What is a use of mass spectrometry?
A Mass Spectrometer allows the analyst to determine the isotopic content of a sample. It does not determine the elemental content. It sees the atomic weight of the ion. If it sees atom mass of 204, it doesn’t know if it is Pb204 or Hg204. It is the job of the analyst to determine if it is lead or mercury. This is done by looked at the results for the masses that are lead or mercury. But all that goes beyond the scope of your question.