producing electrode 16 are metal discs of a relatively fine mesh or grid construction which is sized to permit ions and electrons to permeate or pass through them typically the ions and electrons pass through the mesh or grid electrodes 12 14 16 with about 85 efficiency the spectrometer 10 includes voltage source means 30 whereby the electrodes 12 14 16 can independently be charged to a pre determined potential such as a high positive or negative voltage whereby the ions are influenced by the electric field so produced to oscillate between the reflector electrodes 12 and 14 that is the reflector electrodes 12 and 14 provide electric fields to reflect the ions to and fro therebetween assisted by the potential on electrode 16 to attract the ions other components of the spectrometer 10 are an electronic signal amplification and digitisation means 32 connected to the detector 18 and a computer 34 for controlling and synchronising the operation of the spectrometer 10 and for processing the detected signals and providing an appropriate mass analysis output operation of the spectrometer 10 is illustrated by fig2 a to 2 d fig2 a represents the trajectories of the positive ions and fig2 c represents the trajectories of electrons the plots of fig2 b and 2d each show the one dimension electric potentials drawn to show that the ions or electrons travel u201c downhill u201d that is positive ions travel towards a negative potential and electrons travel towards a less negative potential any positive ion or electron striking the channeltron detector 18 will produce an amplified current signal to.