Virtual image type double focusing mass spectrometer

A double focusing mass spectrometer incorporating a diverging electrostatic field and a converging magnetic field through which an ion beam to be analyzed passes comprising:

• a. an ion source for producing an ion beam;
• b. inner and outer electrodes for producing the diverging electrostatic field, positions within said field being defined by the cylindrical coordinates r, .phi. and z, the central ion orbit having a radius r.sub.e in the z=0 plane, equipotential lines within said field in the z=0 plane being defined by a particular value of r, equipotential lines within said field in an r-z plane which contains the z axis and thereby being perpendicular to the central ion orbit being defined by a radius R, said field satisfying the condition ##EQU1## the ratio of the radius r.sub.e of the central ion orbit and R.sub.e being the radius of curvature of an equipotential line in the r-z plane passing through the central ion orbit satisfying the following range
• 2.