Low-cost, additively manufactured electron impact gas ionizer with carbon nanotube field emission cathode for compact mass spectrometry

Abstract

We report the design, fabrication, and experimental characterization of the first additively manufactured electron impact gas ionizer for compact mass spectrometry in the literature. The device occupies a total volume equal to 2.3 cm3 and is composed of a carbon nanotube (CNT) field emission electron source and a multi-material, 3D-printed multi-electrode ion-generating structure. The ionizer’s electron source is a silicon chip coated with a forest of plasma-enhanced chemical vapour deposited CNTs as emitting substrate and a 100 µm thick stainless-steel stencil with thousands of 100 µm diameter apertures at 150 µm aperture pitch as extraction gate. The device’s ion-generating structure is a set of 3D-printed, finely featured dielectric and metallic parts that kindle interaction between electrons and neutrals and shepherd ions outside the structure; its dielectric components were printed in polymer via high-resolution digital light projection stereolithography (25 µm pixels), while its metallic pieces were binder inkjet-printed in SS 316L with features as small as 340 µm (thickness of the wires of the ion cage). The CNT field emission electron sources were experimentally characterized in high vacuum (2.0 × 10−7 Torr), emitting up to 1.4 mA @ 700 V with 31.5% gate transmission. The ionizers were characterized in air at pressures as high as 5 mTorr, generating up 38 µA ion current with 8.5% ionization efficiency. The ion current is linear with pressure, in agreement with the electron impact ionization model.