Filaments, electron beams from

Ions generated by an electron beam from a heated filament are passed into a cubic cell where they are held by an electric trapping potential and a constant magnetic field. Each ion assumes a cycloidal orbit at its own characteristic frequency, which depends on mJz the cell is maintained under high vacuum. Originally, these frequencies were scanned by varying the electric field until each cycloidal frequency was, in turn, in resonance with an applied constant radiofrequency. At resonance, the motion of the ions of the same frequency is coherent and a signal can be detected. 

These devices are collectively known as electron-column instruments. They all may be likened to elaborate x-ray tubes, in which the specimen is the target and in which extreme measures have been taken to focus the electron beam from the filament into a very small spot. 

A field emission source uses a needle-like tungsten or carbon tip as the cathode, shown in Fig. 14.28. The tip is only nanometers wide, resulting in a very high electric field at the tip. Electrons can tunnel out of the tip with no input of thermal energy, resulting in an extremely narrow beam of electrons. Electron beams from heated filaments have a focal (cross-over) diameter of about 50 p,m while a field emission source has a crossover diameter of only about 10 nm. Field emission sources can serve as probes of surfaces at the nanometer scale (an Auger nanoprobe). 

Energetic (70 eV) electron beam from a heated filament impacts vaporized analytes, displaces an electron, and generates radical cations that frequently fragment... 

This investigation s motivation essentially arises from a discovery of an unwanted real-time in situ nucleation and growth of Ag filaments on a-Ag2W04, Ag3P04, and Ag2Mo04 crystals which was driven by an accelerated electron beam from an electronic microscope under high vacuum [161-165]. 

An electron impact (El) ion source uses an electron beam, usually generated from a rhenium filament, to ionize gas-phase atoms or molecules. Electrons from the beam (usually 70 eV) knock an electron from a bond of the atoms or molecules creating fragments and molecular ions [366,534,535]. Several factors contribute to the popularity of El ionization in environmental analyses such as stability, ease of operation, simple construction, precise beam intensity control, relatively high efficiency of ionization, and narrow kinetic energy spread of the ions formed. 

---