Abstract: The invention relates to an electron impact gas ion with high brightness and low energy spread. This high brightness is achieved by injecting electrons in a small ionization volume (from less than 1 ?m to several tens of micrometers in size) from one side and extracting ions from the other. The electrons injected are produced by a high brightness electron source, such as a field emitter or a Schottky emitter. In one embodiment of the invention the required high electron density in the ionization volume is realized by placing a field emitter close to the ionization volume (e.g. 30 ?m), without optics between source and ionization volume. In another embodiment of the invention the source is imaged onto a MEMS structure. Two small diaphragms of e.g. 50 nm are spaced e.g. 1 ?m apart. The electrons enter through one of these diaphragms, while the ions leave the ionization volume through the other one. The two diaphragms are manufactured by e.g.