Resolution of Isobaric and Isomeric Ions Using Chemical Shifts in an Ion Trap Mass Spectrometer

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 properties not the mass/charge ratios of the isobaric ions by the separation of an isomeric pair, acetophenone and dihydrobenzofuran.