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We report a gas ionizer based on arrays of microfabricated double-gated isolated vertically aligned carbon nanofibers (VA-CNFs) for application in low-power portable mass spectrometers. Field-emitted electrons from VA-CNFs are accelerated to high energy and subsequently collide with neutral gas molecules, leading to ionization/fragmentation of the molecules. Double-gated field-emitter arrays with isolated VA-CNF tips were fabricated using a photoresist planarization technique. Two types of devices were fabricated and characterized. The first type of device has the emitter tip in the same plane as the extraction gate, and the second type of device has the emitter tip 900 nm below the extraction gate. All devices were made using a process that results in gate and focus diameters of 1.7 and 4.2 μm, respectively. When operated as a field-emitted electron impact ionizer (EII), for the same ion current, the ionization efficiency (ratio of ions to emitted electrons) increased from 0.005 to 0.05 as the pressure is increased between 5×10-6 and 1×10-3 torr. In comparison with EIIs based on thermionic electron sources, the power dissipation reduced from >;1 W to 100 mW.