Mass Spectroscopy in Penning Trap

Abstract

While mass spectrometry in Paul traps serves well mainly for molecular analysis in chemistry, Penning traps provide high accuracy and precision. The technique is based on the fact that the ratio of cyclotron frequencies ω _c = (Q/M)B of two ions in the same magnetic field B gives directly the ratio of their masses ω _c(1)/ω _c(2) = M(2)/M(1). If carbon-12 as the standard of the atomic mass scale is used as reference, the mass of the ion of interest is obtained directly in atomic units. Although the cyclotron frequency is not an eigenfrequency of the Penning trap, it can be obtained from combinations of ω+, ω, and ω _z as evident from the set of equations (1.31)-(1.33). In the ideal case of a perfect quadrupole trap and constant and homogeneous magnetic field B no further correction has to be applied. In reality, however, the trap potential will deviate from the ideal shape leading to shifts of the motional frequencies. In the case of several simultaneously stored ions, space charge potential also modifies the ion oscillation as well as image charges induced in the trap electrodes by the oscillating ions. Finally, since the comparison of frequencies has to be made at different times, fluctuations of the magnetic field in time have to be considered.