A method of mass spectrometry comprising: digitising a first signal output from an ion detector to produce a first digitised signal; said method being characterized by comprising the following steps: determining or obtaining a second differential of said first digitised signal; and determining the arrival time of one or more ions from said second differential of said first digitised signal; wherein said step of determining the arrival time of one or more ions from said second differential of said first digitised signal comprises determining one or more zero crossing points of said second differential of said first digitised signal, determining or setting a start time t1 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal falls below zero or another value, and determining or setting an end time t2 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal rises above zero or another value. digitising a first signal output from an ion detector to produce a first digitised signal; said method being characterized by comprising the following steps: determining or obtaining a second differential of said first digitised signal; and determining the arrival time of one or more ions from said second differential of said first digitised signal; wherein said step of determining the arrival time of one or more ions from said second differential of said first digitised signal comprises determining one or more zero crossing points of said second differential of said first digitised signal, determining or setting a start time t1 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal falls below zero or another value, and determining or setting an end time t2 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal rises above zero or another value. determining or obtaining a second differential of said first digitised signal; and determining the arrival time of one or more ions from said second differential of said first digitised signal; wherein said step of determining the arrival time of one or more ions from said second differential of said first digitised signal comprises determining one or more zero crossing points of said second differential of said first digitised signal, determining or setting a start time t1 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal falls below zero or another value, and determining or setting an end time t2 of an ion arrival event as corresponding to a digitisation interval which is immediately prior or subsequent to the time when said second differential of said first digitised signal rises above zero or another value.