We have measured the atomic mass of the deuteron relative to the calibration ¹²C⁶⁺ and ¹²C⁴⁺ ions and arrived at a value of 2 014 101 778 052 (40) pu for the mass of deuterium and 2 013 553 212 744 (40) pu for the mass of the deuteron. The spectroscopy was performed in the new University of Washington Penning Trap Mass Spectrometer (UW-PTMS). The measurement was made by using the experiment to make a very accurate determination of the cyclotron frequency of a deuteron isolated in a Penning trap, followed by the cyclotron frequency of a carbon calibration ion, isolated at the same place in that same trap. The ratio of the cyclotron frequencies is inversely proportional to the ratio of the two ion's mass-to-charge ratio after systematic corrections are made. The deuteron mass along with the proton mass, which also has its best measurement at the UW-PTMS, can be used with the deuteron binding energy[20] to refine the neutron mass to m_n= 1 008 664 916 018 (435) pu.

In addition to discussing the physics of our penning trap spectrometer and the systematic corrections necessary to obtain a mass ratio, improvements to the spectrometer will be described that were used in the deuteron measurement as well as improvements that were made later for future measurements.