MAGNETIC-SECTOR MASS SPECTROMETER
质谱仪 · LORENTZ-FORCE ION OPTICS · LIVE SIMULATION
mass spectrum — m/z
drag to orbit · pinch / ⌘-scroll to zoom
Sample
How a mass spectrometer works
- Ionise. A hot filament boils off electrons that knock electrons off the sample atoms, leaving positive ions of charge q. An element's isotopes carry the same charge but slightly different masses m.
- Accelerate. The ions fall through a voltage V, so each gains the same kinetic energy: qV = ½mv². Lighter ions therefore come out faster — v = √(2qV/m).
- Bend. In the magnet the Lorentz force q·v×B stays perpendicular to the velocity, so it curves the ion into a circle. Setting it equal to the centripetal demand, qvB = mv²/r, gives the radius r = mv/(qB) = √(2mV/q) ⁄ B. Heavier ions swing wider.
- Separate. After a half-turn the beams land back on the entry line, spaced apart by mass — the isotopes are physically resolved. Aston's plate caught neon at mass 20 and 22, the first proof that one element can have isotopes.
- Detect. A photographic plate records every mass at once; a single Faraday cup instead sits at a fixed radius while B is swept, so each isotope crosses it in turn and paints the peak spectrum you know today.
Same energy, different speed, different radius. Because every ion of charge q gains the same energy qV, the magnet sorts purely by m/q: the landing position scales as √(m/q). Raise B to pull the spectrum inward, raise V to push it out.
Try: switch to xenon for a forest of isotopes · drop the field and watch the heavy ions overshoot the plate · open the energy spread and see the peaks blur · run a Faraday sweep.
Try: switch to xenon for a forest of isotopes · drop the field and watch the heavy ions overshoot the plate · open the energy spread and see the peaks blur · run a Faraday sweep.
Pick a sample · drag the field · sweep B · drag to orbit · ⌘/Ctrl + scroll to zoom