Hand-Held Electrostatic Spray Ionization Mass Spectrometry Enables On-Site Metabolomic Analysis of Sophora japonica and Robinia pseudoacacia

Mass spectrometry plays a key role in agricultural and food chemistry, yet direct ionization of plant samples is complicated by endogenous salts and other matrix components that destabilize conventional direct-current nanoelectrospray ionization (DC-nanoESI). To address this limitation in point-of-need analysis, we developed hand-held electrostatic spray ionization (HESSI), which produces stable ionization from nanoliter sample volumes without direct electrical contact with the solution. A compact pulsed-discharge module and dielectric barrier limit electrochemical reactions and reduce the risk of discharge at the emitter while remaining compatible with salt-containing plant extracts. Approximately 50 nL of the sample sustained MS signals for up to 5 min. In a proof-of-concept application, 208 compounds were tentatively annotated from Sophora japonica and Robinia pseudoacacia extracts in negative-ion mode, and a neural network model distinguished the two groups with an accuracy of 83.3%. These results support HESSI as a compact and sample-efficient platform for the rapid mass-spectrometric characterization of botanical materials.