v is applied by means of voltage source v 1 to the cathode 53 relative to the anode 54 by use of the switch s the fence electrode 55 is caused to be at either u2212 190 v or 0 v relative to the anode 54 ions are generated in the sample chamber gaseous space approximately depicted by the area 52 positively charged ions are picked up by the cathode 53 whilst negative ions are picked up by the anode 54 fence electrode 55 provided by the invention is located between them electrodes 53 54 and 55 are conveniently made of an electrically conducting corrosion resistant sheeting such as 1 5 mm thick stainless steel sheet etched so as to contain slats as shown in fig4 cathode slats 53 a are of approximately 0 5 mm in width whilst other slats are 0 15 mm in width ptfe members 56 and 57 ensure separation between electrodes 53 and 55 of 0 25 mm and between electrodes 54 and 55 of 0 75 mm positive ions generated in the vicinity of the gaseous space 52 are caused to migrate to cathode 53 by virtue of its negative potential relative to electrodes 54 and 55 the distance between the cathode 53 and the transmitting material 51 is optimally no more than 2 mm and the voltage applied by source v 1 is of the order of u2212 200 v the fence electrode 55 includes slats 55 a located directly below corresponding slats 53 a in the cathode and the slots 54 a between slats in the anode.