In improved mass spectrometers of the double focussing zero second-order aberration type with first-order spectrograph properties, parameters are related by specific equations as a result of which the five aberration coefficients B.sub.1, B.sub.2, B.sub.11, B.sub.12 and B.sub.22 can all be simultaneously zero.

A double focusing mass spectrometer having a diverging electrostatic field, a converging electrostatic field and a converging magnetic field. The two electrostatic fields are connected with each other without substantial free space therebetween. The ion beam passes through the electrostatic fields coming to an intermediate focus point adjacent to the ion exit boundary of said converging electrostatic field. The beam then passes through the magnetic field to satisfy the double focusing condition in combination with the electrostatic field. Very small image magnification and aberration free focusing are obtained by this mass spectrometer.

Ions emitted from an ion source are dispersed by a dispersion means, and those ions of a predetermined mass number are extracted from the thus-dispersed ions and are selectively implanted into a sample. A plane of incidence of ions in the mass-dispersing means is so formed that the angle of incidence of ions thereon has a negative value, and a plane of emission of ions therein is so formed that the angle of emission of ions of the predetermined mass number therefrom has a positive value.

A magnetic sector for a non-scanning mass spectrometer includes a high permeability yoke with opposing faces to which are attached high energy product magnets and shaped pole pieces separated by a gap so that a high magnetic flux exists in the gap. The high magnetic flux in the gap enables very small surface areas of the pole pices faces forming the gap so that the overall magetic sector volume and weight are reduced.