A voltage turns mass differences into arrival differences
A packet of positive ions enters the space between extraction electrodes. An electrical pulse accelerates the packet through an opening into a nearly field-free vacuum tube. A detector waits downstream.
Equal charges crossing the same voltage drop gain equal electrical energy. A heavier ion moves more slowly at a given energy and takes longer to cross the tube. The detector converts the arriving impacts into electrical signals, spreading the mixed packet into a time spectrum.
The starting region has a size
Some ions begin closer to the exit than others. Their initial velocities can point forward or backward. Acceleration changes those velocities, and crossing the extraction gap takes time of its own.
An ion starting farther back may gain more energy and later catch a neighbour. In 1955, Wiley and McLaren analysed a two-region source whose fields reduce part of the arrival spread caused by different starting positions. The organization of the packet begins before the drift tube.
Sources: [2]
Known ions calibrate the clock
An electronic trigger does not establish an electric field instantaneously. Cables, detectors and timing circuits introduce further delays. Treating the displayed zero as the physical departure time can shift the entire mass scale.
Separate acquisitions of known mass-to-charge references determine the delay and time scale. Other known ions, excluded from that fit, test whether the calibration predicts their arrivals. Unknown events then use the frozen calibration.
Voltage and geometry matter too. A voltage change alters ion speed, and thermal expansion changes the flight distance. Saving the actual voltage readback and reference arrivals helps separate instrument drift from a changing sample.
An ion mirror delays the faster ions
Ions of one species can leave the source with different energies. Faster members arrive earlier and broaden the peak. A longer straight tube increases their separation as well as the flight time.
A retarding electric field slows the ions, converts kinetic energy into potential energy and sends them back. More energetic ions penetrate farther and spend longer in the mirror. A suitable field and drift length compensate for part of the speed difference. This time focus comes from the extra physical trajectory.
Position, width and area answer different questions
Peak position gives mass-to-charge ratio. The width measured at half the peak height is its full width at half maximum. A smaller width helps neighbouring components separate. Mass accuracy asks how close the position is to a known value. A narrow shifted peak still needs calibration.
Peak area counts recorded events. Ions can hit apertures, miss the detector or arrive during its recovery. Independent charge references check efficiency before counts can support an abundance estimate. A stronger beam can increase the losses.
Different ions can share one arrival peak
Doubling both mass and charge leaves their ratio unchanged. Under the same launch conditions, those ions can arrive together. Distinguishing their masses or chemical identities requires separate charge-state or other experimental information. The flight spectrum retains their common ratio.