A fault detection system for protecting a mass spectrometer from the effects of temperature extremes. The system comprises an ion block, a thermal source for providing thermal energy to the ion source block, a temperature sensor providing a reading for the temperature of the ion source block, a temperature regulation means for controlling the thermal source in dependence of the reading and a control system for monitoring the temperature change produced by the energy source.The control system is adapte4d to monitor the rate of change of the reading provided by the temperature sensor relative to the thermal energy provided to the ion block.