The mass-to-charge ratio scale.

Increases in the capacity for accurately measuring the mass-to-charge ratio of specific gas-phase ions justify the reconsideration and standard definition of the gas-phase mass-to-charge ratio scale and the clearly denned connection of that scale to condensed phases. We propose that the chemical mass standard for solids and the gas phase be based upon the mass of carbon-12 buckminsterfulierene ((12)C60). The mass-to-charge ratio scale in the gas phase would be based upon the mass of gas-phase (12)C60, the mass of the electron, and the electron charge in atomic units. As mass measurement accuracy improves, corrections to this mass-to-charge ratio standard are anticipated for the vaporization energy of the 12C60 molecule and its ionization potential or electron affinity. We propose that the positive ion scale be set by the mass-to-charge ratio of (12)C 60 (+) as (+)719.9994514±0.0000004 u per electron charge. We propose that the negative ion mass scale be set by the mass-to-charge ratio of (12)C 60 (-) as (-)720.0005484±0.0000004 u per electron charge.