Mass Spectrograph
We can design an instrument in which we can control the magnetic field, B. If we ionize (almost always singly) the material, we know the charge, q. We can use known voltages to get a known speed for the ions, v. We can then have the beam circle in the field and hit a target, and from that we can measure the radius, r. Hence, we can then calculate the mass using r = mv/qB .