Ion Trap — Basics in Mass Spectrometry

Unlike a quadrupole or TOF where ions fly through, an ion trap holds ions in place inside an RF field, then releases them one mass at a time. There are two common geometries, shown below.

On this page: 3D Paul trap · Linear trap

1 · 3D Paul trap

A doughnut-shaped trap: a ring electrode with two end caps. Ions of all masses are held together, then the RF is ramped so they leave lightest-first. Two synchronized views — a cross-section (left) and a top view (right).

light ion → ejected first heavy ion → ejected last same event from two viewpoints

One device does everything — it both traps and separates. The trap fills with a mixture of masses, then a mass-selective scan ejects them in order toward the detector.

2 · Linear ion trap

Four rods like a quadrupole, but closed at both ends. Ions are held in a line along the axis and ejected sideways through a slot. A long cross-section (left) and an end view (right).

light ion → ejected first heavy ion → ejected last ions held in a line, ejected sideways by mass

The linear trap holds ions along a line instead of a single point, so it can store more ions and offers higher sensitivity — which is why it is common in modern hybrid instruments.

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