A method of mass spectrometry is disclosed comprising passing ions through an ion mobility spectrometer and acquiring first ion mobility drift time data. A calibration function is applied to the first ion mobility drift time data to determine a physico-chemical property (e.g. CCS) of the ions. Second ion mobility drift time data is then acquired and the calibration function is applied to the second ion mobility drift time data to determine the physico-chemical property of one or more known or reference ions. The calibration function is then adjusted.