An IMS or other analytical instrument has a corona discharge needle (20) to ionize sample gases or vapours. A gate (3) is opened or closed to admit or prevent entry of the ions produced by the corona discharge to a drift chamber (4). The operation of the corona discharge needle (20) and the gate (3) are controlled such that the gate is open during at least two discharges, to admit faster ions produced by the most recent discharge together with slower ions produced by an earlier discharge.