The mass of an element or compound is determined experimentally by using a mass spectrometer. A sample of the substance is injected into the mass spectrometer and ionized by an electron beam. The resulting cations are then pulled into the analyzing chamber using a powerful electric field (accelerating plates).

A magnetic field perpendicular to the path of the ion beam is then used to deflect the cations into the detector. Based on the reading from the detector, a computer displays the results (computer output). A peak is displayed for all masses observed. If multiple isotopes are present, a separate peak will be displayed for each. In addition, the height of the peak indicates the abundance of the ion.