Mass spectrometer

A mass spectrometer having:

• an ion source for ionizing a component in a sample;
• a mass analyzer for separating ions generated by the ion source according to their mass-to-charge ratios;
• and an ion detector for detecting the ions separated by the mass analyzer, the ion detector having a conversion dynode for converting ions into electrons and a secondary electron multiplier for multiplying and detecting the electrons, the mass spectrometer comprising:
• a) a voltage supplier for applying high voltage to the conversion dynode, where the high voltage is independent from a voltage applied to other components of the mass spectrometer;
• b) an analysis executer for changing the voltage applied to the conversion dynode by the voltage supplier to a plurality of levels, and for obtaining, for every change in the voltage applied to the conversion dynode, a noise signal under an analysis condition at that point in time and a detection signal corresponding to an ion originating from a target component;
• and
• c) an optimum voltage setter for calculating an SN ratio based on the noise signal and the detection signal obtained by the analysis executer for each different level of the applied voltage, and for determining, based on the calculated result, the voltage to be applied to the conversion dynode in an analysis of the target component.