Mass Spectrometry and Relative Atomic Mass

Because most elements occur naturally as a mixture of isotopes, the "mass" of an element listed on the periodic table is not the mass of any single atom but a weighted average. The relative atomic mass, $A_r$, is defined as the weighted mean mass of the atoms of an element relative to $1/12$ the mass of a carbon-12 atom. Carbon-12 is the reference standard, assigned a mass of exactly 12 atomic mass units. Because $A_r$ is a ratio of masses, it has no units. To find it we need two pieces of information for each isotope: its mass and its relative abundance, the fraction of atoms in a natural sample that are that isotope.

The instrument that provides this information is the mass spectrometer. In a modern time-of-flight mass spectrometer, a sample is first vaporized and then ionized, commonly