Mass ratio of two ions in a Penning trap by alternating between the trap center and a large cyclotron orbit

We have implemented a technique for precision mass comparison of two ions trapped simultaneously in a Penning trap in which each ion is alternately positioned at the center of the trap – where the cyclotron frequency is measured – and in a large cyclotron “parking” orbit. Systematic shifts in the cyclotron frequency ratio have been studied by comparing 13C2H2+/14N2+ and 14N2+/28Si+. Using the method to compare 28SiH3+/31P+ we have obtained a new atomic mass for 31P, M[31P] = 30.973 761 999 7 (61) u.