MASS DISCRIMINATION DUE TO Z AXIS ION CLOUD COHERENCE IN THE FOURIER-TRANSFORM MASS-SPECTROMETRY TRAPPED-ION CELL

A mass discrimination effect common to Fourier transform mass spectrometry (FTMS) experiments involving the introduction of externally generated ions to the analyzer trapped-ion cell is described. The combination of inhomogeneous excitation fields with the coherent z axis motion of focused ion packets is shown to introduce time-dependent variations in apparent relative mass abundances that are as large as 60%. This effect is evaluated for the dual-cell ion equilibration experiment by modeling the z axis motion of isomass packets between adjacent trapped-ion cells during the pulse sequence. Predicted oscillations in the FTMS signal profile with period equal to half the expected cubic cell trapping period are then verified experimentally. Mechanisms for abrupt termination of the mass-dependent oscillations at a time 2-25 ms after initiation are also considered; for example, ion-neutral and ion-ion interactions are discounted, and correlation with the onset of large amplitude magnetron motion is established. Finally, the merits of the dual-cell ion equilibration pulse sequence as a direct probe of z axis evolution of the ion cloud during the FTMS experiment are demonstrated.