Atomic Mass Unit - Terminology

Terminology

The unified atomic mass unit and the dalton are different names for the same unit of measure. With the introduction of the name "dalton", there has been a gradual change towards using that name in preference to the name "unified atomic mass unit":

• In 1993, the International Union of Pure and Applied Chemistry approved the use of the dalton with the qualification that the GCPM had not given its approval.
• In 2003 the Consultative Committee for Units, part of the CIPM, recommended a preference for the usage of the "dalton" over the "unified atomic mass unit" as it "is shorter and works better with prefixes".
• In 2005, the International Union of Pure and Applied Physics endorsed the use of the dalton as an alternative to the unified atomic mass unit.
• In 2006, in the 8th edition of the formal definition of SI, the CIPM cataloged the dalton alongside the unified atomic mass unit as a "Non-SI units whose values in SI units must be obtained experimentally: Units accepted for use with the SI". The definition also noted that "The dalton is often combined with SI prefixes ..."
• In 2009, when the International Organization for Standardization published updated versions of ISO 80000, it gave mixed messages as to whether or not the unified atomic mass unit had been deprecated: ISO ISO 80000-1:2009 (General), identified the dalton as having "earlier called the unified atomic mass unit u", but ISO 80000-10:2009 (atomic and nuclear physics) catalogued both as being alternatives for each other.
• The 2010 version of the Oxford University Press style guide for authors in life sciences gave the following guidance "Use the Système international d'unités (SI) wherever possible ... The Dalton (Da) or more conveniently the kDa is a permitted non-SI unit for molecular mass or mass of a particular band in a separating gel." At the same time, the author guidelines for the journal "Rapid Communications in Mass Spectrometry" stated "The Dalton (Da) is a unit of mass normally used for the molecular weight ... use of the Da in place of the u has become commonplace in the mass spectrometry literature ... The "atomic mass unit", abbreviated "amu", is an archaic unit".
• In 2012, in response to the proposed redefinition of the kilogram, it was proposed that the dalton be redefined as being 0.001/N_A kg, thereby breaking the link with C₁₂. This would result in the dalton and the atomic mass unit having different definitions and differing from each other by a factor of the order of 10-9.