Abstract: The invention provides a mass spectrometer comprising an ion source provided with an electron emitting source and magnets which are cooperable to produce a collimated electron beam within the ion source; a mass analyzer; first and second electrodes which cooperate to limit the angular divergence of the ion beam which emerges from the source along the ion beam axis; and magnetic field screens disposed between the first and second electrode means, which reduce the field due to the magnets along the ion beam axis. In this way the mass discrimination introduced by the magnets in prior ion sources is reduced and the accuracy of isotropic ratio measurements is improved.