A time of flight mass spectrometer that includes a first electrode; and a second electrode that is spaced apart from the first electrode. The ion source is configured to apply voltages to the first and second electrodes to produce an electric field in a region between the first and second electrodes so as to influence ions present in the region between the first and second electrodes when the mass spectrometer is in use. A shield is formed on the first electrode and/or second electrode. The shield is configured to inhibit an electric field formed between edges of the first and second electrodes from penetrating into the region between the first and second electrodes when the mass spectrometer is in use.