About this model
This simulator sketches a classic sector mass spectrometer in two spatial dimensions. The first stage is a velocity selector: uniform electric and magnetic fields are perpendicular to each other and to the nominal beam direction. For a positive ion moving along +x with velocity v, the electric force qE and magnetic force q(v×B) oppose along the transverse direction when E, B, and v are oriented as in standard textbook diagrams; straight-line motion requires v = E/B (magnitudes, with consistent sign conventions). Ions with the wrong speed are deflected and do not pass cleanly through the slit. After selection, ions enter a region with magnetic field only (idealized, uniform). The Lorentz force bends them into circular arcs with radius r = mv/(qB) for perpendicular injection—so at fixed q and selected v, heavier isotopes follow larger radii and strike a position-sensitive detector at different locations. The animation integrates Newton’s law with the Lorentz force for two species sharing the same q but different masses. Pedagogical simplifications include uniform box-like fields with no fringe effects, no collisions, non-relativistic speeds, and schematic units; real instruments add electric sectors, time-of-flight stages, quadrupoles, and much more elaborate optics.
Who it's for: Undergraduate students learning charged-particle motion, velocity filters, and the basics of mass analysis in chemistry and physics laboratories.
Key terms
- Velocity selector
- Lorentz force
- Cyclotron radius
- Mass-to-charge ratio
- Magnetic sector
- Crossed fields
- Ion trajectory
- Mass spectrometry
Frequently asked questions
- Why must the velocities be matched to E/B before the sector?
- The sector radius r = mv/(qB) depends on v as well as m. Without a selector (or another way to fix v), ions of different masses and different energies would land on top of each other, confounding the mass measurement. The crossed-field stage enforces a narrow band of speeds.
- Does the simulator include electric-sector focusing or quadrupoles?
- No. Those elements improve beam quality and resolution in real spectrometers. Here only the minimal physics lesson—selector plus uniform B bend—is modeled.
- What happens when I move the v/(E/B) slider away from 1?
- That mimics a mistuned selector: ions feel a net transverse force in the crossed-field region, so trajectories curve before the magnetic sector, illustrating why matching v ≈ E/B matters.
- Are the radii labeled “estimate” exact?
- The readout r ≈ v/((q/m)B) is the familiar perpendicular-entry formula in pure B. The numerical trajectory also includes the transition from the selector’s combined fields into the sector, so small differences from an ideal semicircle are expected.