Mass spectrometers

Mass spectrometer with an ion source 1 and a mass analyser 17. The ion may be directed along one of two paths between the ion source 1 and the mass analyser 17. Path 1 contains at least one ion optical element 7 with a periodic oscillating electric field to enable mass rejection of ions to occur. Ions travelling along path 2 are subjected to purely electrostatic forces so no mass discrimination occurs. The apparatus includes means 4 to direct beams of the ions along one or other path as desired. Such apparatus provides a reduction in the background ions in order to enhance the abundance sensitivity of the mass spectrometer while providing an alternative electrostatic ion path enabling the same apparatus to be used for analysis of low mass ions with minimal mass biasing. The mass spectrometer may have and inductively coupled plasma as the ion source, and the analyser may be of a double focussing magnetic sector type.