Mass‐specific selection of ions in Fourier‐transform ion cyclotron resonance mass spectrometry. Unintentional off‐resonance cyclotron excitation of selected ions

Mass-specific selection of ions in Fourier-transform ion cyclotron resonance (FT-ICR) ion trap is shown to be hampered by unavoidable off-resonance cyclotron exctron excitation of the ions to be selected. This unintentional off-resonance cyclotron excitation is caused by radio-frequency fields which are applied during the selection procedure to eject unwanted ions by on-resonance excitationof their cyclotron motion. The experimental results indicate that the effective cyclotron motion of the ions subjected to an off-resonance single-frequency RF field is alternatingly excited and de-excited with a periodicity equal to 2π/(ω0−ωeff), where ω0is the frequency of the excitation field and ωeff 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-rfrequency excitation fields is set to k2π/(ω0−ωeff), where k has to be an integer, representing the number of full off-resonance excitation/de-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 50000 can be obtained for the selection of ions with a nominal mass-to-charge ratio of 79.