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Abstract A technique capable of label-free detection, mass spectrometry imaging (MSI) is a powerful tool for spatial investigation of native biomolecules in intact specimens. However, MSI has often been precluded from single-cell applications due to the spatial resolution limit set forth by the physical and instrumental constraints of the method. By taking advantage of the reversible interaction between the analytes and a superabsorbent hydrogel, we have developed a sample preparation and imaging workflow named Gel-Assisted Mass Spectrometry Imaging (GAMSI) to overcome the spatial resolution limits of modern mass spectrometers. With GAMSI, we show that the spatial resolution of MALDI-MSI can be enhanced ~3-6-fold to the sub-micrometer level without changing the existing mass spectrometry hardware or analysis pipeline. This approach will vastly enhance the accessibility of MSI-based spatial analysis at the cellular scale. 

RationaleNew ionization processes have been developed for biological mass spectrometry (MS) in which the matrix lifts the nonvolatile analyte into the gas phase as ions without any additional energy input. We rationalized that additional fundamental knowledge is needed to assess analytical utility for the field of synthetic polymers and additives. MethodsDifferent mass spectrometers (Thermo Orbitrap (Q‐)Exactive (Focus); Waters SYNAPT G2(S)) were employed. The formation of multiply charged polymer ions upon exposure of the matrix/analyte(/salt) sample to sub‐atmospheric pressure directly from the solid state and surfaces facilitates the use of advanced mass spectrometers for detection of polymeric materials including consumer products (e.g., gum). ResultsAstonishingly, using nothing more than a small molecule matrix compound (e.g., 2‐methyl‐2‐nitropropane‐1,3‐diol or 3‐nitrobenzonitrile) and a salt (e.g., mono‐ or divalent cation(s)), such samples upon exposure to sub‐atmospheric pressure transfer nonvolatile polymersandnonvolatile salts into the gas phase as multiply charged ions. These successes contradict the conventional understanding of ionization in MS, because can nonvolatile polymers be lifted in the gas phase as ions not only by as little as a volatile matrix but also by the salt required for ionizing the analyte through noncovalent metal cation adduction(s). Prototypevacuummatrix‐assisted ionization (vMAI) and automated sources using a contactless approach are demonstrated for direct analyses of synthetic polymers and plasticizers, minimizing the risk of contamination using direct sample introduction into the mass spectrometer vacuum. ConclusionsDirect ionization methods from surfaces without the need of high voltage, a laser, or even applied heat are demonstrated for characterization of detailed materials using (ultra)high‐resolution and accurate mass measurements enabled by the multiply charged ions extending the mass range of high‐performance mass spectrometers and use of a split probe sample introduction device. Our vision is that, with further development of fundamentals and dedicated sources, both spatial‐ and temporal‐resolution measurements are within reach if sensitivity is addressed for decreasing sample‐size measurements. 

RationaleThe developments of new ionization technologies based on processes previously unknown to mass spectrometry (MS) have gained significant momentum. Herein we address the importance of understanding these unique ionization processes, demonstrate the new capabilities currently unmet by other methods, and outline their considerable analytical potential. MethodsTheinletandvacuumionization methods of solvent‐assisted ionization (SAI), matrix‐assisted ionization (MAI), and laserspray ionization can be used with commercial and dedicated ion sources producing ions from atmospheric or vacuum conditions for analyses of a variety of materials including drugs, lipids, and proteins introduced from well plates, pipet tips and plate surfaces with and without a laser using solid or solvent matrices. Mass spectrometers from various vendors are employed. ResultsResults are presented highlighting strengths relative to ionization methods of electrospray ionization (ESI) and matrix‐assisted laser desorption/ionization. We demonstrate the utility of multi‐ionization platforms encompassing MAI, SAI, and ESI and enabling detection of what otherwise is missed, especially when directly analyzing mixtures. Unmatched robustness is achieved with dedicated vacuum MAI sources with mechanical introduction of the sample to the sub‐atmospheric pressure (vacuumMAI). Simplicity and use of a wide array of matrices are attained using a conduit (inletionization), preferably heated, with sample introduction from atmospheric pressure. Tissue, whole blood, urine (including mouse, chicken, and human origin), bacteria strains and chemical on‐probe reactions are analyzed directly and, especially in the case ofvacuumionization, without concern of carryover or instrument contamination. ConclusionsExamples are provided highlighting the exceptional analytical capabilities associated with the novel ionization processes in MS that reduce operational complexity while increasing speed and robustness, achieving mass spectra with low background for improved sensitivity, suggesting the potential of this simple ionization technology to drive MS into areas currently underserved, such as clinical and medical applications.