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A nonintrusive tomographic method sensitive to the spatial distribution of trapped ions in the rf quadrupole ion trap is used to investigate ion cooling in the course of collisions with helium. Cooling commences after coherent excitation, by a resonant ac signal, of a mass-selected population of ions and its progress is followed using a short monopolar dc pulse to probe the position of the ion cloud. The amplitude of the probe dc pulse is selected such that it is sufficient to eject ions at some phases and not others. The abundance of the remaining trapped ion population is recorded by a scan of the rf amplitude and thus provides information on the axial secular motion of the original trapped ions. Ions of identical nominal mass, but different chemical composition (krypton, benzene-d 6 , and 1-hexene, all nominal mass 84 Da), are studied using pressures chosen to give cooling periods on the order of 10 ms. The maximum excursion in the axial direction, when plotted as a function of cooling time, provides information on the cooling process. The relative cooling times for the ions examined agree with calculated or experimentally known, velocity-dependent collision cross sections. Cooling times, using 0.46 mTorr of helium when operating the ion trap at Mathieu parameter q z ) 0.278, were 12 ms for krypton, 9.5 ms for benzene-d 6 , and 7 ms for 1-hexene. Simulations of ion motion made using the ion trap simulation program, ITSIM, with ion/neutral elastic collisions enabled, gave results that closely match and augment the experimental data. Methods for increasing the resolution of the experiment are discussed.

Isobaric and isomeric polyatomic ions are resolved for the first time in a quadrupole ion trap. This is achieved using a trap in which the geometry of the instrument can be manipulated under operating conditions. Variation of the z o dimension of the trap allows the chemical mass shifts associated with higher-order field components to be maximized. Resolution of isobaric doublets is illustrated for the molecular ions of nitrobenzene-d 5 and naphthalene and for the 3-ethoxyaniline and p-nitrotoluene molecular ions. The resolution is shown to be a function of the chemical propertiessnot the mass/charge ratiossof the isobaric ions by the separation of an isomeric pair, acetophenone and dihydrobenzofuran.