Mass-specific selection of ions in a Fourier-transform ion cyclotron resonance (FT-ICR) ion trap is shown to be hampered by unavoidable off-resonance cyclotron excitation of the ions to be selected. This unintentional offresonance cyclotron excitation is caused by radio-frequency fields which are applied during the selection procedure to eject unwanted ions by on-resonance excitation of their cyclotron motion. The experimental results indicate that the effective cyclotron motion of the ions subjected to an off-resonance single-frequency RF fieid is alternatingly excited and de-excited with a periodicity equal to 2n/(w0-wen), where wo is the frequency of the excitation field and wen is the effective ion cyclotron frequency. Furthermore, it has been demonstrated that the translational energy gained during off-resonance cyclotron excitation can be predicted quantitatively by theory. The net translational energy can be minimized below 1 eV if the duration of the individual single-frequency excitation fields is set to k2n/(w0-wen), where k has to be an integer, representing the number of full off-resonance excitationlde-excitation periods. The unintentional off-resonance cyclotron excitation is shown to lead to an instrumental upper limit for the mass selectivity of ion selection, the so-called front-end resolution. This upper limit is proportional to the cell diameter, and the square of the magnetic field strength, and inversely proportional to the noise level of the excitation RF-field, and the mass of the ions to be selected. It is demonstrated that an instrumentally dictated maximum mass resolution of better than 50 OOO can be obtained for the selection of ions with a nominal mass-to-charge ratio of 79.
Computer-controlled formal optimization of the tuning of narrow band Fourier transform Ion cyclotron resonance time domain signals has been Implemented. The multivariant modified simplex optimization method has been tailored to the tuning parameters and the instrument response surface. The narrow band signal has been optimized on the basis of mass spectrum resolution, line shape, and signal to noise ratio. The optimization performance and reproducibility have been tested over a wide ion mass range and yielded reliable resolution Improvements. Computer-controlled optimization of a m/z 1066 ion signal from a tris(perfluorononyl)-s-trlazine fragment improved the resolution 50-fold, to 1600 000 In the absorption mode.
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