Abstract: A distance-of-flight mass spectrometer (DOFMS) includes an ion source, a field-free region, an extraction region in which ions are accelerated, and a spatially-selective detector for spatially selectively detecting ions extracted by the extraction region. A method for operating a distance-of-flight mass spectrometer DOFMS comprises controlling a detection time in such a way as to permit ions with progressively greater mass-to-charge (m/z) ratios to enter the extraction region of the DOFMS at positions which will permit the ions with progressively greater m/z ratios to enter the detector of the DOFMS, generating a component mass spectrum at each selected value of detection time, and then assembling a composite mass spectrum by shifting the distance-of-flight axis of each component mass spectrum by a distance corresponding to the change in detection time.