Time-of-flight mass spectrometer with post-deflector filter assembly

The invention relates to time-of-flight mass spectrometry (TOFMS), and more particularly to a system that limits the detection of undesired ions. A time-of-flight mass spectrometer includes a deflector and a filter assembly that is located along a flight path between the deflector and an ion detector. The filter assembly passes incoming ions to the detector when the ions approach the filter assembly along their original flight path, and the filter assembly occludes incoming ions from the detector when the ions have been deflected from their original flight path by the deflector. In an embodiment, the filter assembly includes filtering plates that are aligned such that the major surfaces of the filtering plates are parallel to the original flight path of the ions. In order to remove ions of a particular mass from a mass spectrum of ions, target ions are deflected from their original flight path, causing the target ions to impact the filtering plates while the ions that are not deflected from their original flight path pass between the filtering plates for measurement by the detector.