A mass spectrometer is a device that ionizes molecules or atoms of a sample, then separates the resultant ions according to their mass-to-charge ratio and detects these ions. Various methods for ionizing sample molecules have been developed to date, and electron ionization (EI) is one of the most generally used methods. In electron ionization, sample molecules are introduced into an ionization chamber of a comparatively small capacity, which is placed under a vacuum atmosphere. A filament for generating thermions is provided outside this ionization chamber, and thermions thereby generated are accelerated and injected into the ionization chamber. These thermions come in contact with the sample molecules to ionize these molecules within the ionization chamber. The ions thus produced within the ionization chamber are extracted to the outside by the action of an electric field created by a voltage applied to ion-extracting electrodes (e.g. a lens optical system) provided outside the ionization chamber (for example, refer to Patent Document 1).
In this type of ion source, the filament temperature rises to as high as 2000° to 3000° C., and a portion of the wall of the ionization chamber located close to the filament rises to a considerably high temperature due to the radiation heat from the filament. Normally, the temperature of the ionization chamber is maintained within a range from 200° to 300° C. by a heater that is thermally in contact with the chamber. However, if the a portion of the wall of the ionization chamber is locally heated as explained earlier until its temperature abnormally rises to a specific level, the metallic material constituting the wall of the ionization chamber becomes activated and produces a decomposition product, and this decomposition product may possibly be mixed with the sample molecules and create a noise. The local heating is also unfavorable that it produces an uneven temperature distribution within the ionization chamber, which deteriorates the ion production efficiency.
In the normal electron ionization, a potential difference of approximately 70 V is provided between the filament and the ionization chamber. The application of this potential to the filament creates an external electric field, which penetrates into the ionization chamber and disturbs the electric field present within the same chamber. This disturbance of the latter electric field prevents ions produced within the ionization chamber from being extracted to the outside of the ionization chamber in an intended manner, which causes a decrease in the amount of ions to be analyzed and deteriorates the ion detection efficiency.
Patent Document: Japanese Unexamined Patent Application Publication No. 2002-373616