Abstract: A gas sample is introduced into an ion cyclotron resonance cell enclosed in a vacuum chamber, and ionized. A magnetic field constrains ions to circular orbits. After an optional delay adequate to allow ion-molecule reactions to occur, a pulsed broad-band oscillating electric field disposed at right angles to the magnetic field is applied to the ions. As the frequency of the applied electric field reaches the resonant frequency of various ions, those ions absorb energy from the field and accelerate on spiral paths to larger radius orbits. The excited motion is sensed and digitized in the time domain. The result of the digitization is Fourier transformed into the frequency domain for analysis. If desired, a sequential series of pulsed broad-band oscillating fields can be applied and the resulting change in motion sensed, digitized and accumulated in a sequential manner prior to Fourier transformation.