1. A mass spectrometer in which the mass distribution of orbiting ions within a magnetic field is detected by bringing ion frequencies sequentially into resonance with applied radio frequencies.
2. A device for detecting and measuring the mass distribution of ions orbiting in an applied magnetic field, either by applying a constant radio-frequency signal and varying the magnetic field to bring ion frequencies equal to the applied radio frequency sequentially into resonance, or by rapidly varying the radio frequency and applying Fourier transform techniques (an operation that transforms one complex-valued function of a real variable into another one).

Fourier transform techniques as used in electronics, control systems engineering, and statistics, is a term used to describe the analysis of mathematical functions or signals with respect to frequency, rather than time.