When a molecule in the gas phase passes through a strong electrostatic field (up to 108 V/cm) it may lose an electron and yield a positively charged ion. The field is established by applying a potential difference of about 10 kV between a sharp metal edge (which may be activated by growth of an array of closely spaced microneedles) as the anode, and the cathode, which has an exit slit and is situated 0.5-2mm away from the anode.