564 FRED M. UBER 



field perpendicular to their direction of motion, each particle experi- 

 ences a deflecting force. This force is perpendicular both to the direc- 

 tion of the magnetic field and to the trajectory of the particle. As a 

 consequence the charged particles are forced to move in circular paths 

 (see Fig. 3). For a constant magnetic field and for particles of the 

 same kinetic energy, the radius of curvature of any particle's path is 

 determined by the ratio of its mass to its electrical charge. Hence, 

 a mass spectrometer is an instrument for segregating ionized parti- 

 cles according to their mass-to-charge ratio. 



When a sample of a gaseous compound is subjected to electronic 

 bombardment in a mass spectrometer, usually several different ionized 

 particles are produced (see Table II). All of these ions are acceler- 

 ated by the same constant electrical potential and are then permitted 

 to pass between magnetic poles. Each particle will be deflected by 

 an amount characteristic of its mass-to-charge ratio. Consequently 

 the various particles may be segregated into groups by having them 

 pass through narrow exit slits to appropriate collectors {4, P- 398). 

 Since each particle carries a charge, the electric current through each 

 collector is a measure of the isotope concentration. Usually only one 

 collector is employed and measurements on the various isotopes are 

 obtained by altering the electrical accelerating potential. In this 

 manner, ions having any desired mass-to-charge ratio may be de- 

 flected at will through a single exit slit into the collecting chamber. 



3. When Should Stable Instead of Radioactive Isotopes 



Be Used? 



Isotopes of several important tracer elements are available in 

 both stable and radioactive forms. This is true of hydrogen, car- 

 bon, nitrogen, oxygen, and sulfur. Therefore, a decision must be 

 made as to whether to employ a stable or a radioactive isotope for a 

 particular problem. Some factors to be considered are: access to 

 measuring equipment, relative cost and technical skill required, dura- 

 tion of the experiment, possibility of interference from radioactive 

 radiations, relative difficulty of making measurements, and finally, 

 but often most important of all, the dilution factor that may be en- 

 countered. 



If apparatus is already in operation for making measurements on 

 one type of isotope, for example a mass spectrometer used for N^^, a 

 propcjsed investigation involving isotopic carlion might be made more 



