Time of flight mass spectrometer

A time of flight mass spectrometer (TOFMS) having: an ion source 110; a detector 150; a variable voltage unit and a control unit wherein the control unit is configured to control the TOFMS to perform at least one acquisition cycle that includes operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating the variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycleso that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit. In one embodiment different voltages are applied to the detector at different times corresponding to different m/z values. This method improves the mass resolution of the mass spectrometer.