Atomic mass
The atomic mass and relative isotopic mass are sometimes confused, or incorrectly used, as synonyms of standard atomic weight (also known as atomic weight) and the standard atomic weight (a particular variety of atomic weight, in the sense that is a standardized atomic weight). However, as noted in the introduction, atomic weight and standard atomic weight represent terms for (abundance-weighted) averages of atomic masses in elemental samples, not for single nuclides. As such, atomic weight and standard atomic weight often differ numerically from relative isotopic mass and atomic mass, and they can also have different units than atomic mass when this quantity is not expressed in unified atomic mass units (see the linked article for atomic weight).
Mass spectrum
Early in mass spectrometry research the resolution of mass spectrometers did not allow for accurate mass determination. Francis William Aston won the Nobel prize in Chemistry in 1922. "For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the Whole Number Rule." In which he stated that all atoms (including isotopes) follow a whole-number rule This implied that the masses of atoms were not on a scale but could be expressed as integers. (In fact multiply charged ions were rare, so for the most part the ratio was whole as well.) There have been several suggestions (e.g. the unit thomson) to change the official mass spectrometry nomenclature formula_2 to be more internally consistent.