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Tracer Techniques /30 : 7 



presence or amount of stable isotopes. Instead, it is necessary to use a 

 mass spectrometer to measure the relative isotope abundance. In this 

 analysis, the material must first be converted to a gas. It is then 

 admitted under low pressure to a region in which it is bombarded with 

 electrons; this occurs in- the ionizing chamber, region (1) in Figure 4. 



Gas 

 Inlet 



Figure 4. Essentials of a mass spectrometer. 



(1) ionizing chamber (3) velocity spectrometer 



(2) accelerating region (4) detector 



The bombarding electrons tend to knock a valence electron out of the 

 atoms, which then become positive ions. These positive ions, if the 

 pressure is sufficiently low, are accelerated toward the cathode. They 

 pass through "canals," that is, holes in the cathode, and are accelerated 

 by a high voltage between the cathode and electrode A shown in Figure 4. 

 All the singly charged ions acquire the same kinetic energy E, namely 



E = \mv 2 = eV 



The velocities will be different for each isotope. The ions are then 

 separated according to velocity by bending in a magnetic field or other 

 means and are finally detected. 



Mass spectrometers can be used to separate measurable amounts of 

 isotopes or to detect the ratio of various isotopes present. A mass 

 spectrometer is larger and more complex than a scintillation counter, 

 but in theory it presents no additional problems. 



7. N 15 



Many stable isotopes are used as tracers in biological problems. The 

 specific example discussed here utilizes nitrogen isotope N 15 . Nitrogen 



