Mass spectrometers

Mass spectrometer apparatus is described in which means are provided to direct the ions along one of two paths between an ion source 1 and a mass spectrometer 10. One path (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 the other path (Path 2) are subjected to purely electrostatic forces so no mass discrimination occurs. The apparatus includes means 4 to direct the ion beam along one or other path as desired. Such apparatus provides a reduction in the background ions to enhance the abundance sensitivity of the mass spectrometer while providing an alternate electrostatic ion path to allow analysis of low mass ions with minimal mass biasing. The apparatus also increases the sensitivity of the mass spectrometer by reducing the argon background caused by the use of an argon gas in the ICP ion source. The invention is of particular value in ICP-MS arrangements. The first path may have a high pass quadrupole 7, a radiofrequency controlled collision cell 16 or both, or alternatively a mass resolving quadrupole, a notch filter quadrupole, a radiofrequency controlled hexapole or higher order multipole.