Most elements are present as a number of isotopes.
The atomic mass given on the periodic table for a given element is the average of the isotope masses weighted according to their relative abundances. In the laboratory the relative abundances of the isotopes for a particular element can be measured using a mass spectrometer. The instrument that we will use has the mass spectrometer coupled with a gas chromatograph. The sample is injected onto the top of a heated column where the mixture is vaporized. This causes each molecule to lose an electron, forming a positively charged molecular ion.