The kilogram is the sole base unit in the International System of Units (SI) system defined by a physical artifact, and recent popular science news articles (with varying degrees of accuracy) describe changes in the relative masses of the standard prototype and its replicas around the world. These articles imply a potential problem exists, which is untrue, because the changes are documented and tracked. However, proposed redefinitions of the kilogram that do not involve the physical prototype are now being considered by the metrology community, and these involve defining the kilogram in terms of a natural constant.

In 1793, the term "grave" was proposed by an early committee looking at metrological standards for the mass standard we know today as the kilogram. The term was not adopted and "kilogram" became the approved term just a few years later. But how to define the kilogram and provide it with the authority of a standard has been a topic of continuing interest for more than 200 years now. Eventually, a physical prototype was constructed and became the consensus kilogram. Davis (1) provides a brief history of the kilogram standard unit and the creation of the prototypes and its copies, and discusses how they are compared in verification protocols.

Of course, the definition of mass is central to mass spectrometry (MS), and redefinitions are not only theoretical but may have a practical consequence. Here, we describe the two distinct redefinitions of mass. First, we review the transition in the 1960s from the ¹⁶O mass standard to the ¹²C mass standard; the former is known as the physical atomic mass scale, and the latter as the chemical atomic mass scale. This transition had immediate effects on MS, and defined the unified atomic mass unit, u. Lingering variations in nomenclature such as amu, u, and daltons (Da) have been peppered throughout the research literature and persist to this day. Secondly, we review some current research and proposals for redefining the kilogram in terms of natural constants, and not the current physical artifact. The anticipated consequences on practical analyses in MS may seem distant, but any redefinition percolates through the SI hierarchy, and we best be prepared. A previous column discussed the transition between physical and chemical atomic mass scales (2); the reiteration here will be brief and will amplify different points. The development of mass standards for commerce and industry (within the range of milligrams to kilograms) proceeded independently and is not covered here.

The term Da (3) honors the early contributions of John Dalton, who studied chemical reactivity of elements and formulated the atomic theory that attempted to explain how they combined. As the theory was developed (with a few notable missteps derived from mistaken assumptions of formula), Dalton and others observed that reacting elements react in simple and consistent integer ratios. Assigning a value of 1 unit to the mass of hydrogen allowed many of the chemical reactions to be conveniently expressed as ratios of mass. As more and more relative masses were determined through the study of chemical reactions, it was suggested by The International Union of Pure and Applied Chemistry (IUPAC) in 1920 (through the creation of an International Commission on Atomic Weights) that the mass of oxygen be set at exactly 16, and that this standard be used as a scale definition. However, the oxygen isotopes ¹⁷O and ¹⁸O were discovered in 1929, and this discovery resulted in the bifurcation of mass scales and a variance in the values of constants associated with them. Physicists used solely ¹⁶O as mass 16 and chemists used the average mass of oxygen in its natural state (which included all the isotopes). Therefore, the physical atomic mass scale and the chemical atomic mass scale differed by a nominal factor of 1.000275. Actually, that factor varied somewhat; oxygen drawn from different sources varies in its isotopic abundances.

The role of A.O.C. Nier in his service on the Atomic Weights Commission from 1947 to 1961 is emblematic of the extended effort required to resolve the dispute (4,5). Nier and A. Ölander each proposed to use ¹²C as the basis for the atomic weights table, and that suggestion was adopted in 1961. Mattauch was an early champion (6) and succinctly discussed the reasons for adopting the ¹²C standard. In the blur of a 50-year historical retrospective, it is not often noted that ¹⁹ F was also proposed as a mass standard (7). The agreement to create the unified atomic mass unit u was indeed a unification of the disparate research communities at the time and accommodated the need for a standard that reflected the precision with which measurements could be made. Duckworth's memoir (8) provides a great deal of personal insight into those communities, and the development of high precision MS in isotopic research.