The University of Washington Penning Trap Mass Spectrometer (UWPTMS) is now producing measurements with uncertainties approaching 10 parts per trillion (ppt). We have recently published (VanDyck, Jr. et al., Int. J. Mass Spectrom. 251: 231-242, 2006) detailed analysis of several systematic shifts which can be important at this level of accuracy. Experimental studies of these effects in our older PTMS, combined with preliminary analysis of H-2 data, and re-analysis of the previously reported He-4 (Van Dyck, Jr. et al., Phys. Rev. Lett. 92: 220802/1, 2004) and O-16 (Van Dyck, Jr. et al., Hyperfine Interact. 132: 163-175, 2001) data, gives more accurate atomic mass values for O-16, He-4, and H-2. Currently we are taking data for a new measurement of the He-3 atomic mass, and working on some improvements to the PTMS, including a new amplifier system for phase-sensitive detection of the ion's axial motion, and a new computer-controlled ultra-stable voltage source for the Penning trap's ring electrode, used to adjust the ion's axial frequency. These new systems will allow us to simultaneously manipulate individual ions in two nearby Penning traps, and some sources of noise will be the same for both traps. We plan to investigate several techniques which should reduce measurement time and improve accuracy by working with the two ions simultaneously.
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