Chemical Ionization Mass Spectrometry
The chemical ionization technique relies on the presence of a relatively high pressure in the ion source region. This high pressure promotes various ion molecule reactions which result in a highly sensitive method for ionization of sample molecules via ion molecule reactions. To obtain the high pressures desired in the ion source, one has to design and construct the source such that it will allow the internal pressures to reach several Torr without interferring with the electron beam or the ion beam exiting from the ion source. Pressures inside the source may reach several Torr while pressures outside the ion source are required to be below 10−3 Torr. Outside the source reduced pressure is required to provide maximum ion transfer from the source to the analyzer. Once the ion beam enters the analyzer region, pressure should again drop below 10−4 Torr. Hence, there should be three different pressure regions. These regions are satisfied by the presence of a dual pumping system and a differential pressure barrier. This barrier consists of a small hole between the source and analyzer region which will allow the ion beam to enter the analyzer region and also will allow the analyzer pump to provide a high vacuum in the analyzer region. Figure 1 demonstrates the proper pressure requirements for a chemical ionization system.
KeywordsChemical Ionization Source Pressure Inlet System Chemical Ionization Mass Spectrometry Electron Impact Source
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- 3.S. P. Markey et al., Proceedings 20th Annual Conf. Mass Spect. Allied. Topics, 318 (1972).Google Scholar