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 The International Union of Pure and Applied Physics (IUPAP ) is an international non-governmental organization whose mission is to assist in the worldwide development of physics, to foster international cooperation in physics, and to help in the ...

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 cha ...

. (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 The mass excess of a nuclide is the difference between its actual mass and its mass number in daltons. It is one of the predominant methods for tabulating nuclear mass. The mass of an atomic nucleus is well approximated (less than 0.1% difference fo ...

is usually indicated in mu or mmu. In

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

the

mass accuracy Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

of a mass analyzer is often indicated in mu, even though a more correct unit would be mTh (milli

thomson Thomson may refer to: Names * Thomson (surname), a list of people with this name and a description of its origin * Thomson baronets, four baronetcies created for persons with the surname Thomson Businesses and organizations * SGS-Thomson Mic ...

) since mass spectrometers measure the

mass-to-charge ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity relating the ''mass'' (quantity of matter) and the ''electric charge'' of a given particle, expressed in units of kilograms per coulomb (kg/C). It is most widely used in the electrody ...

, 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