The time-of-flight mass spectrometers which must demonstrate a high constancy of the calibrated mass scale even under changeable ambient temperatures and thermal loads due to pumps or electronics. Time-of-flight mass spectrometers calculate the masses of ions from the measured time of flight in a long flight tube that is normally manufactured of stainless steel. These flight tubes are subject to temperature-related length changes which affect the flight time and therefore the mass determination. The thermal expansion of spectrometer parts between ion source and ion detector, thus keeping the flight path for the ions at a constant length. Length compensation can be produced by design of the spacing system made of materials of different thermal expansion coefficients, the length changes of which balance out in opposite directions.