A pulsed ion analyser, such as a TOF, can only  sample an ion beam for a fraction of time, defined by the period it takes for the detector to complete one complete analysis. The overall efficiency of detection is referred to as the duty cycle and expressed as a percentage of the total signal admitted to the analyser. Also used to express the  limited time (dwell time) spent during a scanned spectrum on any single m/z value compared to that for a single ion SIM or MRM experiment (in this sense duty cycle refers to the ion collection or detection time divided by the total scan time x 100 %)