Abstract: A mass spectrometer for ion ratio detection. The ions are produced and initially focussed in the conventional way. After focusing, the ion beams are refocused by a magnifying focusing assembly which is located past the focal plane. The magnifying lens magnifies the beam spacing, and the focal plane. A series of staggered conversion dynode assemblies, each of which have a side entry that reflects the ions toward an electron multiplier, is located along the magnified focal plane. Each assembly receives one of the ion beams, and the others pass to the subsequent assemblies. The space between the bottom of each assembly and the slit into which the ion beam enters, must therefore be smaller than a spacing between beams.