A CDMS may include an ion source to generate ions from a sample, a mass spectrometer to separate the generated ions as a function of ion mass-to-charge ratio, an electrostatic linear ion trap (ELIT) having a charge detection cylinder disposed between first and second ion mirrors, wherein ions exiting the mass spectrometer are supplied to the ELIT, a charge generator for generating free charges, a field free region between the charge generator and the charge detection cylinder, and a processor configured to control the charge generator, with no ions in the charge detection cylinder, to generate a target number of free charges and cause the target number of free charges to travel across the field-free region and into contact with the charge detection cylinder to deposit the target number of free charges thereon and thereby calibrate or reset the charge detection cylinder to a corresponding target charge level.