A mass spectrometer is able to very accurately determine the relative atomic mass of an element by measuring the mass and abundance each of the isotopes of an element in a sample (and this may vary depending on where the element sample has come from). We can also use the technique to help identify organic molecules.
Here you will find links to everything you need to know for A level Chemistry 😊.
Time of flight mass spectrometer including ToF calculations
Calculating the relative atomic mass of an element from a mass spectrum
Calculating the relative abundance of isotopes
Interpreting the mass spectrum of a molecule