High resolution mass spectrometry can be used to:

• determine relative atomic masses by determining the relative abundances of different isotopes in a sample of an element
• determine the rmm of the molecule by identifying the peak with the biggest m/z ratio in a spectrum of a molecule

However, it is also possible to deduce the complete molecular formula when you examine the mass spectrum under high resolution.

For example, if the molecular ion peak of a compound is a m/z ratio of 58 in a mass spectrometer then it could be either C₄H₁₀ or C₃H₆O. However, these molecules do not share the same mass. The exact relative isotopic mass of each atom is as follows: