A method of fabricating miniature quadrupole electrostatic mass filter has been previously described. The electrodes are metallised cylinders, mounted in grooves etched in oxidised silicon substrates, which are held apart at the correct spacing by cylindrical spacer rods. This invention concerns an ion source mounted on extensions of the spacer rods, which project beyond the mass filter. The ion source consists of a cold-cathode electron emitter, which emits electrons with energies sufficient to cause impact ionisation, and electrostatic optics suitable for coupling the ion flux into the mass filter. Methods of constructing a single self-aligned electron source and a similar dual source are described. Arrangements for mounting the electron source and the ion coupling lens so that the electron and ion beams travel at right angles to one another for efficient separation are described. A method of fabricating a self-aligned one-dimensional einzel electrostatic lens from metallised cylinders mounted in the silicon substrates using etched grooves is described. A method of fabricating a-self-aligned two-dimensional einzel lens from metal plates is also described.