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 g eometry of the instrument can be manipulated under operating condition s. Variation of the z(o) dimension of the trap allows the chemical mas s shifts associated with higher-order field components to be maximized . Resolution of isobaric doublets is illustrated for the molecular ion s of nitrobenzene-d(5) and naphthalene and for the 3-ethoxyaniline and p-nitrotoluene molecular ions. The resolution is shown to be a functi on of the chemical properties-not the mass/charge ratios-of the isobar ic ions by the separation of an isomeric pair, acetophenone and dihydr obenzofuran.