The existence of different isotopes of an element in a sample is primarily determined using a technique called mass spectrometry. Mass spectrometry is an important analytical tool in chemistry that indicates the mass of atoms or molecules in a sample. A sample is injected into the mass spectrometer (see the schematic to the right) and a stream of electrons bombards the particles in the sample. When this occurs, each particle ejects an electron and becomes a positive cation. The cations are then accelerated through a magnetic field onto a detector. The mass spectrometer generates a spectrum displaying various peaks corresponding to the mass-to-charge (m/z) ratio of the particles that were present in the sample. Usually, if an atom is analyzed by mass spectrometry, the m/z ratio is equal to the mass number (A) of the atom. For example, hydrogen-1 would be found at an m/z ratio of 1.
To learn how mass spectrometry differentiates between different isotopes of an element, open the Mass Spectrometer Learning Tool and work through the following steps.
- Select oxygen from the 'Select Element' submenu under the 'Options' menu.
- Leave the isotope ratios at the default settings (these are the ratios that are commonly found in nature) and click scan.
- When the scan is complete, view the spectrum. Note the very large peak at an m/z ratio of 16. This peak corresponds to the oxygen-16 isotope.
- Click the 'Magnifier' button and select the regions just above 17 and 18 to see the peaks at these m/z ratios. You may need to zoom in a few times. You will see a small peak at 18, produced by oxygen-18. Also, a very small peak should be visible at 17, corresponding to oxygen-17.
Each of the peaks in a mass spectrum of a pure element corresponds to an isotope of the element. Therefore, the number of peaks indicates the number of isotopes and the m/z ratio on the X-axis of the mass spectrum represents the atomic mass of each of the isotopes of an element. Use the Mass Spectrometer Learning Tool to explore how many isotopes three elements other than oxygen have.