How are isotopic abundances determined experimentally?
Thanks for your question about the relative abundance of isotopes.
A mass spectrometer can be used to determine the masses of elements.
Let's say you wanted to test some magnesium. Pure magnesium can be placed in the mass spec. Peaks will let you know what masses are present. The heights of the peaks tell you the amounts of each mass. Please keep in mind that isotopes of an element differ in mass only (because of differing number of neutrons). All magnesium atoms will have 12 protons, but they do not have to have the same number of neutrons.
I was trying to find a good image that would show this but so far not. If you have access to a textbook on line, try looking in the sections for mass or isotopes. Perhaps they will have a good image.