Pulsed ion extraction diagnostics in a quadrupole ion trap linear time-of-flight mass spectrometer.

Pulsed extraction techniques are investigated for a quadrupole ion trap (QIT) interfaced to a linear time-of-flight (TOF) mass analyzer. A nonfocusing short-pulse mode of operation is developed and characterized. The short-pulse mode creates a near-monoenergetic ion packet, which is useful for reaction kinetics experiments and for making diagnostic measurements of the ion cloud size in the trap. Monopolar and bipolar pulsing modes, with the voltage pulses applied to one or both QIT endcaps to extract the ions into the TOF region, are compared. Ion TOF peak distributions are characterized experimentally and by ion trajectory simulations. Also, first-order spatial (Wiley-McLaren) focusing of ions is characterized for the conventional long-pulse extraction mode. The nonparallel fields in the QIT, which serves as the first acceleration region in the linear-TOF mass spectrometer, are shown to degrade spatial focusing and mass resolution.