A mass spectrometer is disclosed comprising a time-of-flight mass analyser comprising an ion detector comprising an analogue to digital converter (ADC). Signals from the analogue-to-digital converter are digitized and the arrival time and intensity of ions are determined. The arrival time T0 and intensity S0 of each ion arrival event is converted into two separate intensities S(n),S(n+1) which are stored in neighbouring time bins T(n),T(n+1). Preferably, second differentials of the digitized signals from the analogue to digital recorder are obtained and used to determine the arrival time and intensity of ions.