Magnetic Sector Mass Spectrometer

Instrument Settings

Accelerating voltage V 2000 V

Magnetic field B 0.30 T

Ion Source Mixture

Live Readouts

Detector radius R

12.0 cm

In focus (m/z)

—

Ions launched

Ions detected

How it works

Ions are first accelerated from rest through a potential difference V. All the electrical work goes into kinetic energy:

qV = ½mv²  →  v = √(2qV / m)

The beam then enters a region with a uniform magnetic field B, perpendicular to the ion velocity. The magnetic (Lorentz) force supplies exactly the centripetal force for circular motion:

qvB = mv² / r  →  r = mv / (qB) = √(2mV/q) / B

Combining the two: the radius of curvature depends only on the mass-to-charge ratio m/z (and the tuned V, B) — not on the ion's charge sign convention or its starting angle. A 180° sector has a special property called direction focusing: any two ions of the same m/z that enter with slightly different angles reconverge after exactly half a circle, which is why a narrow detector slit placed on the source's axis, at 2r from the entry point, can cleanly separate masses.

Voltage / Field sliders — change v and r for every ion in flight; only ions whose radius matches the fixed detector radius R make it through the slit.
Ion source mixture — toggle which species (by real m/z) the source emits.
Auto-scan — sweeps B from low to high while the source fires continuously, reproducing how a real scanning sector instrument records a mass spectrum: a peak in detector counts each time B brings a species into focus.

This is the original mass-analyzer geometry (Dempster/Nier-type 180° and 60–90° sectors) still used today in isotope-ratio and noble-gas mass spectrometers, and the same r = mv/(qB) relation underlies every magnetic-sector instrument in use.

He⁺ (m/z 4) H₂O⁺ (m/z 18) N₂⁺ (m/z 28) Ar⁺ (m/z 40) CO₂⁺ (m/z 44)

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