Abstract: A method of operating an ion mobility spectrometer comprises the following steps: drawing a sample of gaseous fluid into a reaction region of the ion mobility spectrometer; providing a first pulse of a pulsed ionisation source to ionise the sample of gaseous fluid, thereby to obtain first sample ions; after a first gate delay following the first trigger opening an ion shutter to permit a portion of the first sample ions to leave the reaction region; providing a second pulse of the pulsed ionisation source to further ionise the sample of gaseous fluid, thereby to obtain second sample ions; after a second gate delay following the first trigger opening the ion shutter to permit a portion of the second sample ions to leave the reaction region, wherein the second gate delay is different from the first gate delay.