A mass spectrometer, more particularly for simultaneously measuring beams of a number of species of ions, has a special homogeneous magnetic sector field, the exit boundary of said field forming a straight line which extends through the point of intersection between the central ray of the incident object-ray pencil of ions and the straight entrance boundary of the sector field, and at least one of the emergent image-ray pencils of ions undergoing second-order directional focusing. In this spectr...