A method includes: accumulating ions having a plurality of m/z values in an ion trap during a time interval; deriving from the accumulated ions a respective intensity value for each of the m/z values; and adjusting each of the intensity values as a function of the time needed by the ion trap to begin collecting ions with the corresponding m/z value. According to a different aspect, an apparatus includes a first portion with an ion trap, and a second portion. The second portion causes the ion trap to accumulate ions with a plurality of m/z values during a time interval, derives from the accumulated ions in the ion trap a respective intensity value for each of the m/z values, and adjusts each of the intensity values as a function of the time needed by the ion trap to begin collecting ions with the corresponding m/z value.