Function: Mass spectrometers determine the abundance of ISOTOPES.An electron gun ionises the atom by firing high-speed electrons at it, causing the atom to lose electrons. The ions have lost an electron and are positive, so they are highly attracted to the negatively charged electric plate. Because of this high attraction, the ions pass through the electric plate at high speed and as a beam. The ions are then deflected by the magnetic field in the direction of the detector, which detects the ions and measures their mass over charge. This is usually expressed like this; (an example)

This graph is an example for the spectrometer reading for chlorine (CL {I know what youre thinking tho~}). The results are in the form of a line graph, with the mass/charge on the x-axis, and the abundance up the y-axis.

See how that happened?Basically, just multiply the mass number of each isotope by its abundance (%) number and divide by 100.