time-of-flight
/ time-of-FLITE /
Imagine firing a starting pistol and releasing runners of different weights down a hallway, each given the same shove. The light runners reach the far wall first; the heavy ones arrive last. If you just measure the arrival time of each runner, you can rank them all by weight. A time-of-flight analyzer does exactly this with ions.
In a time-of-flight (TOF) mass analyzer, all ions are accelerated by the same electric push so they carry the same kinetic energy, then sent down a field-free tube. Lighter ions (lower m/z) travel faster and reach the detector sooner; heavier ions arrive later. By precisely timing each arrival, the instrument converts flight time into mass-to-charge ratio.
It matters because TOF analyzers can record an enormous mass range very quickly and, in modern reflectron designs, with high resolution and accurate mass — making them workhorses for large molecules and for fast coupling to separations. The caveat is that timing must be exquisitely precise: tiny spreads in starting position or energy blur the arrival times, so good resolution depends on careful instrument design.
In a MALDI-TOF instrument, a laser pulse launches ions, a clock starts, and an ion of m/z 5000 strikes the detector a few microseconds after one of m/z 1000 — that timing gap is read straight off as their mass difference.
Lighter ions fly faster, so flight time encodes mass-to-charge ratio.
Time-of-flight pairs especially well with pulsed ionization like MALDI, since each laser pulse gives a clean start time. Modern reflectron and multi-pass designs lengthen the effective flight path to sharpen resolution without making the instrument longer.