Abstract: The calibration of an ion mobility spectrometer is corrected for the actual operating pressure and temperature conditions within the system. An absolute pressure transducer is used to monitor absolute atmospheric pressure. This information is used in conjunction with the measured calibrant drift time to pre-determine expected drift times of target ions, compensating for any pressure and/or temperature differences from the reference conditions under which the original calibration has been carried out. This improved calibration allows the unit to be operated with a significantly lower waiting period after start-up, and also under significantly different atmospheric pressure conditions. It also improves the precision of operation of the system.