A Wiley–McLaren-type time-of-flight mass spectrometer is improved for ions spatially spread over the ion-extraction region. We examine the case where a continuous ion beam is introduced coaxially to the flight path. The mass resolution, M/ΔM, is degraded because the ions are extensively spread along the flight path, when a conventional linear electric field is applied to the extraction region. It is pointed out that this is mainly due to the fact that the ions initially located near the exit of the extraction region arrive much earlier than the others as revealed by flight-time simulation. The simulation further identifies an ‘ideal’ nonlinear electric field, which compensates for the flight-time difference and thus maximizes the mass resolution. We demonstrate that such a nonlinear electric field close to the ‘ideal’ one is materialized by simply removing a grid mesh from the exit electrode of the extraction region, which results in significant improvement in the mass resolution.
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