A method for determining a compensation factor parameter, c, for controlling an amount of ions ionised that are injected from an ion storage unit into mass analyser, where c is an adjustment factor that is applied to optimized injection times that are based on an optimized visible charge of a reference sample, the method comprising: detecting at least one mass spectrum for at least one amount of injected ions; determining from the at least one detected mass spectrum, a slope, s(sample), of a linear correlation of a relative m/z shift with visible total charge Qv of detected mass spectra; determining the compensation factor c as c=s(reference)/s(sample) where s(reference) is the slope of a linear correlation between reference-sample relative m/z shift values and reference-sample visible charge values determined from a plurality of mass spectra detected from a plurality of respective pre-selected amounts of a clean reference sample.