RADIAL DISTRIBUTIONS AND EJECTION TIMES OF MOLECULAR-IONS IN AN ION-TRAP MASS-SPECTROMETER - A LASER TOMOGRAPHY STUDY OF EFFECTS OF ION DENSITY AND MOLECULAR TYPE

Photodissociation of mass-selected populations of trapped ions is used as a tool to determine the spatial distributions of the ion clouds un der a variety of trapping conditions. These ion tomography experiments are performed in both the axial and radial dimensions, and the result s show that the ion cloud expands significantly in the radial dimensio n as the number of trapped ions is increased. This expansion correlate s with an increasing error in mass assignment due to delayed ion eject ion. Furthermore, both effects appear to be related to the occurrence of compound-dependent (rather than mass/charge ratio-dependent) effect s on ion ejection. The molecular ions of nitrobenzene and n-butylbenze ne, and the benzoyl cation, examined under fixed conditions using the same number of ions, each displays different mass shifts which correla te with differences in the magnitudes of their radial distributions. T hese results demonstrate that the spatial distribution of a collection of ions depends on their physicochemical properties. Furthermore, alt erations in the geometry of the trap are shown to be a means of contro lling the compound-dependent positional distributions as well as the c orresponding mass shifts. Ion tomography measurements of the size of t he ion clouds are made for all three types of ions as a function of th e number of trapped ions. They show that the compound-dependent mass s hifts can be eliminated by symmetrically increasing the spacing of the end cap electrodes, a procedure which deliberately increases the posi tive octapolar field component. The implications of these results for exact mass measurements using ion traps are considered.