We describe a simple experiment intended for didactic laboratory vacuum classes of undergraduate courses, using a residual gas analyser (RGA): the determination of the natural abundance of the different isotopes of a given element in a gas sample. In addition to the understanding of the RGA operation and handling principles, the experiment allows the students to perform direct quantitative measurements of isotopic abundances and compare with the results tabulated in the literature. As an example, sets of measurements are presented for xenon and neon using a quadrupole mass spectrometer. The experimental results obtained are in good agreement with the values presented in the literature.