2-DIMENSIONAL COULOMB-INDUCED FREQUENCY-MODULATION IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE - A MECHANISM FOR LINE BROADENING AT HIGH-MASS AND FOR LARGE ION POPULATIONS

Fourier transform ion cyclotron resonance (FTICR) spectra generated fo r large ion populations exhibit frequency shifts and line broadening, apparently due to Coulomb forces between ions. Although previous two-d imensional (2D) models of Coulomb effects in FTICR accounted for frequ ency shifts, they did not account for spectral line broadening. In thi s article, a 2D model is proposed that predicts line broadening due to Coulomb-induced frequency modulation. The model considers the case of two different-mass ions orbiting at their respective cyclotron freque ncies around a common guiding center. A mutual modulation of the cyclo tron frequency occurs at the difference frequency between ions. If the modulation period is much shorter than the FTICR observation time, th en sidebands spaced at intervals approximately equal to the modulation frequency are predicted. However, if the modulation period is similar in duration to the FTICR observation period, the sidebands can no lon ger be resolved, which results in spectral line broadening. This latte r case is a necessary consequence for isotopic peaks in the high mass region around m/z 2000, where deterioration in FTICR performance has b een observed. Computer simulations are used to confirm the mass depend ence and to demonstrate other features of the model, including a stron g dependence of the modulation on ion number. In support of the model, experimental FTICR spectra for large populations of methylnaphthalene ions at m/z 141 and 142 exhibit constant frequency sidebands correspo nding to multiples of the difference frequency for the two ions extend ing from nominal values of m/z 136 to 147.