3D-Printed Field-free Ionization Source for Mass Spectrometry

Abstract

Most ambient ionization methods for mass spectrometry require externally applied high-voltage fields to generate charged droplets, limiting their portability and energy efficiency. We report a fully three-dimensional (3D)-printed ionization source, in which a standalone atomizer functions as the droplet-generating ionizer by enabling spontaneous charging through gas–solid triboelectric interactions. The device features a coaxial flow configuration, in which high-velocity sheath gas interacts with a BaTiO₃-doped poly(lactic acid) (PLA) nozzle surface, generating interfacial charges that are transferred to emerging droplets. Computational fluid dynamics simulations confirm sustained wall shear stress near the outlet, supporting the proposed charge-generation mechanism. By tuning the nozzle’s dielectric composition and outlet geometry, droplet charge density and size can be precisely modulated up to 0.3 nC/μL under optimized conditions. Faraday cup measurements reveal a clear correlation between the structural parameters and total droplet charge. Using methyl viologen (MV²⁺) as a single-electron probe, mass spectra revealed efficient electron transfer and secondary product formation, confirming enhanced interfacial electron availability. These results demonstrate that triboelectric enhancement via a 3D-printed design enables voltage-free ion generation and controllable electron transfer, offering a structurally simple, low-cost, and power-free approach to ambient chemical analysis.