Abstract: A mass spectrometer has an ion source in having two nested frusto-conical members between the narrow ends of which the splayed out end of an inlet pipe is disposed. The inner frusto-conical member is of mesh form and its wide end leads to the inlet of an ion filter. R.f. and d.c. are applied between the frusto-conical members. The ion filter has four elongate hyperbolic plates of conductive material supported within a tubular ceramics support member. The elongate hyperbolic plates are symmetrical with respect to an axis towards which they project and define a passage within the support member. The outlet of the filter leads to an ion detector.