Separation of ions by electric and magnetic fields

The most common conventional gas source is an electron impact (El) source. This consists of a metal chamber with a volume of a few cm3, through which the sample flows in the form of a gas. Electrons produced by thermionic emission from a heated tungsten filament are passed through this gas, and accelerated by a relatively low voltage ( 100eV), causing ionization within the sample gas. A plate inside the chamber carries a low positive potential (the repeller ) which ejects the positive ions into a region which contains a series of plates (called lenses) and slits, which serve to focus, collimate, and accelerate the ion beam into the next part of the system 

These methods require that the sample is either a gas or, at least, a volatile substance which can be easily converted into a gas (this explains the utility of mass spectrometry in the field of organic chemistry). In inorganic chemistry it is often more difficult to obtain a gaseous sample, and so other ionization sources have been developed. If the sample is thermally stable, it may be volatilized by depositing it on a filament and heating the filament (thermal ionization mass spectrometry - see below). In restricted cases (e.g., organometallic chemistry), chemical treatment of the sample may give a more volatile sample. 

An ion of mass m with a charge of e is given a kinetic energy of E when it is extracted from the ion source by an applied voltage V, where  

The equation of motion of a charged particle in a magnetic field of strength B is given by the following, where r is the radius of the circular track taken by the ion  

For many applications in geochemistry and archaeology, the information desired from mass spectrometry is a precise measure of the abundance ratio of two or more isotopes of the same element - 12C/13C, or 160/180, or 206Pb/207Pb and 208Pb/207Pb at the heavier end of the mass scale. In these 

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