Peak Confluence Phenomenon in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry.

Abstract

When two ions with very small mass difference are analyzed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, the two peaks in the mass spectrum approach to each other by increasing the number of ions and finally merge into a single peak, even though the width of the peaks is sufficiently small compared to the distance between the peaks. It seems that Coulomb force between the two ion packets modulates their cyclotron motions produced by an rf electric field in the FT-ICR trap. We studied the phenomenon theoretically using a model of two charged particles which are confined in a plane perpendicular to a uniform magnetic field. In the case that the two ions appear as a single peak the two ion packets are coupled and rotate around the center of mass with oscillating radius and the position of the peak in the spectrum is determined by the frequency of rotation of the center of mass which corresponds to the weighted average of the mass of the two ion packets. It is found that there is an oscillating force acting between the two ion packets and when the force is resonant with the oscillating radius, the two ion packets are decoupled and appears as two separate peaks in the spectrum. Various factors which affect the phenomenon are discussed.