heater 35 and the molecules absorbed are desorbed operation of the heater 16 and the heater 35 will be explained using fig2 in a condition waiting for measurement the heater 16 and heater 35 are heated to a high temperature together thereby the temperature of the ion trap space is set up at a high value of 200 to 300 u00b0 c and the water molecules in the ion trap space are expelled in addition to above in the same way when the equipment is turned on by switching the power supply on the heater 16 and the heater 35 are heated to the high valve together and the temperature of the ion trap space is set to be in 200 300 u00b0 c furthermore in a preparation stage of the measurement the heater 16 and heater 35 are heated to a low condition together and temperature of the trap space falls gradually the temperature is reduced to about 100 u00b0 c just before starting the measurement as a result the orbit and adsorption of the sample may be prevented even if the buffer gas is introduced in the trap space it is preferable to continue maintaining the temperature therefore the quadrupole electrodes are heated synchronizing with the buffer.