DYNAMICS OF ION COUPLING IN AN FTMS ION-TRAP AND RESULTING EFFECTS ON MASS-SPECTRA, INCLUDING ISOTOPE RATIOS

We have studied the coupled cyclotron motions of ions at moderate to high density in the ion trap of a Fourier transform mass spectrometer. For the case of two ionic species of similar mass in the ion trap (CO+ and N2+) we present image current detection results over a range of ion densities. At low ion densities the ion motions are independent, resulting in two distinct image current frequencies. At higher ion densities the ion motions are strongly coupled. In the high density limit only one of the coupled motions has the symmetry required to generate an image current. Thus, the FT mass spectrum of the CO/N2 mixture at high density consists of only a single peak at the frequency of the image current active, coupled cyclotron motion. By analyzing the image current transients in detail we can follow the development of the collective normal modes of the ions in the trap. FTMS spectra of naphthalene (C10H8) as a function of ion density demonstrate that the coupled ion motions can have a substantial effect on the measurement of C-12/C-13 isotope ratios.