I was reading an article on the AP Chemistry course in Khan Academy about mass spectrometry, which had this:

... ions are then accelerated through electric plates and subsequently deflected by a magnetic field

My question is, why do the ions need to be accelerated? Why can they not move without acceleration?

  • 5
    $\begingroup$ Ions need to reach the detector, not just dwell in ionization unit. Plus there are numerous analyzers such as ToF-MS that utilize velocity of accelerated ions. $\endgroup$
    – andselisk
    Oct 5, 2022 at 6:19
  • $\begingroup$ @andselisk as porphyrin points out thermal velocity already guarantees non-dwelling; it's mass resolution that drives the need for a well-defined and carefully controlled acceleration. There are no mass analyzers that don't use velocity, be it free motion in a vacuum or diffusion in a gas, liquid, gel, or other medium. $\endgroup$
    – uhoh
    Oct 7, 2022 at 23:54
  • $\begingroup$ People are not generally mind-readers, so it is difficult sometimes to understand where the confusion that leads to a question lies. In this case you ask "why can they not move without acceleration?" Ions in a gas do move without needing to accelerate them with an external potential source. The reason for the plates has to do with separating different ions. If you take a gas consisting of a mixture of different types of molecules, how to know which are there (without using chemical reactions)? $\endgroup$
    – Buck Thorn
    Mar 8, 2023 at 7:34
  • $\begingroup$ @BuckThorn so if I edit my question to remove the part where I ask why they can't move with acceleration, and just keep the first question, can my question be considered for reopening? $\endgroup$ Mar 8, 2023 at 10:48
  • $\begingroup$ The use of plates (and the associated electric and magnetic fields) to accelerate and deflect charged particles is what characterizes mass spectrometry as a mass determination tool. Your question is a bit like asking, "why do we need gravity to measure the mass of an object with a balance?" There are in fact other ways to determine mass, it just happens that mass spec uses magnetic and electric fields in a clever way to provide us with a powerful tool. There are other ways of measuring average molecular mass of a gas, for instance by measuring effusion en.wikipedia.org/wiki/Effusion $\endgroup$
    – Buck Thorn
    Mar 8, 2023 at 15:28

1 Answer 1


The field helps capture almost all of the ions and accelerates them towards the detector. These ions initially have a statistical distribution of velocities (due to Boltzmann/thermal distribution plus inhomogeneities in the source) and this can lead to wide mass peaks. Accelerating the ions (with the same charge $z$) gives them approximately the same kinetic energy, independent of mass, as $KE=eV=(1/2)mv^2$ where $V$ is the acceleration voltage (typically a few thousand volts) and $v$ the velocity on leaving the accelerating plate. The heavier ions move more slowly than lighter ones and thus become separated in proportion to $m/z$ while moving along the drift tube. The speed is now such that the initial distribution becomes a smaller fraction of the kinetic energy than it was and is therefore less important. When entering the magnetic field they are deflected by differing amounts (i.e. follow a different radius) and so strike the detector in different places. The change in radius $\delta r$, and so splitting of mass peaks, is proportional to $\sqrt{V}$.

  • $\begingroup$ Just a caveat: though perhaps not as prevalent in a typical analytical lab, there are also spherical electrostatic and electric quadrupole mass spectrometers that don't use magnetic fields; they disperse as $\Delta E / E$ rather than $\Delta p / p$ for magnetic dispersion. $\endgroup$
    – uhoh
    Oct 7, 2022 at 23:49
  • 1
    $\begingroup$ @Uhoh and of course also time of flight $\endgroup$
    – porphyrin
    Oct 9, 2022 at 16:10

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