells in the FNA sample. Altogether, these results demonstrate the robustness and applicability of low-flow DESI-MS for high spatial resolution imaging of tissue sections, which could in the future potentially be implemented across a variety of biomedical and clinical studies.