NS Notes / Basics in Mass Spectrometry / Time-of-Flight

Mass Analyzer · 2 / 3

Time-of-Flight (TOF)

Separating ions by how long they take to fly · from the basic linear tube to reflectron and spiral designs

In a time-of-flight analyzer, all ions are accelerated with the same energy. Since kinetic energy is E = ½mv², lighter ions end up faster and heavier ions slower, so they separate by arrival time as they fly down a field-free tube. TOF does not throw any ions away — it detects them all, just at different times.

On this page: Linear · Reflectron · Spiral

1 · Linear TOF
The basic design: one straight tube. A packet of ions is released together; light ions (small dots) pull ahead of heavy ions (large dots) and reach the detector first.
a fresh packet also releases automatically each cycle
light ion → fast, arrives first heavy ion → slow, arrives later all ions reach the detector — only timing differs

The relationship is t ∝ √(m/z) — a mass four times larger takes twice as long to arrive. But real ions of the same mass leave with a slight spread of energies, so they arrive a little apart and the peak is broadened. The reflectron fixes exactly this.

2 · Reflectron
An ion mirror corrects the energy spread. Two ions of the same mass but different energies enter; the faster one penetrates deeper into the mirror and spends longer there, so both reach the detector together and the peak sharpens. Toggle the mirror off to see the difference.
same mass, different starting energy
higher energy → faster lower energy → slower

The mirror does not make different masses arrive together (that is impossible and would break TOF). It brings ions of the same mass but different energy back into step — the extra distance inside the mirror exactly offsets the extra speed. The result is much higher resolution.

3 · Spiral TOF
To resolve masses even better, the flight path is folded into a long figure-8 spiral, giving a very long flight distance in a compact space. The longer the flight, the more the masses separate.
5
light ion → laps ahead heavy ion → falls behind folded path = long flight distance in a small space
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