Accurate atomic mass measurements from Penning trap mass comparisons of individual ions

We report measurements of mass ratios of 20 pairs of molecular ions with a single ion Penning trap mass spectrometer having an accuracy exceeding one part in 1010. The dominant source of error is random magnetic field fluctuations which cause a 2.6x10-10 rms scatter in measurements of the cyclotron frequency. Robust statistical analysis of the data ensures that non-gaussian outliers are weighted less heavily in a smooth and consistent manner. Systematic errors are estimated to be 2x10-11 or below for doublet mass comparisons. The ratios form an overdetermined set, such that the atomic masses of nine isotopes can be derived from at least two independent groups of ion mass ratios, providing many consistency checks for systematic errors at the 10-10 level. At this level of precision, certain mass measurements have important implications in fundamental metrology. Results presented here are essential for defining a practical atomic standard of mass, for calibrating γ-ray wavelengths and for determining the molar Planck constant and the fine structure constant