Mass spectrometer
Every time a target sample is injected from an injector ( 12 ) of an LC unit ( 1 ) and a mass spectrometry for a target component in the sample is performed, a CD voltage applied to a conversion dynode of an ion detector ( 29 ) is switched. For each of the multiple CD-voltage levels, a data collector ( 32 ) collects noise data during a period of time where no component is present and intensity data of an ion originating from the target component, while the SN ratio calculator ( 33 ) calculates an SN ratio. After the actual measurement is completed, an optimum CD voltage determiner ( 34 ) compares the SN ratios calculated for each CD voltage, finds the CD voltage which gives the highest SN ratio, and stores this voltage in an optimum CD voltage memory ( 42 ) as an optimum CD voltage for the analysis conditions at that point in time and for the m/z of the analysis target. According to this method, even when the flow rate of the mobile phase is particularly high or when a hard-to-vaporize mobile phase is used, the CD voltage is appropriately set and a detection signal is obtained with high SN ratios.
1. 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.
2. The mass spectrometer according to claim 1 , wherein the ion source is an atmospheric pressure ion source for ionizing a component in a sample in liquid form.
3. The mass spectrometer according to claim 2 , wherein a liquid sample separated into components by a liquid chromatograph is introduced into the atmospheric pressure ion source.