Hyperfine Interactions 108 (1997) 227–238 227 Accurate mass spectrometry of trapped ions M. Bradley, F. Palmer, D. Garrison, L. Ilich, S. Rusinkiewicz and D.E. Pritchard Research Laboratory of Electronics, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA The Penning trap Ion Cyclotron Resonance (ICR) method we use to weigh atomic masses is reviewed, and our plans for future measurements, new methods, and apparatus improvements are discussed. Our ultimate goal is to develop a new technique for measuring atomic masses with an accuracy of a few parts in 10 12 . We will do this by comparing the cyclotron frequencies of two simultaneously trapped ions. In order to successfully implement this new method we are developing a quieter, more sensitive DC SQUID-based detector and a new more harmonic trap, and we plan to use our classical squeezing techniques to reduce the effects of thermal noise. With our improved apparatus we will weigh Cs and Rb to help determine the fine structure constant α, weigh 29 Si and 30 Si as part of the current effort to replace the artifact kilogram standard with a Si crystal containing a known number of atoms, and measure the 3 H– 3 He mass difference to help set a limit on the mass of the electron neutrino. Our higher accuracy will also enable us to “weigh” the neutron capture gamma rays of 28 Si, 32 S, and 48 Ti to help determine the molar Planck constant NAh and the fine structure constant α. Finally, with a mass measurement accuracy ∼ 10 −12 we will be able to “weigh” chemical bonds. 1. Introduction Of the three basic physical quantities – mass, length, and time – mass is currently measurable with the least accuracy. The fact that atomic masses cannot be compared with more precision is unfortunate because mass comparisons of initial and final states can measure the energy available for a variety of interesting physical and chemical processes (e.g., emission of a gamma ray, neutrino, neutron, or electron, and chemical reactions). During the last few years the MIT ICR (Ion Cyclotron Resonance) group has brought together a wide variety of new techniques for making, trapping, and cooling single ions, comparing cyclotron frequencies, and minimizing the effects of the tiny perturbations of the electric and magnetic fields. These techniques have allowed us to produce a table of ten atomic masses (see table 1) important for metrology or for the determination of fundamental constants [1]. The fractional accuracy of the masses in the table, typically 10 -10 , represents one to three orders of magnitude  J.C. Baltzer AG, Science Publishers