The milli mass unit or (mmu) is used as a unit of mass by some scientific authors even though this unit is not defined by the IUPAP red book nor by the

IUPAC green book ''Quantities, Units and Symbols in Physical Chemistry'', also known as the ''Green Book'', is a compilation of terms and symbols widely used in the field of physical chemistry. It also includes a table of physical constants, tables listing the pr ...

. It is a short form of the more formally correct "milli unified atomic mass unit" (mu) and equivalent to of the

unified atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

(u). A more modern name is the millidalton (mDa) since the "unified atomic mass unit" is more and more displaced by the unit

dalton Dalton may refer to: Science * Dalton (crater), a lunar crater * Dalton (program), chemistry software * Dalton (unit) (Da), the atomic mass unit * John Dalton, chemist, physicist and meteorologist Entertainment * Dalton (Buffyverse), minor ch ...

. (1 Da = 1 u) Since 1961 the unified atomic mass unit "u" has been defined as the mass of C. Before that the atomic mass unit "amu" was defined as the mass of O (physics) and as the mass of O (chemistry). Thus the publication date in literature ought to be heeded when reading about the milli mass unit as its name does not reveal whether it refers to the old amu or the newer u. The mass excess is usually indicated in mu or mmu. In mass spectrometry the mass accuracy of a mass analyzer is often indicated in mu, even though a more correct unit would be mTh (milli thomson) since mass spectrometers measure the mass-to-charge ratio, not the mass. The relative mass accuracy is often indicated in ppm, even though this is no longer supported by the

which suggests using units like μTh/Th instead of ppm.

References

{{reflist Units of mass