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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?
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 $\sqrt{V}$.
