A time of flight mass spectrometer including: an ion source including: a first electrode, wherein the first electrode includes a sample plate for carrying a sample; and a second electrode (9, 9') that has an aperture formed therein and is spaced apart from the first electrode; a sample plate carrier on which the sample plate is mounted; a mechanism configured to translate the sample plate carrier laterally with respect to an ion optic axis so as to laterally offset the sample plate carrier and therefore the sample plate with respect to the ion optic axis, wherein the ion optic axis extends between the first and second electrode and through the aperture in the second electrode; wherein the ion source is configured to apply voltages to the first and second electrodes to produce an extraction electric field in an extraction region between the first and second electrodes so as to extract ions from the extraction region through the aperture in the second electrode when the mass spectrometer is in use; wherein a shield (10, 10') is formed on a surface of the second electrode that faces the first electrode so that the shield extends from the surface of the second electrode towards the first and electrode, wherein the shield (10, 10') surrounds the ion optic axis and is outwardly spaced apart from a boundary between the second electrode (9, 9') and the aperture, wherein the shield is configured to inhibit an electric field formed between edges of the first and second electrodes from penetrating into the extraction region between the first and second electrodes so as to inhibit changes in the extraction electric field in the extraction region when the sample plate is laterally offset with respect to the ion optic axis when the mass spectrometer is in use. an ion source including: a first electrode, wherein the first electrode includes a sample plate for carrying a sample; and a second electrode (9, 9') that has an aperture formed therein and is spaced apart from the first electrode; a first electrode, wherein the first electrode includes a sample plate for carrying a sample; and a second electrode (9, 9') that has an aperture formed therein and is spaced apart from the first electrode; a sample plate carrier on which the sample plate is mounted; a mechanism configured to translate the sample plate carrier laterally with respect to an ion optic axis so as to laterally offset the sample plate carrier and therefore the sample plate with respect to the ion optic axis, wherein the ion optic axis extends between the first and second electrode and through the aperture in the second electrode; wherein the ion source is configured to apply voltages to the first and second electrodes to produce an extraction electric field in an extraction region between the first and second electrodes so as to extract ions from the extraction region through the aperture in the second electrode when the mass spectrometer is in use; wherein a shield (10, 10') is formed on a surface of the second electrode that faces the first electrode so that the shield extends from the surface of the second electrode towards the first and electrode, wherein the shield (10, 10') surrounds the ion optic axis and is outwardly spaced apart from a boundary between the second electrode (9, 9') and the aperture, wherein the shield is configured to inhibit an electric field formed between edges of the first and second electrodes from penetrating into the extraction region between the first and second electrodes so as to inhibit changes in the extraction electric field in the extraction region when the sample plate is laterally offset with respect to the ion optic axis when the mass spectrometer is in use.