Abstract: An ITMS includes an inlet for receiving a sample that will be tested for a substance of interest. The inlet communicates with an ionization chamber and a drift chamber communicates with the downstream end of the ionization chamber. A first grid electrode extends across the downstream end of the ionization chamber and a second grid electrode is slightly downstream from and parallel to the first grid electrode. A slight potential bias is applied to the first grid electrode to hold the ions in the potential well between the first and second grid electrodes. However a pulse is applied periodically to the first grid electrode to accelerate ions into the drift chamber. The accumulation of the ions in the potential well prior to generation of the pulse results in a thinner band of ions ejected into the drift chamber and hence achieves higher resolution.