This Demonstration models a mass spectrometer with a sector-type analyzer. After undergoing ionization, fragments of molecules are accelerated through an electric potential and enter a magnetic field. The fragments then follow curved paths through the magnetic field, depending on their charge to mass ratios, and are detected as signals on the analyzer. The curvature is proportional to . You can vary the electric and magnetic field strengths and see the effect on the trajectory of an arbitrary particle of selected charge and mass. You can also choose to observe the mass spectrum of one of several preset molecules.