A mass spectrometer is an instrument that enables scientists to determine the relative atomic (or molecular) masses of substances present in a sample. In one type of mass spectrometry known as Electron Impact mass spectrometry, the sample is introduced into the spectrometer as a gas.The sample is first ionised by an electron beam to give one or more cations depending on the composition of the sample.
M(g) + e– 
M+(g) + 2e–
Cations with higher positive charges are formed to a much lesser extent due to the difficulty of removing an electron from a species that already bears a positive charge.
M+(g) + 2e– Cations with higher positive charges are formed to a much lesser extent due to the difficulty of removing an electron from a species that already bears a positive charge.
The cations formed in the ionisation are separated on the basis of their mass to charge ratio (m/z) through use of electric and magnetic fields.
Because the charge on most ions generated is +1, m/z is equal to the mass number of an atom (or the sum of the mass numbers for polyatomic species).
A mass spectrum shows the mass to charge ratio of the cations generated in the ionisation and the relative numbers of these present in the sample (% relative abundance). All abundances are expressed as a percentage relative to the most abundant ion which is assigned a value of 100%.
The mass spectrum of krypton gas shows that it consists of four isotopes because atoms with the mass to charge (m/z) ratio of 82, 83, 84 and 86 have been detected. The % natural abundance of the isotopes can be calculated from % relative abundances in the mass spectrum.
The ions formed in electron impact mass spectrometry are very reactive and short-lived, therefore it is important that the spectrometer be under high vacuum (less that one-billionth of atmospheric pressure).
Many substances that are not gases under ordinary conditions are vaporised at high vacuum especially if the sample is heated slightly through the filament present in the ionisation chamber. Therefore use of mass spectrometry is not limited to substances that are gases at ordinary pressures and temperatures.