We investigates ionic clusters, assemblies of ions and molecules, using mass spectrometry. We couple electrospray ionization (ESI) with ion mobility spectrometry–mass spectrometry (IMS–MS) to generate weakly bound ionic clusters in the gas phase and to quantify collision cross sections (CCS), which provide conformational information of ionic clusters. By integrating CID/MSⁿ experiments with quantum-chemical calculations (DFT) and trajectory-method CCS predictions, we predict the structures and thermodynamic stability of ionic clusters and host-guest complexes including alkali halide ions, amino acids, saccharides, and N, O-donor ligands. Together, these measurements provide reproducible structural constraints that inform electrolyte design, ion transport, and supramolecular assembly across solution, interfacial, and gas-phase regimes.

The structure–function relationship is a fundamental principle underlying all biological systems. Consequently, the study of the molecular structures of biological macromolecules, with a particular focus on proteins and nucleic acids, is essential for comprehending biological processes. Since the 20th century, the development of mass spectrometry (MS) has encompassed techniques such as electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI), enabling the transfer of delicate biomolecules from solution into the gas phase. In our research, we couple MS with ion mobility spectrometry (IMS) to obtain additional information called the collision cross section (CCS), which reflects the surface area of biomolecules. This CCS has been demonstrated to serve as an indicator of structural preservation or unfolding. By employing it as an index, it is possible to interpret structural stability and features that were previously inaccessible.

Charged micro- and nano droplets generated in electrospray ionization (ESI) form during solvent evaporation. These charged droplets possess large surface-to-volume ratios, high surface charge densities, and strong interfacial electric fields that create pH/composition gradients, enrich reactants, and accelerate interfacial chemistry. As a result, product distributions may deviate from bulk behavior in some systems—for example, regio-, chemo-, or enantioselectivity can shift, and protomer/tautomer ratios may differ. We use ESI–ion mobility–mass spectrometry, which resolves isomeric and protomeric ions by their collisional cross sections, together with tandem mass spectrometry to obtain structure-specific fragments, and we interpret these measurements using density functional theory to propose and evaluate candidate structures and protonation sites. Studying chemical behavior in charged droplets matters because it can change the signals we measure and sometimes allows reactions that don’t appear in bulk; measuring these effects makes ESI–MS results more trustworthy and helps us control selectivity.

We have been developing SLIM(Structures for Lossless Ion manipulations)-based, high-resolution ion mobility platforms that enable IMS-photo-IMS experiments for structure-selective analysis in the gas phase. Using a few meter-long ion mobility paths and switching components, ions with specific strcutrues are captured in the trap region. A tunable laser introduced between the tandem IM stages drives wavelength-dependent photodissociation and photoisomerization, creating diagnostically useful product ions. By reinjecting photo-induced products into a second IM separation and coupling to MS, we obtain accurate collision cross sections, isomer-resolved photochemistry, and unambiguous connectivity between precursor and product ions. This workflow clarifies 3D structures and dynamics of biomolecular ions, ionic clusters, and reactive intermediates that are difficult to resolve by IM–MS alone.

We also pursue a broad range of mass spectrometry–based studies. These include developing new analytical methods to observe and quantify large clusters such as nanoclusters and quantum dots(QD). Photochemistry-mass spectrometry enables characterizing photoisomeric species. We are doing comprehensive mass spectrometric analyses and structure elucidation of diverse organic/inorganic molecules and clusters.