用語を通して学ぶ質量分析基礎の基礎第３回「イオンや分子の質量の単位，ｕ， Ｄａ， ａｍｕ， ｍｍｕ」

Abstract

The unified atomic mass unit (unit symbol: u) is a non-SI unit of mass, defined as one-twelfth the mass of a single 12C atom in its ground state. [SI: Le Systeme International d'Unites; The International System of Units] This definition was agreed upon by both the International Union of Pure and Applied Physics (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC) in the early 1960s to resolve a longstanding difference between two scales of the atomic mass unit. The term “atomic mass unit” (unit symbol: amu) has been used as a unit of mass defined as one-sixteenth the mass of a single atom 16O [m(16O)=16 amu] in physics and as one-sixteenth the isotope-averaged atomic mass (equivalent to the atomic weight) of oxygen [Ar(O)=16 amu] in chemistry.The unit dalton (unit symbol: Da) is also a non-SI unit of mass defined as “1 Da=1 u,” and is accepted as a unit for use by the SI in the 8th edition of the SI brochure (2006). Therefore, both the unified atomic mass unit and dalton are authorized units for mass of ions and molecules. It is a common mistake to use the deprecated term “atomic mass unit” and the deprecated unit symbol “amu” for the unit of mass defined as one-twelfth the mass of single atom 12C. The unit symbol “mmu,” meaning a millimass unit, is also an appropriate unit in SI. Instead of “mmu,” “mDa” or and “10-3 u” should be used.