Question

A Time of Flight Mass Spectrometer is a powerful tool utilizing electric fields to determine the mass of ions emitted/ejected from the surface of a sample. As shown in figure 1 below, a laser can be focused onto a sample of ejected ions can be accelerated by an electric field to a high transparency grid after which they continue with a constant velocity to the detector.
If the sample -> grid distance is 10mm and a potential difference of 50 volts is applied between them,
(i) calculate the electric field accelerating the ions.
If negative ions are to be detected as n figure 1 and the grid is grounded.
(ii) what is the potential of the sample with respect to ground?

Answer 1

i) Electric field$|\overrightarrow{E}|=\frac{\Delta V}{d}$

$|\overrightarrow{E}|=\frac{50}{10\times 10^{-3}}V/m$

$\Rightarrow |\overrightarrow{E}|=5\times10^{-3}V/m$

ii) Potential of sample with respect to ground is 50V because grid is grounded.