Theory for broadband detection of ion cyclotron resonance signals

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Abstract
A complete line shape theory is developed for the transient response of a new type of ion cyclotron resonance (ICR) detector circuit. The detector is basically a balanced capacitance bridge which is sensitive to the abundance of gaseous ions stored in a static magnetic ion trap. For the first time, the equations of motion of ions in the ICR analyzer cell are shown to be coupled to the circuit equations of the detector. Also, the effect of nonreactive ion–molecule collisions on line shapes and on the transient response of the detector are analyzed and shown to allow measurement of ion–molecule collisions frequencies as a function of ion translational energy. One of the most important features of the capacitance bridge detector is its broadband sensitivity to a wide range of ion cyclotron resonance frequencies. This allows a mass spectrum of ions stored in the ICR analyzer cell to be obtained by scanning the frequency ω1 of the irradiating rf electric field at a fixed magnetic field strength. The capacitance bridge circuit can serve not only as a direct replacement for marginal oscillator circuits traditionally used in ICR experiments, but also as the detector for recently developed Fourier Transform and Rapid Scan ICR experiments.