This invention provides a method of aligning a capillary needle, a set of electrodes, and an input to a mass spectrometer. The electrode system is formed as an assembly of two separate chips and forms an ionisation device. Each chip is formed on an insulating plastic substrate. The first chip carries mechanical alignment features for the capillary electrospray needle together with a set of partial electrodes. The second chip carries a set of partial electrodes. The complete electrode system is formed when the chips are assembled in a stacked configuration.